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SOIL F ACTORS AFFECTING CROP SUIT ABILITY FOR U PLAND CROPS IN C~~ODIA
Seng Vang', Richard Bell, Hin Sarith, Noel Schoknecht, Wendy Vance and P.F. White
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Camlx>dian Jownal of Agricullure: Volume 9, January-December 2009
and mungbean were most h1:ely to be profitable in the MWS. Generally yields on Prateah Lang soils were too low for profitable crop production. Howeve-r, there are very large areas of upland soils in Cambodia that have not been examined in the present study and hence the present paper should be treated as a preliminary assessment of crop suitability.
Keywords crop suitability, land quality, soil constraint, crop yield
ranking, upland and lowland soils.
Introduction Upland crop production has expanded markedly in
Cambodia in the last decade. Indeed even in the last few years~ major increases in crop production have been reported (MAFF, 2007). Maize, cassava and soybean account for most of the increase, and the provinces of Battambang and Kampong Cham have experienced greatest increases in output of non-rice crops. Market opportunities in Vietnam, Thailand and ebewhere (e.g. China, J. Spriggs, personal communication) are major driving factors in this expansion..
The main non-rice crops in Cambodia are maize, cassava, soybean, mtmgbean, and sugar cane (MAFF, 2007). Rubber is also an important crop in eastern Cambodia. Prestunably all these crops are suitable for growing in Cambodia, but not all are equally suited on all soils. Diversification of cropping in Cambodia will probably involve the expansion of more than one of these existing crops. Identifying the soil conditions that suit each crop and the most probable locations of such soils will improve the success of crop diversification in Cambodia. It is also necessary to understand the impacts of climate variability and potential climate change on crop diversification and the choice of!" suitable soiL.. The rate and scale of diversification are likely • to depend, in addition to soil and climatic fac tors, on mart.t factors and profitability.
Based on Landsat imagery of 1993, 1.25 million ha ofl land is potentially available for crops other tlian rice in Cambodia, and another 2.3 million ha of shru~lands may also have potential (World Bank/ UNDP/ FAq 1996). Only about 0.2 million ba is currently used for upland crops (MAFF, 2007), so this preliminary assessment suggests the potential for a 6-20 fold increase Of \!Pland agriculture, including cropping, agroforestry and grazing. Security considerations have until recentlY. lu;;'/ted the utilisation of ... ~ these areas. Now that those concerns have eased, and the standard of roads bas improved, population pressure and market access factors are likely to be key drivers of the e.'\.-pansion of upland cropping as they were in northern and north-eastern Thailand in. the 1960-70s (Chiang Mai University/ Chulalong!iom Social Research Unit 1983; Ruaysoongnem and-.5uphancbaimart 2001). However, the land suitability of these are.as in Cambodia has not been as."iiessed, apart from an estimate of the area that could be suited to rubber-J1roduction (FAO 1999).
In the present paper the aim was to synthesise findings from studies in Bauan district, Battambang pro\<ince) Ou Reang Ov)district, Kampong Cham pro•ince, and Trarnkak district, jakeo province to identify soil factors affecting crop suitaoilily for upland crops in Cambodia. The study areas represent, respectively, upland soils developed on sediments of various ages in north-west Cambodia, basaltic soils in eastern Cambodia and sandy soils in southern Cambodia.
Materials and Methods In the present paper we consider land qualities affecting
cropping land use, the extent to which they limit crop production in Cambodia and their variation among soil groups. We then review the soil-related crop requirements, based on literature information and results of on-farm trials in Battambang, Kampong Cham and Takeo pro•inces. Crop suitability is assessed based on trials conducted on farmers• fields with maize. mtmgbe.an, peanut, sesame and soybean. Finally a brief consideration is given to the potential gross margins for crops based on expected yield outcomes.
011-fnrm trials TriaL. were sown in the early wet season, EWS (May
August) and in the main wet season, MWS (July-October), in 2004 and 2005. Since there was no access to supplementary irrigation at all sites trial'{_eued completely on rainfall. Seed of the following species apd varieties were used for the on-fann triaL.; mungbean (11ig11a radiala) cv. Cardi Chey; soybean (Giyci11e rna!) cv. DT&4; maize (Zea mays) cv. Composit; peanut (Ar.ac~is hypogaea L.) Local variety, and; sesame (Sesamuw indicum L.) Local variety (white). TriaL. were conducted .in ':rrarnkak district (Takeo province), Ou Reang Ov district (Kompong Cham pro•ince) aod Banan district (Battarnbang province). There were 3-4 sites per district as determined by prior survey of soils (Hin et al. 2006 a, b, o). Soil profiles were inspected and described in detail at niost sites with fertilizer chemical characterization carried out on at least one site of each soil type .. If the e.~perim~ts were repeated on the same soils in both seasons. different sites were used The following Soil groups from the
t" Cambodia Agronomic Soil Cla.sification were included: Prateah Lang (clay subsoil); Prey Khmer (deep sandy); Kome?ng Siem (non-gravelly); Ou Reang Ov ; Laban.<iek; Kompong Siem (Calcareous); Toul Samroung; Kein S'"'Y·
,.._ All crop varieties received the following rates of fe-rtilizer nutrients (as modified from Dierolf et al. 2001; and ClAP, 1999). After installing seeding beds, 50% ofN and K and all
, of the remains fertilizers were mixed thoroughly and applied basally by spreading evenly, and then incorporated by hoeing (0-1 0 ern) into soil of each bed. These fertilizers were applied 48 hours before sowing seed The remaining 50% of N (as urea) and K (as KCI) were top-dressed within 3-4 weeks after sowing. Top-dressing of fertilizer was carried out after weeds had been removed.
The experimental field were ploughed (20-30 em deep) and properly leveled to control water and ensure e.ven soil fertility and uniformity. Drains were c.onstructed to remove e.xcess water from the field to avoid inundation. After harrowing, bed• I 0 m long x 1.5 m wide x 0.15 m high were installed In the main wet season, a deep drain of 15 em depth was dug arotmd each bed and the soil material heaped on the bed to raise its level for improved drainage. In the early wet season, only a shallow 10 em drain around the bed was set up.
The e.'\.--periment plots at each site were arranged in a randomized complete block design with 5 crop varieties and 4 replications. Crops were sown at recommended spacing. seed rate and depth. All e"perimental fields were kept free of weeds and insects during the progress of crop growth and developme.nt
At each site, soil samples were taken from 0-15 ern and 15-30 em depth before fertilizer application and after harvesting of crops in 2004. Soil samples were air dried, crushed to remove any plant residue, and sieved to pass through a 2-mm sieve. Soil analysis includes pH (water and CaC!,), organic C, N (Total and inorganic forms), Total P, K, S, Cu, Zn, Mn, Fe, B, exchangeable Ca, Mg, Na, K, and AI. These dsta are presented in Hin et al. (2006 a, b, c).
25
Smg Vang st aL Soil Facton Ajf.alng Crop Sl1irability for Upland Crops in Cambodia
Dry matter of shoot at harvest and }ield of pods and seeds wen dettrmined by rmdomly sampling 15 pLuus per plot. T oral seed yield per plot (pod }ield fur pe.mU1) was also dotmnmed at ban·m by maturity all plants in the ploL After seed weight was .-.corded, seed samples were taken to assess I 000-seed wetgbt, e:tcept for sesame.
Results and Discussion Land qualilfts lfmiting crop suitability
Broadly, land qualities limiting crop suitability fall into four c.aiegories:
• Tillage. hard setting, soil strength, stoniness, stitlciness wbeo wet;
• Gennioation and emergence- crusting, soil strength; • Growth· nutrient availability, acidity, water supply
(drougbl), waterlogging, inundation; • land and rMOOJce degradation risk- slope, dispersion,
leaching.
A modified rating of Lu!d qnalities for non-rice production m Cambodia was recen!ly proposed by Bell •t aL (2005, 2006), on whkh the discussion below is based. In addition, detailed laod eapability assessments for Bao.m district (Seng 11 al. 2007), On Reang Ov district (Bell et aL 2007&) and Tramkak district (Bellst al. 2007b) contain additional details to those discussed below.
Tillag• Land qualities that relate to tillage are particularly
significant in Cambodia where draft animal< are used to prepare Lu!d for most cropping. When dry, many of the soils of Cambodia, especially in the lowlands, have high strength or are hnd setting (White et aL 1991). They are therefore difficult to plough until the first one or two rainfall e\UI:! moisten ond soften the soils (White et aL 1991). The lowland futeah Lang soil bas high s~<>1h even when moist ~lie its sandy surface tel<tllre. On tbe other hand after fi]D$ ran, cloy sotls may be too stitlcy for efficient tilloge. Jncrea.SlDgly 2-wheel and 4-wheell!'llctors are being used for Lu!d prepo· ration, especially in upland areas of Cambodia, and so some revision of the ntings of limiting land qualities for tillage may be necessary for tractor tillage, High -.oil strength and bard setting may be less of a limitation for tractor drawn tillage, and may allow earlier laud P"'l'"~atiou and sowing in the early wet season. On the otheJ hand,. excessive we-tness and low load bearing capacity of clayey soils may be more of a limitario11 for tractor tillage,thau for draft animal<.
Crop tmt,tencl Tbe erratic rainfall over most of Cambodia creates
constraints for crop emergence .. April rain£all is critical for early wet season sowing of field crops but is extremely variable (Vaoce Ill <Jl. 2004). Inlense rainfall coupled with low soli struCNnl stability causes slalcing of surface soils m•bna them prone to fonn hard crusts as they dry. Crusting ~>ill uupede seedling emergeDCe. Crusting probably rel.ores to low orpnk motter le\•els and latlc of retention of crop resu!ues, but in addition cloys of Prateah Lang and Bakao soils exhibit dispersion (White et aL 1991). If the seedbed of these soils dries, the hard setting and dispersive tendency of soils is ••pressed as high soil strength that further limits crop establishmenl Poor soil structure is especially likely on paddy fields due to the repeated tillage of wet soals to prepare them for transpl.u!ting rice.
The constraints of intense rainfall and poor soil structure are exacerbated by the use of low quality seed. Fanners
26
commonly report that soybean seed has to be rt-S0\1111 if moisture conditions are less than ideal after sowing since rhe seeds have low vigour and c:anDOI tolerate either excess moisture or drying of the seedbed during emergence. In on-farm Dial< in 2004, poor emergence ond establishment of sesame and variable establishment of soybean, and mungbean were obserYed (Seng, unpublished dora). Only peanut and maize reliably established under most conditions.
Nutrient m•ailability Soil nutrient supply, per s•, is n0! con.sidered as a laud
quality for land capability, since fertiliser application enables these constraints to be overcome. HoWt\'tr, fe.niliser use on non.rice crops remains very low, based on smveys in Battambang and Kampong Cham (F arquanon " al. 2006) so for the time being crop performance will reflect the inherent nutrient supply of soils. E.'trernely low ._,tractable P le\'els are common in rice soils m Cambodia (White .. DL 1991). or the crops used in the present study, soybean is least tolerant of l<>w P levels (Dierolf 11 aL 2001). Hence either the lock of P f'eriiliser or the use of low le\'tls of fertiliser will limit SO}'bemmore thao other crops on low P soils. The land qualities related to nutrient supply and availability of nutrients are. generally those which are diffitult to chaoge, such as pH, AI toxicity, P sorption capacity and nutrient leaching.
Acidity Soil acidity is likely to be a significant limiting !'actor
for non-rice crops on many Cambodian soils. It has been largely overlooked previou•ly because flooding of soils for rice production neutralises acidity (Kirk 2004). However, Seng st al. (2005) showed strong responses by upland rice to Inn• application on the acid Prateah lang and Kolctrap soils (pH CaCh =4; AI saturation= 80%) when maintained in m aerated state. whereas oo response was found when these soils were flooded. Flooding increased soil pH to 5.5 or greater regardless of lime appliconon, whereas only the highest lime rate in this experimenl mcreased pH of unllooded soils to 5.5 and tbe unlimed, un.Oooded soil had pH < 5. From preliminary analysis of a range ofuplond soils from Kampong Cham and T akeo, AI toxieuy appears to be. a signilicam potential limiting factor for a range of field crops ()iin sr aL 2006a, 2007a). Among the five main upland soil< in On Reang Ov and Pouhea Krek districts in Kampoug Cham province, AI saturation was > 20% m the On Reang Ov, Prey Khmer, Prateah Lang, and the Labansiek non-petroferric phase (Hiu •I a/. 2007a,b). In the Labansiek non-petroferric soil, significant exchangeable Al was present in the surface layers, whereas in the Ou Reang Ov soil, it was confined to the sub-soiL In Prey Khmer soils in the uplands of western T akeo pro>ince and on sandy rises in Ponhea KreJ.: district, Kampong Cham province, AI saturation values of 50-80"1. were found in the sub-soil (Hin or aL 2006b, 2007b). According to tbe Fertility Capability Ct..sification, > 60% AI saturation is likely to limit most crops by AI toxicity, whereas only sensitn·e crops will be restricted in root gro,.ih at I 0- 60% AI saturation (Sanchez ., aL 2003).
The most AI toleraot of the crops grown m Cambodia are cassava, cowpea, upl.u!d rice ond rubber (Dierolf sf al. 2001). Peannt has low-moderate AI tolerance, whereas mungbeau, maize and soybean are generally low in AI tolerance. However, aD these species exhibit genotypic variation in AI toxicity tolerance so that the species ranking depends on tolerances of the particular varieties under consideration.
Camlx>dian Jownal of Agricullure: Volume 9, January-December 2009
Manganese toxicity and Mo deficiency are additional constraints that may limit crop production on the acid soils of Cambodia.
Alkalinity Alkalinity is not a significant limitation in Cambodia,
except on the soils that are developed from calcareous marl and limestone such a'i those in Battambang and the west of Cambodia (White et al. 1997). On these soiL<, maize and soybean are reasonably well adapted, but peanut, mungbean and sesame are prone to Fe deficiency on such soils. Unlike most- nutrient deficiencies, Fe deficiency is difficult to correct with fertilisers and the best prospects usually come from selection of suitable cultivars or species for growth on sucb bigb pH soils.
Waterlogging Intense. rainfall events will often lead to waterlogging in
both the EWS and MWS because the infiltration rate for soils i< mucb less than rainfall intensity. This i< beneficial for rice cultivation because it stores water than can be used later by the crop. Shallow groundwater during the M\VS can also induce waterlogging. For non-rice crops, waterlogging is usually harmful, especially if prolonged, and within 30 em of the soil surface. In addition, lowland soils may be prone to inundation whe-n pending of wate-r on the soil surface occurs either from intense rainfal~ or from river flooding.
Sesame) mungbean and peanut are all very sensitive to waterlogging or inundation events (Sys et al. 1993). Soybean is regarded as being better adapted to wet soiL< and waterlogging than other non-rice crops; however even this crop has poor tolerance compared to rice. Risks of inundation or flooding on lowland soils for more than a few days woulc[,., usually prohibit the use of a parcel of land for non-rice crops •
' tmless extensive drainage was \Ulde.rtaken.. Shallow groun<i-water levels beginning towards the end of the EWS IUJjf a!So be a risk for non-rice crops. Hence it is imporcaoqoiSelect freely-drained soils for field crops. Lowland rice soils pose. an extra waterlogging risk due both to bunding of tie.lds that limits surface drainage and to the development of a plough pan at 10-20 em depth which limits drainage within the profile. For example, the Bakan soil whicb has a loamy to clayey texture and occurs in rhe low lY!n?'liarts of the old terrace is particuLarly risky for non-~e crops (White ef al. 1997). Hence, the prospects for ~pwmg non-rice crops on the low lying Bakan soil durin~'tbe early wet season are considered to be low (White .!It aL 1991; Bell et al. 2006) because inundation and waterlogwg risk are very high.
'
Soil water storage Water supply (drougb.t) is a key Lintiting factor for most
areas of Cambodia -~cause of the monsoonal rainfall pattern
~~e~g~;;~~ e:'::L~~:~~!~~~L ~~o~).tM~~~~ the field crOJ?s g(OWD in the EWS receive. less than optimal rainfall in tqtal in most years. Hence the water storage capacity of"the soil would have a large bearing on the regnlation~of water availability to crops especially during peqc>as pflittle or no rainfall. Based on ave.rage transpiration rares-of 6-8 llllllfday (Fukai ef aL 1995), and farmers' reports that 5 days or more without rain cause wate-r stress in non-rice crops. > 35 mm water storage in the top metre of the profile is considered to be a minimum for non-rice crops. Greater than 70 mm soil water storage would allow crops to grow with minimal water stre.o;s for 10 days or more.
Whereas paddy rice is very shallow rooted and cannot e."Ploit water stored deeper in the sandy profiles, this may
not be a limitation for other crops. Hence deep sands (75-100 em) will have a higher potential for production of deeprooted fie.ld crops than for rice. By contrast, on soils bke the Prateah Lang, the de.nse sub-soil may impede root penetration so that the available stored wate-r is vezy low. making non-rice crops very prone to either waterlogging following intense rain, or drought following a short ,period without rain. The growth of non-rice field crops in"Paddy fielda during the EWS may be particularly"!'r~ne to water shortages. Another fotm of pan for non-nee crops is the ferricrete pan in the shallow phase of,the Prateab Lang and the petrofenic phase of Labansiek SUb-soil AI toxicity may also impede root growth and act as--a limit on root acces.<; to stored sub-soil water.
Crops vary in drought --tolerance, with soybean and maize generally less tolerant than sesame~ muugbean and peanut (Sys ef al. 1993;JBell et al. 2005). Hence. low soil water storage is a morl severe constraint for soybean and maize that other cro~, ~nd probably •'Plains why farmers' tend to grow these <;£<lps in the M\VS. The availability of soil water especially1.llrring the EWS dependa on the magnitude of rainfall ev.J/aitd soil texture. Light rainfall events may be sufficie~t to\rovide sufficient plant available water for crop eS)ablishmenl on sandy soiL< but not on heavy clay soils. On the latter soiL<, soaking rainfall events may be nece~ary to create sufficie.nt soil wate.r storage for successful gennination, emergence and crop establishment. Preliminary modelling of water balance using DSSAT for a feiection of Cambodian soils suggest< that the shallow depth
J">..of1soil wetting in the E\VS may be a limiting factor for crop ··- -production since it limits the amount of wate-r stored in the
root zone (Chen et aL unpublished data). lian'dand wafe'l' degradation
The key land degradation hazard that needa to be as<essed in upland soils is erosion potential. Slope angle and slope length in landscape elements are the main propenies that should be assessed for rating erosion risk. Soil proper-
11 ties that affect erosion include infiltration characteristics and surface soil strucmral stability. Anecdotal evidence suggests that sandy upland soils on slopes have an bigb risk of severe erosion if cleared and left bare in the wet se-ason. However, sunreys of farmers in three provinces of Cambodia suggest a moderate risk of erosion overall, and very low risk on thebasaltic terrain of Ou Reang Ov district in Karnpong Cham province (Cbea et aL 2007). In general, the fields were of moderate slope or less, and no steep land wa~ encountered in the surveys. The absence of reports on erosion in Ou Reang Ov district is consistent with the good soil structure and drainage of the soils present in the survey area (Hin et aL 2007a). The uphmd Labansiek soil bas s trong stable soil strucmre (\1/hite ef al. 1997) and would represent a lower erosion risk for land of similar slope to the sandy soils. The sloping land in basaltic terrain is co!ll!llonly occupied by Ou Reang Ov Soil group which has very good drainage. Similar conclusions were reached regarding low erosion risk of upland soils developed on basalt compared to those on metamorphic rocks in the Central Highlands of Vietnam (D'haeze et aL 2005). However, it is important that erosion risk be assessed in land capability classification to ensure that unsustainable land uses for field crops are not proposed. On the other band the emergence of minimum tillage and stubble retention systems for non-rice crop establishment would greatly reduce the erosion hazard.
Phosphorus leaching potential is also selected as a land quality since the export of P into water bodies may cause eutrophication that has harmful effects on food harvesting from these water bodies (Bell et al. 2001). Whilst this may
27
SBIIg Va11g eta/. Sail Factors Affocting Crop Suitability for Upla11d Crops ill Cambodia
not be an imminent risk in Cambodia due to low fertiliser usage, it could become a significant consideration in the medium term. Given the heavy dietacy dependence of Cambodia's population on freshwater fisheries and other aquatic resources, any degradation of their productivity would have-serious consequences for food security.
Soil ~'P• based l1md quality rati11gs Assuming that typical soil properties e.xist at a site, it is
possible to rate the land qualities based on severity of limitation likely for non-rice crops. Below is a summary of the land qualities for the basaltic soils asse .. sed in Ou Reang Ov district, Kampong Cham province, sandy soils assessed in Tramkak district, Takeo province, and the soils formed on sediments in Banan di..trict of Battambang pro\oince (Tables 7-10). For more detail on the land capability assessments in each di<trict the reader can refer to Seng et al. (2007) and Belle! aL (2007a,b).
Crop requirements Gene-ralised species differences in tolerance to various
constraints are outlined in Table 4. More specific discussion follows on the tolerances of peanut, soybean, maize, mungbean and sesame to soil constraints, based on published literature and on-farm trials in Tramkak district, Takeo province, Banan di..trict, Battambang province and Ou Reang Ov district, Kampong Cham province from 2003-2005. While the soil type rating of land capability provides a u'l.eful generalised assessment of limiting factors for crops, indi\.;dual crops, indeed individual varieties of a crop, may exhibit different tolerances of limiting factors which cause their suitability on a soil to vary from the ge-11eralised one.
Peanut Once established, peanut is Iegarded as dro)l~t
resistant. Generally in Cambodia, the total growing .§ea§on total rainfall is adequate for peanut both in the EWS\and in ) the MWS. However, periods of > 5 days withou~iu. occur -almost every year in the early wet season (Vance et al. 2004). Hence on sandy, gravely or shallow ~oi)s with less than 35 nun water storage in tbe root zone. crop'v.•ater stress occurs with a high level of probability In the early wet season and with late planting in the main'we( season. Overall the drought risk is lower in the MWS beCau.<e there is more reliable soil water availability {for J establishment and emergence than the EWS. This is reflected in on-farm trials conducted in 2004 and 2005 when 30% of EWS peanut crops failed, but none of ·the MWS crops. However, mini-droughts are also common in the MWS, and when these coincide with podding and seed filling, peanut yields may be impaired.
Peanut has broad soil adaptation in Cambodia because of its tolerance of at¥ soils and of drought. Peanut tolerates a wide range ofpf\: It is relatively tolerant of acid soih , but at pH (CaC1,},?" 4 its yield is greatly decreased, probably due to aluminium (AI) toxicity. Peanut is quite tolerant of manganese (Mn) toxicity on acid soils compared to other crops. P~anut rarely faih to establish even under dry conditions and generally produces an acceptable yield even when other crops fail due to drought.
The most common soil constraint of peanut is waterlogging: it is not as tolerant as soybean or maize as reflec-ted in the relatively poorer perfonuance of peanut on the clayey Kompong Siem soil in the main wet season compared to early wet season (Table 5).
For ease of pegging and seed production, soft soils are preferred whereas bardse.tting or massive smface- layers
inhibit pod production. Generally this means that soils with < 10% clay are favoured for peanut crops, but self-mulching clay soils such as the Kompong Siem Soil group may still produce high yields of peanut and allow satisfactocy podding. Similarly, for harvesting peanut pods, sandy soils, soft soils, or friable well-structured soils are ~referred to fac-ilitate the preparation of clean, soil-free pods.
Peanut is prone to iron (Fe) deficiency on'alkaline soils such as tbe Kompong Siem calcareous. phAs.e! However, it can still prodnce acceptable yields on these'soils tmlike crops such as mungbean and sesame which fiik:ompletely.
Average yields of peanut on si~s c!=ified according to the Cambodian Agronomic Soil G~ification (White et a/. 1997) are shown in Table 5, b~d on EWS and MWS crops in 2004 and 2005. Highest yi.!fds were fotmd in Kein Svay, Kompong Siem and T oul S3mronng Soil groups. These are the most favourable soils fo/
1
peanut, as they are for many othe-r crops. Hence., o~ese soils peanut will have to compete for profitability) with other crops. Using a DSSAT mode~ the water.!imited ·yield potential of peanut at Battambang was estimated"''o be 3134, 2874 and 3075 kglha for sowing on 15 A{gus~ 30 June and 18July, respectively for 2002, 2003 and>2004 (W. Chen, personal communication). The simulated' yield potential in 2004 (3075 kgtha) was similar to the yield (30()4 kg!ha) achieved in on-farm triah in Banan distric.t with the best input management of the crop. Henc~ this sugge..t that 3 tiha is a reasonable yield potential to '3iln for in peanut production on good quality soils in Cambodia. 'i. Yield was decreased by 0.5 tiha in the calcareous phase
of Kowong Siem soils compared to the basaltic phase. The Prex Kilmer and Ou Reang Ov Soil groups, that have low soil water storage, also had moderate yields. Lowest yie.lds, tljatl were comparable to those of farmers ' yields in Cambodia, were obtained on Labansiek and Prateah Lang Soil groups. The yie.lds in Labansiek in 2005 were especially low due to extreme soil acidity. The low }ield< on Prateah Lang Soil group are attributed to low soil water storage and root depth, coupled with waterlogging ri<k.
Main wet season yields were generally similar to or higher than those in the EWS. This may be. attributed to greater rainfall in the M\VS and better distribution of rainfall which decreases the risk of drought. High yield obtained on Prey Khmer soil with irrigation in 2005 (data not sown) is strong support for the impact of drought on peanut production in the EWS. The main exceptions we-re the lower MWS yie.lds on Kompong Siern soil in both years, which may be associated with greater waterlogging and the Labansiek soil in 2005 which was probably =ociated with the location of the trial on a strongly acid soil.
The main limiting factors in Soil groups are sununarised in Table 6. However, it should be noted that the limiting factors refer to the typical properties of each Soil group. Variation from site-to-site can occur, so the same limiting factors may not occur on all fields of the Soil group and the degree of limitation may vacy. For example, acidity varies considerably among sites of Prey Khmer and Labansiek Soil groups.
Soybean The preference of soybean i< for fertile, loancy soils.
Howeve.r> such soils favour a range of other crops, so soybean bas to compete with alternative crops on the better quality soils based on profitability.
Soybean lacks drought tolerance. Soybean roots are relatively shallow and have a low surface area to weight
"' ~
Tab I• I. Land capa~i[ity ratin~ for upland cropping basoo on the typical land quality values for the basaltic soils belonging to the Labansiek, Ou Reang Ov and Kompong Siem Soil groups. The overall land caRabilhy rating is baso<l on the rating of the most limiting land quality . Source; Bell et a/. (2007a)
Land qualities
Soil workability
Surface condition
-~ Ot
Surface soi I structure decline susceptihi liLy
pH (CaCI,) (0·20 em)
pH (CaCI,) (20.50 em)
Nutrient avallabi li ty
Waterlogging
lnundation
Soil water stOrage. (muvm)
Rooting depth (ern)
Water erosion risk
P export
Overall land capability
Labansiek
Value
;)()Qod, fa ir
Soft,Jirm
Low
4 .3-4.5
4.3-4.5
·"" ....
L''
Strongly acid, mod<r· ate leaching, high P retention
Very low
Very inFrequent
> 70
>SO
Moderate
Low
Low pH
Capability
Wry high
Very high
Very high
Fair
Fair
High
Very high
Very high
Very high
Very lligh
High
Very high
Fair
~$ \ ,
c
N«~ .'lbe: lat¥1 ~lity ndin8$ for$(*:ific (f()p$WiiJ VftY de:pblinaonind1vid\GJ «<>preqU.rell)IDCs.
Ou ReangOv
Value Capability
Poor
Firm, setf·muk hing with tendency to cn.JSt• ins
Moderate
4.5 to>S
4.3 to>S
Moderate to high leach-~i[jg._ti:s k, moderate to "t{igb P .retention, moder· ate acid~
Lo'v/ 1 ~ " ' Very in(requent
< 35
Fair
Very high -fair
High
Very high . high
Very high ·fair
High
High A
ye1y high
FaiY
>SO Very high C. i': I
Moderate High
Low Very high
Lo\\1 soil water storage Fa lr
Kompong Siem
Value. Capobaity
Poor Fair
Self·mulcbing Very lligh
Moderate High
>S Very high
>S Very high
Low lea<:hing risk Very high · high Moderate P retention
Moderate Fair
Very InFrequent Very High
> 70 Very lligh
35 tO >50 Very lligh ·high
hQ\V • Very high
Low Very high
Waterlosgtng.a.nd Fair soil workabilitY·
:n)e lflld q.aaUty~uet~re ltWni$C$ f<>c SOD~ •~I fiOual vai~Srufl)' vary ¢01$id«Jtbly at a $-ii/Ql s:ile.ln pw~icubr, ~pH varitlioo in ~k S<il $fe)Jp i.S" IJ:.etv (()pr«luce t.ianifiefOc._ln Cf()CibiJ•y 111ti:);a$ k> tboere te~~ here.
~ [ § ~ s a
<2,
i ~ ~
f _oo ;;;!!
i J "' ~
I ~ I Table 2. Land c:apabili'r ratings for upland cropping based on the typi<:al land quality values for tl1e sandy soils of t he Tramkak District in Takeo Province. Tbe overall land capability rating is based o n tlte rating of the most limiting land qualit y. Source: Beller al. (2007b) -
~
~-Prey Khmer soil Prey Khmer Soil group Prateah Lang Prateah Lang
j fine sand phase) (mediUill sand phase) (clay sub-soil phase) (loamy sub-soil phase) Land quatities • ---
" e$ Capability Values Capability Values Capability Values Capability ~ Valu •. :;·
Soil workability Good, fui r •Nery high Good, fair Very high Good· poor Fair Good· poor V cry h igh· fui r ... ~
' ~ ' .. Surfuce condition Soft .. frrm 'Very)ligh Soft, frrm Very high Hard setting Low Hardsetting Low ., ~
" r-
Su rfa.ce soi l structure dedi ne Hish High High Fair High Fatr ~ Moderate Moderate I ~ S\1 sctp tib il ity s:t
pH(CaCI,)(0-20 em) 4.3·4.5 Fair 4.6-S'or 4.3<'"+.-.;i High-fuir 4.6-5 High 4.6·5 10<4.3 High tO low I g
pH (CaCJ,) (20.50 em) <4.3 Low 4.6-5 or 4.3· 4,5 ~ High-fair 4.3-4.5 Fair 5-S to4.3-4.5 Very high to I ~ S' fair •• Nutrient availability High leaching, Fair High ~e.e~tillg . • pift, ll. Moderate leaehing High Moderateleaehing High It low PRJ low P retent.ioD
Very~igh §:
Waterlogging VeJy low Very high Nil~ very low "1 Moderate Fair Moderate to high Fair to low .,.
.....,-;. . A J • , "'V s "< A
"" Inundation Low Very high Nil, low / >SO em I) Vety high Moderate High Very hogh ...
" ~ Soil water StOrage (mm'm) 35-50 High 35-50 Fair High ·L Low Low S'
" ... ..., Rooting depLh >50 em Very high >50 em Very high Moderate- Hish
·~ >50 em Very high I ~ 'Fa1r~Low
:;· Water erosion risk Moderate to high High·fuir Moderate High Very low Very high- Low Very high I ~ Pe;q»rt Hish Fair High Fair Low ·Moderate Hish 'Lojv to moderate Very high to I ~ high Overall laod capability SulMoil acidity Low High leaching, Fair Hardsetting, water .. Low Hardse~ihg; water· Low
low soil watel' logsingand lnunda· Ioggins. acid.it)o and stOrage and acidity tion low soil water SJ,9r-
age
N«e:: ·The 1!00 capebility nrtiq;s fa spccifieerop will v.y depending oa indivi<btl crop~q.ri«m«U -The land <pllity valOOJareaverag03 fotsoil ~and a«ual valuatmay vwy eonsi<knblyat a given site.
"' -
Tablt J. Land capa!?jlizy ratin!]ll ror upland cropping based on Lhe typical land quality w lues ror lhe soils or the &nan District. B3uambang Province. The overall land capability rating is oosed on the rating Of t.he most limiting land quality. Sour~; Seog eJ a/. (2007) . -
~
:0 Kompong Siem soli Toul Samroung KeinSvay
Land qual it H>s 0,-y. (Calcareous phase)
Values Capability Values Capability Values Capability
Soil workability Poor • Rtir Good Very high Good Very high
Surface condition fim1 ~Gr. ~
Very high Soft, fimt Very high Soft.finu Very high
SurfaObsoil structuredoctinesuscep-tibility
Moderate t~lgb Low Very high Low Very high
pH(CaCI.) (0.20 em) 7.2-7.3 Very lu;h' 4.2-4.5 Low to Fair 5.6 Very High
pH (CaCI2) (20·50 em) 7.6-7.7 Fair ,J
I 4.3 Fair 5.3 Very high
Nutrient availability High P retention, low Fair ( LOw ~aching, moder· High Moderate leaching High Fe availability a\0 P re!,en\ion
~
') ~tsrr) W atel'logsi 11g High Low
~ Fair Low Veoy high • ,,
I lnu ndat ion Low Very high High ..,.Fair (early wet) to Low Very high Lo\v 1 ikin wet season)
<. Soil water storage (mm/m) Fair Fair - High
~/a Rooting deptlt 25·35 em Fair 40cm High >50 em Very high ~ Water erosion risk High Fair Low Very high ~ Very high
P export Low Fair Low Very high Moderate High
Overall land capability Alkalinity in subsoi~ fai•·· low lnundation. wate.·log· Fair lO LO\V Leachi•lS. null'ie'n1 ex· High pOOr workability. water- giug, acidity port. low soil water logging storage
N«e: -lbe I.DI capeblily raCings foe f4XCific crops wiD vary <kpa::ld.ilg en Odh·idual aq> rcqui"e:ruerKJ. -The land quality \elooaaro average~ for scil &fOlPS. ani ac«tal ~UoO:l may vary eomid<nbly at a given s:i~ . The assesst'llMt beloo> ofToul S111Uou~ Soil !J<q') reb10sto ()(:QJf'roi'CCS oo gi.W.Ie riSES in tb8 plat\ thM ~ Jess proro «)
inmdaticn and watedoggjng d!Sllhotn<reeorumonoecurencesofthe Soil gt<:lq).
l;l 11 [ §' ~ 5 :; .. -~ ~ ~· §' ~ < 0 r " .'<> -~ = " ~ ~ b " m if ~
"' 0 0 '0
Seng Vang et aL Sail Factars Affecting Crop Suitabilily for Upland Crops ill Cambodia
Table 4. Geuenilised tolerances of crop species to limiting land qualities. The main sources of information used to rate tolerances were Sys et aL (1993) and Dierolf et aL (2001) Land quality Maize Mungbean Peanut Sesame Soybean Acidity- Mn toxicity S Acidity-AI toxicity S Alkalinity S Seed eme-rgence T Waterlogging S Drought S Shallow root depth VS VS-wry seositiw;. ~ sensiti'\te; T- tolenun; VT- \tery toleraJll.
vs vs vs T
vs T T
T T s
VT s
VT s
s s
vs vs s T s
s s
vs T
VT vs T
Table 5. Yields of peanut pods (tlba) from on-farm trials on various Soil groups in 2004 and 2005 in earl}' wet season (EWS) and main wet season (MWS) planting
Soil gt·oup EWS 2004
Kompong Siem 2.75 Kein Svay 2.50 Toul Samroung 2.25 Kompong Siem calcareous 2.00 Prey Khmer 2.50 OuReangOv 1.50 Labansiek 0 Prateah Lang 0 Mean 1.44
EWS200S 3.37 136 2.42
2.65 028 1.96 1.49 1.13 1.85
MWS2004
2.09 3.04 1.84 1.79
1.59 1.48 2.02_. 1.28 1.84
MWSZOOS ' ~.73 ' 3.55
3.02 2.47 2.04 2.01 1.15 1.72 2.25
Mean
2.72 2.64 2.38 2.23 1.67 1.53 1.17 1.03 1.85
Table 6. Main limiting factors for peanut on various Soil groups when planted in early wet season (E\VS- April to mid May planting) and main wet season (M\VS- July to August planting)
Soil group E\VS Kein Svay Moderate waterlogging T oul Sarnroung Moderate acidity
' 1\>fWS Moderate waterlogging Moderate acidity, waterlogging
Kompong Siem
Kompong Siem calcareous
High soil moisture required for crop eslllblisbment, hard when dry, sticky when wel Alkalinity, bard when dr)( ,su'cl-y wbeno.wet. high soil moisture required for cr~ e$tablishment Moderate to severe 'f.cidity, ~w nutrient retention, low soil \fater storage
Waterlogging, hard when dry, sticky when wet
Alkalinity, bard when dry, sticky when wet, waterlogging.
Prey Khmer Moderate to severe acidity~ low nutrient retention,. low soil water storage
OuReangOv
Labaa<iek
Hard setting at s\uface, low soil water storage Mod-erate to -severe acidity. moderate soil water storage
Hard setting at surface, low soil water storage Moderate to severe acidity. moderate soil water storage
Prateah Lang Hard serting,"low soil water storage. waterloggmg
Hard se-tting. ac-idity. low soil water storage, waterlogging
ratio, both of which limit its water absorption during dry periods. Few soybean crops even established successfully when planted in May, and those that did rarely produced satisfactory yield< (Table "?· Soybean performed poorly on sandy soils (Prey Khmer and Piateah Lang Soil groups) and on soils with low soil water storage such as gravely or shallow soils (e.g. Ou Reaug<Ov Soil group). Largely this can be attributed to drou@t ri§li. With supplernenlllry irrigation or with favourable rainfall distribution, reasonable yields could be possible on salllf, shallow or gravely soils.
Soyb~h~good tolerance of waterlogging compared to other non-nCe crops. Hence it is best grown in the M\VS. It may also tolerate flood irrigation techniques more effectively than/.?tber crops. On clay soils (Kompoug Siem and La¥,D'sie'k Soil groups), good soaking rains are needed for reliablel'soybean establishment. Low rainfall on clay soils will result in limited soil water availability for seed germination and plant establishment of soybean.
Soybean bas a relatively narrow pH range compared to peanut. It is not tolerant of strongly acid soils and commonly suffers Al or Mn toxicity on such soils.
Soybean cv. DT84 is prone to Fe chlorosis on alkaline soils. Other varieties may be less susce.ptible but there is
32
insufficient knowledge of cuJtivar differences among Cambodian cultivars.
Average soybean yields in Cambodia were- low compared to international ave-rages. They also only reached about 50% of what is regarded as maximum yield potential for soybean with high inputs (F arqubarson et al. 2006). Howeve-r~ with good management, on the better quality soils, soybean yields in Cambodia can exceed 3 tlba (Table 7). Using a DSSAT model calibrated for DT84 soybean and 1998 rainfall at Pochentong airport, Vance el al. (2006) found a ma:wnurn simulated soybean yield of 2.9 tlha, based on 30 May sowing on a Kompong Siern Soil profile. By contrast, on a Prey Khmer profile, the maxim\un simulated soybean yield was 2.5 tlha. This suggests 3 tlha is a reasonable yield lllrget for soybean on good quality soils in Cambodia, but most crops currently fail to reach this potential.
In the MWS, highest soybean yields were obtained on Kompong Siem and Kein Svay Soil groups. The high yields were- more consistent on Kein Svay soil Satisfactory soybean yields were also obtained on T oul Samrouug Soil group. All other soils produced modest yields, comparable to average farmers' yields in Cambodia despite the high level
Camlx>dian Jownal of Agricullure: Volume 9, January-December 2009
of inputs and management Variable yie-lds were obtained on Ou Reang Ov, Labansiek and Prey Khmer Soil groups, probably due to variation in drought stress and soil acidity. On a moderately acid form of Labansiek Soil group, high soybean yields were obtained in 2004, in contrast to the crop failuxe on a very acid form of Labansiek in 2005.
The main limiting factors for soybean in Soil groups are summarised in TableS. However, it should be noted that the limiting factors refer to the typical properties of each soil
group. Variation from site-to-site can occur, so the same limiting factors may not occur on all fields of the soil group and the degree of limitation may vary. For example, the Ou Reang Ov and Prey Khmer soil groups that have low soil water storage may produce reasonable soybean yield in seasons when rainfall is high and its distribution is even throughout the growing season. But on the sa.ti.~oil, poor rainfall or extended periods without rainfall may result in ...
Table 7. Yields of soybean seed (tlha) from on-farm trials on various Soil groups in 2004 and 2005 in early \l:,et season (EWS) and main wet season (M\ll S) planting "' S~il group EWS 2004 EWS 2005 MWS 1004 MWS 2005 Mean A Kompong Siern 0.09 0 3.32 1.46 2.39 Kein Svay 0 0 2.03 2.65 2.34 Toul Sarnroung 0.53 0 1.20 2.'!>2 1.61 Kompong Siem 0 14 0 I 17 13- I 25 (calcareous) · · · ) · Ou Reang Ov 0.36 0 1.4 0.77 1.1 Labansiek 0 0 2.19 ( 0 1.1 Prey Khmer 1.03 0.26 0.35 1.23 0.7& Prateah Lang 0 0 0.90 ) 0.53 0.72 Mean 0.35 0.2 1.43 1.20 1.31 "' Calculated fot mafo wa seasoo Otlly.
very poor yields. ..Maize
I
Mungbean Maize is fairly broadly adapted to a wide range of soils Mtmgbean are not broadly adapted to a wide range of with pH from 5.0-S.O. However, maize is sensitive to
soils. Yields vary considerably from year to year and site to drought. Hence low soil water storage ic; a more severe con-siie (Tables 9). Their prime asset is short duration that allows sfr:lint for maize than other crops apart from soybean. Sandy, them to be grown opportunistically when there is insufficient t" grave!>' and shallow soils increase the risk of drought in time to complete the life cycle of other crops. Camliodia and good yields are unlikely on these soils with-
Mungbe.an are not acid tolerant Manganese toxicity is out supplemental)' irrigation or favourable rainfall distribu-oue of the main limiting factors on acid soils. but AI toxicicy_ • fion during the growing season. By contrast. maize has rea-and molybdenum (Mo) deficiency may also be associa~ sonable tolerance. of waterlogging. but tolerance is lowest at with poor acid soil performance. Yields were generally (poor the tasselling stage (Zaidi et al. 2004). on Labansiek Soil group, probably due to soil acidity., Maize is not as acid tolerant as peanut. The level of AI
Mungbean prefers good drainage and does .nol tolerate ' saturation in soil for maize should be < 40% (Dierolf et al. waterlogging as well as maize or soybean. Their bighe.st seed 2001). The Composit variety of maize is tolerant of alkali yields we.re obtained on the sandy Prey Khme6 and well soils and does not e.xhibit Fe chlorosis when fanners, varie-drained Ou Reang Ov Soil groups (Table. 9):.)ly contrast ties in the same field did. The tolerance of hybrid red maize complete failure was consiste-ntly fotmd on the-waterlogging- to Fe chlorosis on alkaline soils is not knowt1. prone Prateah Lang soils. Highest mean yields of maize in on-farm trials were
Mungbean is prone to Fe. chloroSis on alkaline soils. obtained on Kien Svay and Kompong Siem Soil groups Complete crop failure may be foun~ on.alkaline soils like the Table 10). However, yields in both soils were quite variable Kompong Siem Soil group, calca~ phase using cv. Cardi among s ites and seasons, and low compared to average yield Chey. Other varieties may beje.~ susceptible but there is obtained by fanners in the border districts of Battambang on insufficient knowledge of culfivar· differences among Cam- recently cleared forest soils. Low yields were obtained on bodian cultivars. Prey Khmer soils despite reasonable rates of fertiliser addi-
Table 8. Main soil limiting factors for soybean on various Soil groups when planied in early wet season (EWS- April to mid May planting) and main wet season (MWS- July to August planting). Note: drought is a major limiting factor for soybeao in the early wet season Soil group Kein Svay r T oul Sarnroun!)
Kompong;s}m
KomPOng Siem (calcareous)
Prey Khmer
OuReangOv
Labansiek
Prateah Lang
EWS Moderate soil water storage Acidity High soil moisture required for crop establishme.nt, sticky when wet Alkalinity, hard when dry, sticky when wet. high soil moisnxre required for crop establishment, low soil water storage on shallow soilc; Acidity~ low nutrient retention, low soil water storage Hard setting, Low soil wate-r storage Moderate to severe. acidity. moderate soil water storage
Hard setting, low soil water storage, acidity
Waterlogging Acidity
MWS
Waterlogging, sticl-y when wet
Alkalinity, sticl-y when weL
Acidity~ low nutrient retention, low soil water storage Hard setting. Low soil water storage-Moderate to severe acidity. moderate. soil water storage Hard se.tting. acidity, low soil water storage, wa-
33
Sm1g Vang •t aL Soil Factors Ajfocting Crop Suitability for Upland Crops in Cambodia
lion and most of the maize crops on Prateab Lang soils failed below a reasonable yield potential for Cambodia and suggest to produce any yield. Using a DSSAT model cabl>rated for further improvements in agronomy are needed to achieve the maize in Cambodia, Vance et al. (2006) found a maximum yield potential. simulated yield of 6.1 tlba on a Kompong Siem profile with Supplying adequate amotmts of nutrients is required to I July sowing, and 5.1 Uba on a Prey Khmer proflle with 1 achieve bigb yield of maize. The rates of nutrients to be May sowing. Hence the present on-farm trial yields fell well supplied to a crop depend on soil types, climate, cultivar, r Table 9. Yields ofmungbean seed (tlba) from on-farm trials on various Soil groups in 2004 and 2005 in early wet season (EWSMay) and main wet season (MWS- mid-July to mid-August) planting Soil group EWS 2-004 EWS 2005
Prey Khmer 1.76 0.65 Ou Reang Ov 1.32 0 Kompong Siern Kein Svay Toul Samrotmg
0.06 0.92
Kompong Siem (calcareous) Labansiek Prateab Lang
0 1.03
0.27 0 0
0 0
0 0 0
Mean 0.6 0.22
target yield and management practice.s. Preliminary estimat~s of maize fertiliser requirement<; for Cambodia are reponed in Belfield et al. (2007).
In Cambodia, on lowland paddy sandy soil (Prateab Lang soil) maize responded strongly to an application of N at 60 kglba or P alone at 60 kg P20,1ha (Seng tmpublished data). Maize responses to P application vary widely because soils are variable in available P as well as their capacity to adso!b added phosphate. In Cambodia, on lowland paddy sandy soil (Prateah Lang soil) maize had no marked response to applied potassium (K) alone. However, when K was add-ed at 60 kg K20 Jha together with N and P, it produced 40% 1' additional grain yield. Zinc deficiency is common on calcay eous soils. In Cambodia, calcareous soils can be fotmd_.;r Battambang (Banan district and districts along the border with Thailand) but there are presently no confirmed reports of Zn deficiency. -
MWS 2004
0.3 0.66 1.53 0.61 0.42
0.64 0.42
0 .5. 0.56
~!WS 2005
.{
1.03 ' 0.48 0.9 2.02 'l'o1 0.06
0 0
0.71
Mean
0.93 0.9 0.85 0.66 0.63 0.24
0.11 0
Little. is known about sesame's soil preferences. Like mungbean, it-grew very poorly on alkaline Kompong Siern calcareQU~pbase soils in Banan district Growth on acid Prey Kluner soiltin Tramkak was reasonable suggesting that acid tolerance was satic;fac.tory, however, generally sesame is regarded as being sensitive to acid soils (Table 4). Given that many farmers grow sesame on Kompong Siem soils, it ap'jlears to be waterlogging tolerant once established, althougb overall sesame is regarded as sensitive to waterlogging (Table4). ,.
Soil factors affecting crop yield and profitability " • On-fatm trials indicate that the yield of crops varies with soils and season.< (Tables 5, 7, 9, 10). In order to account for differences in maximum yield among species, seasons, and years, yields were ranked based on relative yields (Table II). The ranking of soils \<aried between E\VS and MWS planting. Since crop yields were generally higher
Sesame for the M\VS crop, most emphasis is placed on the ranking The preference of sesame is for fertile, well-drained for this season. In the MWS, Kompong Siern and Kein Svay
loamy soih. Most current production is oulo'amy and clayey Soil groups were ranked highest amongst those assessed. soils in Karnpong Cham (Ou Reang OV J~nd Kompong Siem However, for non-rice crops, the parent material for Kom-Soil groups), and Battambang. Howev~trial results indicate pong Siem soils bas a major bearing on crop productivity. that reasonable yields are possibll' on sandy Prey Khmer The calcareous phase of Kompong Siem that forms from soils with appropriate fertiliser inputs. limestone or marl in Banan district (Belief aL 2007) was one
Achieving reliable emergence and seedling survival is a of the least productive soils while the Kompong Siem soil on major constraint to sesame. Sesame emergence and early basalt in Ou Reang Ov district, Kampong Cham province establishment appears to be especially damaged by intense was the most productive. This illustrates how suitability of rainfall and insect damag~to eme.rging seedlings, and fam - soils for non. rice crops can vary from that of lowland rice. on ers tend to sow it very early before frequent heavy rainfall is the same soiLs. Generally crop yield performance in the likely. MWS was poor on the Prateab Lang, Prey Khmer and
Table 10. Yiel~\,f maize seed (Uba) from on-fann trials on various Soil groups in 2004 and 2005 in early wet season (EWSMay) and main we season (MWS- mid-July to mid-August) planting Soil group EWS 2004 EWS 2-00S
Kein Svay 2.75 1.16 KompongiSiern 1.75 0 Toul Samrotmg Kompong Siern (calcareous)
Labansiek OuReangOv Prey Khmer Prateab Lang Mean
34
2.25 1.5
2.25 3.0 1.5 0
1.88
1.3 2.52
0 0.67 1.13
0 0.86
MWS 2004
2.85 4.09 1.87 1.4
4.29 1.85 1.1 4
0.78 2.13
MWS 2-005
3.19 2.09 1.92 1.34
0 0.79 1.40
0 1.31
Mean 2.49 1.98 1.84
1.69
1.64 1.58 1.29
0.2 1.55
Cambodian Jow'llal of Agricullwe: Volume 9, January-Deoonber 2009
Labansiek Soil groups. Crop productivity dropped in the EWS relative the
MWS on Kompong Siem, Kein Svy and Ou Reang Ov Soil group, while relatively it increased in Toul Samroung, Prey Kluner and Kompong Siem calcareous.
For individual species, the ranking on particular soil types varied from the overall ranking. Indeed all crops bad much lower yield in 2005 than in 2004 on the Labansiek soil on account of lower soil pH in 2005, which was U'J'ressed in prevalent Mn toxicity symptoms in plants. Mungbean performed well on the Prey Khmer Soil group compared to other species which may indicate sen."Sitivity to waterlogging intheMWS.
Crop yields on Ou Reang Ov soil in the MWS of 2004 were generally 30--50% lower than those on Kompong Sie.m and Labansiek Soil groups in maize, mungbean, peanut and soybean (Seug eta/. 2007). Major limiting factors on the Ou Reang Ov soils were considered to be low soil water storage, and possible AI toxicity in the snb-soil. Crop yields in the MWS of 2004 on Kompong Siem soil outstripped those of Ou Reang Ov and Laban.siek Soil groups (Tables 5, 7, 9, 10), both of which have better internal and eK!emal drainage. This suggests that waterlogging did not develop strongly in the 2004 main wet season on Kompong Siem.
Yields on Prey Khmer soil tended to be similar or greater in the EWS than the MWS, by contrast with results in other soils. Peanut yields on Kompong Siem, Kompong Siem calcareous and Toul Samro\Ulg soils were highe.r in the EWS than MWS. This suggests that waterlogging may be a constraint for peanut on the clayey textured soils in the main wet season..
gross margins calculated by Scott (2006), the likely profit from growing these crops was assessed and comparisons made among them. The above rating of land qualities and crop yield performance presumes that no technology bas been applied to alleviate or overcome the limitation. Clearly there are often opportunities to do so. Waterlogging, for e.xarnple can be alleviated by raised beds aJI!' ·<!§ins: when this is done., the seve.rity of the limitation ic;~ecreased,. and the crop producti,;ty increased accordingiY"Similarly, with erosion control measures are implemente~ the capability of sloping land for croppiug ";11 be upgraded. Poor surface soil structure may be alleviated by minimum tillage and crop residue retention. Finally surface ..._soil)acidity can be treated with lime. Hence many land qualities are not fixed properties of soils.
Profitability of the crop~ was estimated using crop yields obtained in the on-faml trials (Table 12) and the gross margins calculated by .Scott (2006) for fanners ' crops in Thong Khmum and Chamkar Leu districts, Kampoug Cham province, and Rattaiiali Mondo! <listric~ Battambang province. These assume prices for grain as (ollows: maize US87.50/t; mungb'ean US325-3751t; peanut US200-250/t, and; soybean US200-235ft. At these prices, break even yields were:aft!h for maize; 0.4-0.5 tlba for mungbe.an; 0.8-1.0 tlba for soybean, and; 1.2 tlba for peanut.
'Early ~et season cropping was generally not likely to prodUce profit at the. yield levels obtained except with peanu( Ouly the Prey Khmer soils grew profitable peanut in the EWS. Mungbean was a possible alternative to peanut on the Prey Khmer soils, but was not profitable on other soils. Even ~anut was not be e.xpected to be profitable on Prateah Lang
soils>i!> the EWS. None of the soybean or maize crops pro-Land suitability duced profitable yields in theEWS of2004 and 2005.
The overall land suitability depends on both the limiting ( ~ Main wet season cropping was much more profitable factors as assessed by land qualities, and the socio-economic beCause of higher yields. Hence there were more suitable factors that determine which land use is most likely to be crop options on most soils in the M\VS. On the soils with selected (Bell et aL 2006). Beyond the land use choice; are lower yields, Prateah Laug, Prey Khmer, soybean ceased to choices of wbat specific crop(s) to grow? The crop yield - be a profitable option, ouly peanut appeared to be capable to rankings outlined above (Table I I) indicate the ~ield poten- produciug profit on these soils in the MWS. As with the tial for non-rice crops determined by soil andrseason. Using EWS cropping. maize was an unprofitable option on all sites Table 11. Ranking of yield performance of ero/. on soils (I =highest; &=lowest), based on on-farm trials in 2004 and 2005 in the early wet and main wet seasons. Ranl:s~r~determined as follows: firstly, mean yield for each crop on each soil was calculated; secondly, relative yields across soils (~;:Yo' of the maximum obtained in any one soil in each year) were ranked from I (highest) to 8 (lowest) for each crop; finally, ranks were summed across crop species to determine mean soil ranking
Maize Aluncrbean Soybean Peanut Soil mean Main wet season 2·004+2005
Kein Svay "?:U) 2 I 2 2 1.8 Kompong Siem I 2 I 3 1.8 OuReangOv ..... 3 3 3 I 2.5 Totti Samrong 5 4 4 3 4.0 Kompong Siem (calcareou.s) 6 6 5 5 5.5 Labansiek I 4 7 6 7 6.0 Prey Khmer 7 5 7 6 6.3 Prateah Lang 8 8 8 8 8.0
Eal'ly w.r season 2004+2005 Kein Svay 3 6 6 5 5.0 Kompo.ng Siem 7 3 5 I 4.0 OuReangOv 6 4 3 6 4.8 Totti Samrong 4 2 2 2 2.5 Kompong Siem (calcareous) 2 5 4 3 3.5 Labansiek I 6 6 7 5.0 Prey Khmer 5 I I 4 2.8 Prateah Lang 8 6 6 8 7.0
35
Se11g Va11g eta/. Soil Factors Affecting Crop Suitability for Upla11d Crops ill Cambodia
at the yield levels obtained in the MWS. Maize yields of 4 t1 ha or more. were needed to reach profit leveL-s comparable to the other crops evaluated. Sucb yield• are obtained on farm. ers' fields in the western border districts on soils recently cleared from forest. The sustainability of such yields without fertiliser and other inputs is still untested. However, it is cle.ar that substantial research investment is still needed to determine the comtraints to maize yield elsewhere in Cambodia, aud to find profitable technologies for overcoming the constraints. Inadequate nutrients, soil acidity, waterlogging and low soil water storage are probably all contributing factors to the low maize yields. As discussed above, DSSAT modelling suggests that yields of 5 tlba are obtainable on Prey Khmer soils in Cambodia with only rainfall as a limiting fllctor.
Conclusions In the present paper we identified soil factors affecting
crop suitability for upland crops in Cambodia. The key limiting soil factors for maize, mtmgbean, peanut, soybean and sesame were revie\\ted. A soil type-based framework for rating land qualities indicated that soil acidity, all:alinity, waterlogging and profsle \vater storage are most common constraints, but hard setting and excessive wetness may limit tillage and seed emergence .. Overall crop trials indicated that peanut was the most reliable species that rarely tailed to produce barvestable yield, while sesame was least reliable. Most crops apart from peanut produced low and tmprofitable yield in the E\VS. Soybean produced many crop failures in the E\VS and would generally not be recomme--nded for planting in May or earlier. From crop trials in the MWS, Kein Svay, Kompong Siem (on basalt), Ott Reang Ov and T oul Samrotmg Soil groups bad the highest capability for, upland crops although the latter soil, like others on the mar; gins of the rainfed lowlands are risky environments for w· land crops in the E\VS and MWS due to the probability of waterlogging and inundation. Peanttt followed by ~cry,8ean
profitable crop production. However, there are still very large areas of upland soils in Cambodia that have not been exantined and hence the present paper should be treated as a preliminary assessme.nt of crop suitability.
Acknowledgements The attthors gratefully acknowledge. A<(!AR for fund
ing, the Cambodia Agricttltttre Research an~ Development Institute for support, the Provincial Department of Agriculture Forestry and Fisheries in Battamban~, Karnpong Cham and Takeo for coordinating trials and fat\ners for proving land for the trials and for carrying .oiltJand preparation and crop maintenance operations.
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