5 J. Agronomy & Crop Science 172, 113—118 (1994)© 1994 Paul Parey Scientific Publishers, Berlin and HamburgISSN 0931-2250

Faculty of Agriculture, University of Peradeniya, Peradeniya, Sri Lanka

Response of Sweet Potato (Ipomoea batatas L.)to Different Planting Times

U. R. SANGAKKARA

Author^s address: Dr. U. R. SANGAKKARA, Faculty of Agriculture, University of Peradeniya, Peradeniya, SriLanka.

With 3 tables

Received March 4, 1993; accepted July 14, 1993

Abstract

Agricultural seasons of the tropics are associated with rainfall, which provides the principal limiting resourcefor crop production. However, as tuber crops are sensitive to temperatures and moisture, the time of plantingcould have a profound influence on yields. Thus a study was carried out over a period of 12 months todetermine the effect of different planting times on establishment, tuber initiation and yields of sweet potato.The trial was planted on similar soils in two agroecological zones as this species is a popular home garden cropin most regions of the tropics and subtropics.

Planting sweet potato with the onset of rains in October produced the highest yields. This is attributed tothe receipt of adequate rainfall over the growth cycle, along with the higher diurnal variation in temperatures.Planting in the dry season or later in the wet season, which receives a lower quantity of rainfall with lowdiurnal variations in temperatures delayed tuber initiation and reduced yields. The study highlights differ-ences in growth patterns of sweet potato when planed at different times. The importance of planting sweetpotato in agricultural systems at appropriate times to produce higher yields is presented.

Key words: Sweet potato; planting, seasonal effects, yields.

Introduction

Sweet potato (Ipomoea batatas L.) is a widelydistributed tropical tuber crop, grown in dif-ferent ecological zones due to its adaptabilityto climatic and edaphic conditions (FAO 1990).It*s success also stems from the relatively shortgrowth cycle in contrast to most other tropicaltubers, and the lack of seasonality in terms ofclimate, where adverse weather does not resultin complete crop loss.

Most studies identify the relatively easymanagement of sweet potato (ONWUEME 1977,FAO 1990). However, research does illustratethe response of this species to climatic variablessuch as temperature (KIM 1961, SHAMSUDDIN

and PAUL 1988); water stress (INDRA andKABEERATHUMMA 1988, SMITTLE et al. 1990) and

light (BISWAS et al. 1989). In field studies, theeffect of planting times on the growth of vinesand tubers of sweet potato have been reported,primarily under controlled or summer condi-tions of the temperate regions (e.g. SAJJAPONGSE

et al. 1988). These field studies demonstratethat sweet potato produces low yields undervery high temperatures and wet conditions inthe field.

Sweet potato is ranked among the populartuber crops of Sri Lanka (Department of Ag-riculture, 1990), due to its widespread cultiva-tion in the different ecological zones of theisland. The species is predominantly a small-holder crop, and it is planted at different timesof the year in small allotments in order toensure the presence of some food at times of

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114 SANGAKKARA

need. Yields of this crop in Sri Lanka varywidely, which could be due to poor manage-ment or to differences in the climatic condi-tions of the year. However, these differencesin yields have not been adequately quantifiedin Sri Lanka and other parts of the humidtropics, although earlier studies (SANGAKKARA

1989) show differential responses on thisspecies to fertilizer in wet and dry seasons.Thus, a study was conducted under field con-ditions, to evaluate the response of sweetpotato in terms of establishment and yieldparameters, when planted at the beginning andlatter parts of major agricultural seasons of SriLanka in two distinct agroecological zones.

Materials and Methods

The study consited of similar experiments conductedat two locations in the wet and intermediate ag-roecological zones of Sri Lanka. These locationswere at the experimental units of the University ofPeradeniya at Peradeniya (9° N, 80.5° E, 413 mabove sea level) and at Kundasala (9° N, 81° E,410 m above sea level). The former site (LI) was inthe wet zone, with two distinctive rainy season,giving 2110 mm rainfall per annum, while the latter(L2) was located in the intermediate zone, withdistinct rainy and dry seasons receiving 1790 mmrainfall per annum.

The trials were sited on an Ultisol, with the fol-lowmg general characteristics, which were similar atboth sites — pH 6.83 ± 0.12 (1: 2.5 H2O); % TotalN —0.16 % ± 0.12; Available P 32.41 ± 2.44 ppm;Exchangeable K 31.09 ± 1.12 ppm and a CEC of18.19 ± 2.14 m eq/lOOgsoil.

Vegetative propagules of sweet potato (VarietyC26) consisting of four nodes and leaves (mean drywt 0.341 g -I- 0.12) were obtained from a singlesource, and planted at a spacing of 75 X 50 cm onwell prepared beds of dimensions 5 X 3 m. The

times of planting at both locations were 10 Octoher,10 December, 10 April and 10 June. These datescorresponded to the normal onset of the wet season,latter part of wet season, onset of rains in the dryseason and later part of the dry season respectively.Each planting date consisted of five replicates withina Randomized Block Design.

All plots were managed uniformly after establish-ment. Thus no irrigation was provided as per normalfarmer practice, and hand weeding was done at 20and 40 days after planting. The fertilizer applied wasas per local recommendations (GUNASENA 1973),and each planting date received 20 kg N, 70 kg P2O5and 80 kg K2O at planting followed by 10 kg N and25 kg K2O at 30 days after planting.

The data collected at all planting dates were —— Percentage establishment and percentage survival

at 15 and 25 days after planting— Days to tuber initiation, by uprooting plants at

three day intervals beginning 25 days after plant-ing, until all plots showed tuber initiation

— Tuber bulking rates (g/wk) by uprooting plants at12 day intervals until the final harvest and re-cording tuber weights corrected to 65 % mois-ture content

— Tuber per plant at 80 days after planting— Final harvest at 175 days after planting, along

with the percentage marketable tubers. The yieldper plant was corrected to 65 % moisture con-tent, which is the normal moisture content ofmarketed tubers (FAO 1990).

The rainfall over the experimental period, thedaily temperatures, humidity and pan evaporation ateach site was obtained from meteorological stationslocated within one km of each location. The com-puted day lengths of the experimental seasons wereobtained from the Meteorology Department, Col-ombo, Sri Lanka.

Results and Discussion

The season beginning in October, which cor-responds to the major agricultural season of Sri

Table 1. Selected climatic parameters of the wet and dry seasons at experimental sites

Cumulative rainfall (mm)MeanMeanMeanMeanMean

daily temp. (°C)diurnal variation (°C)monthly RH (%)daylength (hours)par evaporation (mm/day)

Wet

95227.1 ±6.6 ±

71.4 ±

1.46 ±

season

1.640.944.2

0.12

(Oct.-

29,4,

76,

-Feb.)L2

714.2 ± 2.06.2 ± 1.01.5 ± 3.99—10

2.84 ± 0.34

Dry seasonL,

56228.6 ± 2.41

4.6 ± 0.5869.6 ± 2.8

3.42 ± 0.21

(Apr.—Aug.)

u314

30.6 ± 1.952.8 ± 0.21

64.4 ± 1.910—11

4.62 ± 0.31

and L2 are wet and dry agroecological zones respectively.

Response of Sweet Potato to Different Planting Times 115

- s

Table 2.

Locatioi

L,

s.U

Effect of season

i'^ Season

Wet

Dry

Wet

Dry

and date

Date ofplanting

EarlyLateEarlyLate

EarlyLateEarlyLate

of planting on establishment

% estab-lishment

94.695.190.681.8—

85.676.975.868.5—

and tuber initiation

o//osurvival

88.487.681.770.6—

80.170.963.850.4—

in sweet potato

Days totuber initiation

36384544

2.0440434645

1.96

l and L2 locations are wet and dry agroecological zones respectively.

Lanka has a good climate for arable crop pro-duction. The rainfall received at both locationsis sufficient for crops, while the temperaturesare relatively milder with a higher diurnal vari-ation (Table 1). In addition, the shorter day-length, which is a prerequisite for most tropi-cal crops (ONWUEME 1977) is also observed.Although a higher humidity is seen in the wetseason, this is not considered a problem insweet potato.

In contrast, the dry season places consider-able heat and water stress on crops, especiallyin the dry location (L2). As presented inTable 1, L2 receives a lower quantity or rain-fall, which is significantly lower than the panevaporation over the period. Thus rainfed ag-riculture often fails in such locations due towater and heat stress. Furthermore, the greaterdaylength could have an adverse effect ontuberization in root crops (BISWAS et al. 1989),although BoNSi et al. (1988) report the absenceof effect of light on sweet potato.

Planting in the dry season in Ll does notsubject crops to moisture stress, due to theadequacy of rainfall (Table 1). The adequacyof water could also overcome the stress placedby temperature. Thus rainfed agriculture ispossible in the wet regions in both seasons.

Time of planting of sweet potato has a sig-nificant effect on establishment, survival andtuber initiation (Table 2). Generally, the per-centage of establishment and survival is greaterin Ll at all plantings. This is attributed to thepresence of adequate water, which is a require-ment for sweet potato establishment.

Planting of sweet potato at the onset of rainsensures a high rate of establishment and survi-val in all locations in both seasons. The excep-tion is the wet season of Ll, where there is nosignificant difference between the two plantingdates. This again could be considered a resul-tant effect of the availability of sufficient mois-ture in this season and location. The lack ofmoisture at the end of the season places thepropagules under stress, resulting in poor es-tablishment and more importantly survival ofestablished plants. This clearly demonstratesthe importance of water at the time of plantingsweet potato.

Time of establishment plays a deterministicrole in tuber initiation of tropical tubers (ON-WUEME 1977). This is demonstrated in sweetpotato (Table 2), where planting in the wetseason, which subjects the propagules to short-er days and greater diurnal variations in tem-peratures initiate tubers earlier. The reportedimpact (WILSON 1982) of lower temperatureson greater cell division and expansion could beconsidered the causal factor. This is furtherconfirmed by the delayed tuber initiation (byA—5 days) at the dry location (L2) due higherambient temperatures and lower diurnal varia-tions.

Planting late in both seasons delays tuberinitiation as propagules are subjected to mois-ture stress even in Ll, which receives a higherquantum of rainfall. Thus the study demon-strates the significant impact of moisture, al-though this is coupled with higher tempera-

116

Table 3. Relationship

Location''' Season

L,

%

Wet

Dry

Wet

Dry

between

Date ofplanting

EarlyLateEarlyLate

EarlyLateEarlyLate

location, season,

Tubers/plant

6.66.34.02.30.825.34.63.01.30.46

date of planting

% marketabletubers/plant

8 5 %6 5 %7 0 %5 5 %

70%4 5 %5 5 %4 0 %

SANGAKKARA

and yield parameters in sweet potato

Bulkingrate (g/wk)

74.6552.1461.4441.089.88

60.4638.4345.6526.8416.94

Yield/plant

(g)

1256711

806526242.5840412618306195.6

Yield/m'

(kg)

3.251.902.141.380.082.231.091.670.820.03

i and L2 locations are wet and dry agroecological zones respectively.

tures and differences in diurnal variations. Thisit warrants further study.

The time of planting influences yield com-ponents of sweet potato significantly(Table 3). In LI, planting early in the wetseason has no major impact on tuber numbersper plant. In contrast, late planting in L2 evenin the wet season reduces tuber numbers by13 %.

The impact of time of planting is signifi-cantly greater in other yield components,which determine final yields. Planting early inLI in the wet season increases the percentagemarketable tubers by 20 %, while the differ-ence at L2 in the same season is 35 %. Asimilar phenomenon is also observed in bulk-ing rates. The difference in the bulking rates ofsweet potato when planted early in the wetseason at LI and L2 are 29 % and 37 % re-spectively. This indicates that bulking rates aremore sensitive to the time of planting than thenumber of tubers or percentage marketabletubers. The impact of delayed planting is great-er in L2 which subjects the plants to a greaterdegree of moisture and heat stress due to itsclimate.

The impact of planting time is reflected bet-ter in the data derived from the dry season. InL2, all yield components are reduced, irrespec-tive of the time of planting. This is due to thewarmer and drier climate in this season, whichinduces stress conditions in sweet potato.However, a comparison of the planting datesillustrates significant differences. In LI, earlyplanting produces 42 % more tubers, while the

difference in L2 is 52 %. In contrast, the per-centage marketable tubers are decreased by15 % due to late planting at LI and L2 respec-tively. This could be attributed to the climateand to delayed tuber initiation when plantedlate.

Planting late reduces the bulking rates by33 % and 42 % in LI and L2 in the dry season.The impact of water stress and higher tempera-tures which is greater in the latter part of thedry season in both locations is seen, whichconfirms the earlier studies (e.g. SMTTTLE et̂ al.1990) on water stress in sweet potato. The dataalso demonstrates the greater reduction inbulking rates in dry locations at times corre-sponding to higher moisture stress.

Productivity of sweet potato is determinedby yields, which in turn are affected by yieldcomponents. Yields per plant and yields perunit area (Table 3) are significantly affected bythe time of planting. In overall terms, plantingin the wet regions represented by LI, producesgreater yields. The difference in yield by earlyplanting in LI over that of L2 is 33 %, whichclearly illustrates the suitabihty of sweet potatoto wet regions with relatively lower tempera-tures.

Late planting in the wet season reducesyields of sweet potato at LI and L2 by 42 %and 51 % respectively. This is again a reflec-tion of the effect of climate, and illustrates theadvantages of early planting. The adverse effectis greater in the dry location (L2) which hashigher temperature and lower moisture. How-ever, the impact of the time of planting is lower

Response of Sweet Potato to Different Planting Times 117

in the dry season. The reductions in yields dueto late planting at LI and L2 is 34 % and 50 %respectively. This phenomenon is similar tothat observed in percentage marketable tubers.It also suggests that bulking rates, which deter-mine tuber growth, are affected to a greaterextent by late planting in the dry season thanpercentage marketable tubers and tuber yieldsof sweet potato.

There was a positive significant (P = 0.05)correlation between bulking rates and yieldsper plant in the early and late plantings of bothlocations. The correlation coefficients were0.84 and 0.67 for early and late plantings re-spectively. This shows the positive relation-ships between climate and yields, along withthe greater variability of tuber production inthe late plantings, which again is an attribute ofthe more stressful climatic conditions. In addi-tion, the study demonstrates the positive rela-tionship between climatic conditions andyields of sweet potato in the tropics, as sug-gested by IzuMi (1989) in the temperate re-gions.

Farmers in the tropics plant one crop afteranother. Thus sweet potato often follows othercrops which are more susceptible to dry condi-tions, due to its wide adaptability. However,some farmers do plant sweet potato with therains. The impact of the time planting is seen inthis study. Farmers could harvest high yieldsof a marketable product if sweet potato isplanted at the onset of rains in agriculturalseasons having relatively low temperatures.However, yields are generally decreased indrier regions, and also when planted towardsthe end of seasons when the rainfall receivedduring crop growth is lower. The impact oflate planting is caused by water stress, whichaffects tuber growth and development in sweetpotato (SMITTLE et al. 1990), although the ef-fects of temperatures cannot be ignored. In wetseasons, the lower temperatures and greaterdiurnal variations cannot be ignored in ex-plaining yield reductions in sweet potato.

Practically, farmers need to be encouragedto plant sweet potato at the beginning of sea-sons, and higher yields could be obtained inmore suitable regions. It will also be advan-tageous to plant a short duration and droughtresistant or avoiding crop such as some grainlegumes at the latter part of a season, whenearly crops fail. However, it may also be ad-

vantageous to farmers to plant sweet potato incase of crop failure in the latter part of aseason, especially in a wet region, due to itsadaptability. Under such conditions, thefarmer may obtain some yield by utilizing landthat may not be planted until the next season.

Zusammenfassung

Reaktion von Siifikartoffeln {Ipomoea ba-tatas L.) gegenliber unterschiedlichenPflanzzeiten

Die landwirtschaftliche Anbausaison in denTropen ist mit dem Rcgenfall assoziiert; Re-genfall ist die grundlegende limitierende Um-weltwirkung hinsichtlich der Pflanzenproduk-tion. Da allerdings KnoUenfrlichte empfindlichsind gegeniiber Temperatur und Feuchtigkeit,kann die Pflanzzeit erhebliche Auswirkung aufdie Ertrage haben. Die Untersuchung wurdedurchgefiihrt iiber eine Periode von 12 Mona-ten, um den Einflul^ unterschiedlicher Pflanz-zeiten hinsichtlich des Aufwuchses, der Knol-lenanlage und der Ertrage von Siiftkartoffelnzu untersuchen. Die Versuchsanlage wurdeunter vergleichbaren Bodenbedingungen inzwei agrookologischen Zonen lm Hinbhck aufdie verbreitete Verwendung der Suf^kartoffel ingartenbaulicher Nutzung in den meisten Ge-bieten der Tropen und Subtropen angelegt.Die Pflanzzeit fiir Siif^kartoffeln mit dem Ein-setzen des Regenfalls im Oktober ergab diehochsten Ertrage. Dies wird zurUckgefiihrt aufdie Verfiigbarkeit ausreichenden NiederschlagsUber den Wachstumszeitraum lm Zusammen-hang mit den hoheren diurnalen Schwankun-gen in den Temperaturen. Auspflanzen in dertrockenen Saison oder spater in der Feuchtsai-son, wodurch eine geringere Menge von Nie-derschlag zusammen mit geringen diurnalenSchwankungen in den Temperaturen wirksamwird, verzogerte die Knollenanlage und redu-zierte die Ertrage. Die Untersuchungen bele-gen die Unterschiede in dem Wachstumsver-halten von Siifikartoffeln bei einem Anbau zuunterschiedlichen Zeiten. Die Bedeutung desAnbaus von Siifikartoffeln in ackerbaulichenSystemen hinsichtlich der geeigneten Aus-pflanzzeit unter Beriicksichtigung der Ertragewird durch die vorstehenden Untersuchungenbelegt.

118 SANGAKKARA, Response of Sweet Potato to Different Planting Times

Acknowledgements

The authors are grateful to Messrs E. R. PlYADASAand N. GALAHITIYAWA for research assistance and toMs S. N. W. MEEGAHAKUMBURA for secretarialwork.

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