6Chemical Characteristicsof Soilless Media
6.1 Charge Characteristics
6.2 Specific Adsorption and Interactions betweenCations/Anions and Substrate Solids
6.3 Plant-induced Changes in the Rhizosphere
6.4 Nutrient Release from Inorganic and OrganicSubstrates
AEC anion exchange capacity;CEC cation exchange capacity;HA [Ca5(PO4)3OH];OCP [Ca4H(PO4)32.5(H2O)];PW solution P concentrations;PZNC point of zero net charge;PZSE point of zero salt effect;SAI specifically adsorbed ion;TCP [Ca3(PO4)2];ZPT suspension pH prior to the addition of protons or hydroxyls.
210 Chapter 6 Chemical Characteristics of Soilless Media
The surfaces of particles and other solids in horticultural substrates bear permanentand/or variable electrical charges. The surface charge properties and the dissolutioncharacteristics of the substrate solids are important, since they affect the ionic com-position of nutrient solutions. Ordinarily, the chemical properties of substrates areestablished prior to their use for growing plants; therefore undisrupted, homogeneoussolid phases are addressed. However, throughout the growth period organic compoundsexcreted from plant roots or resulting from decomposition processes accumulate in thesubstrate (Tate and Theng, 1980; Huang and Violante, 1986; Tan, 1986; Silber andRaviv, 1996). Thus, the surfaces of the newly formed solids become heterogeneous andthe chemical properties of the mixture may be significantly different from those of thesingle, well-defined component that was characterized before use (Silber and Raviv,1996). Plants growing in soilless culture typically have smaller root-system volumethan plants growing in soil culture. Therefore, the root density of soilless-grown plantsis higher so that such plants have a greater impact on the rhizosphere than soil-grownplants. Despite extensive research relating to irrigation and fertilization managementor plant growth in soilless culture, the literature regarding the chemical properties ofsoilless media is scarce. Although peat, perlite, stone wool and mixtures of organicwith inorganic materials are abundant in the horticulture industry, the literature regard-ing the chemical properties of these media is insufficient. Tuff is a volcanic materialused as a substrate for horticultural crops in Italy, Spain, France, Turkey and Israel,and had been the subject of considerable research with respect to chemical propertiesas a horticultural substrate. Thus tuff is used in this chapter as a model for illustrationand elucidation of the chemical processes taking place in soilless media systems.
6.1 CHARGE CHARACTERISTICS
The surfaces of solids in horticultural substrates of volcanic or organic origincarry permanent and/or variable positive and negative electrical charges. Permanentnegative charge results from isomorphous substitutions (Gast, 1977; McBride, 1989,2000; Sumner and Miller, 1996; Sposito, 1989, 2000), that is substitution of structuralcation by lower valency cation that has the same coordination number and size inlayer-silicates (common examples: Si4+ by Al3+ or Al3+ by Mg2+). The extent ofcation adsorption to the surfaces is referred to as the cation exchange capacity (CEC;cmol kg1) and is used to characterize the cation exchange properties of the medium(Gast, 1977; Sposito, 1989).
The CEC of several inorganic and organic horticultural substrates are presented inTable 6.1. These values represent CEC measurement of intact materials prior to use,and it is important to note that plant growth may affect the chemical properties of thesubstrate solids. CEC values of soil components are usually referred by weight (i.e.,cmol kg1). However, soilless production system involve a relative small fixed rootzone volume and therefore it is more practical to relate substrates CEC to volumeunit (cmol L1). By using volume-based comparison it is possible to properly compareboth heavy and light media.
6.1 Charge Characteristics 211
TABLE 6.1 Cation Exchange Capacity (CEC) of Several Horticultural Substrates
Substrate cmol kg1 cmol L1 Reference
Perlite 2535 24 Dogan and Alkan (2004)Stone wool 34 5 Argo and Biernbaum (1997)Tuff 1060a 1060 Silber et al. (1994)Clinoptilolite (zeolite) 200400 400800 Mumpton (1999)
Coconut coir 3960 24 Evans et al. (1996)Peat 90140 711 Puustjarvi and Robertson (1975)Pine bark 98b 10 Daniels and Wright (1988)Compost 160180 1520 Inbar (1989)
a CEC in pH 7, tuff has variable charge surfaces as shown in Fig. 6.2.b CEC in pH 7, pine bark has variable charge surfaces as shown in Fig. 6.4B.
The variable charge (both negative and positive, depending on the solution pH)is generated mainly from the adsorption of H+ and OH on solid surfaces suchas metal oxides, hydroxides, microcrystalline silicates (allophane and imogolite), oron functional organic groups (Stevenson, 1994). The effects of solution pH on themagnitude of the surface charge may be determined experimentally by measuring theCEC and the anion exchange capacity (AEC) over a range of pH values. The pH atwhich the AEC is equal to the CEC is referred to as the point of zero net charge(PZNC) (Parker et al., 1979; Sposito, 1981, 1984, 2000) and is frequently used tocharacterize the charge properties of the medium. Note that in all cases, pH is referredto the solutions. An alternative experimental method is to conduct a pH titration curveusing an indifferent electrolyte (Gast, 1977; McBride, 1989, 2000; Sparks et al., 1996).A typical potentiometric titration for woody peat is presented in Fig. 6.1 (Bloom,1979).
The pH value at which a series of titration curves for various electrolyte concentra-tions intersect is referred to as the point of zero salt effect (PZSE; Parker et al., 1979;Sposito, 1981, 1984, 2000). However, common substrates (compost, peat moss, tuff,stone wool) rarely consist of a single, well-defined component. They typically containa mixture that presents both permanent and variable surface charges. The interpreta-tion of the experimental CEC, AEC or pH titration data obtained from these mixturesis, therefore, complicated. Moreover, the existence of additional sources or sinks forH+/OH derived from the dissolution/precipitation of minerals during the CEC, AECor pH titration analyses makes direct interpretation of analytical results even moreambiguous.
212 Chapter 6 Chemical Characteristics of Soilless Media
pH = 3.9
0.1N NaCl0.01N NaCl
FIGURE 6.1 Acid-base potentiometric titration curves of woody peat. Reprinted from Bloom (1979),with kind permission from the Soil Science Society of America Journal.
The difficulties involved in the determination of the actual surface charge of aheterogeneous material that contains both pH-dependent and permanently chargedsurfaces, together with primary and secondary minerals, can be seen in the case oftuff (Figs. 6.2 and 6.3). Yellow, Red and Black tuff, commonly used in greenhouseproduction, are characterized by the degree of weathering, with Yellow tuff beingmost weathered and Black tuff being the least weathered. The CECs of Yellow, Redand Black tuff were found to be pH dependent, with the CEC increasing by 6.6, 4.2and 1.9 cmol kg1, respectively, for every unit increase of pH (Silber et al., 1994).
5 7 9 11
FIGURE 6.2 Cation exchange capacity versus pH for black, red and yellow tuffs from northern Israel.Reprinted from Silber et al. (1994), with kind permission from Elsevier Science.
6.1 Charge Characteristics 213
7 9 115pH
FIGURE 6.3 Potentiometric titration curves of yellow tuff: (A) acid/base addition (0.1N HCl andNaOH, respectively) as functions of pH and NaCl concentrations, tuff/solution ratio of 1/40, equilibriumperiod of 1 week. ZPT is the suspension pH prior to the addition of acid or base; and (B) net surface chargecalculated with Eq. (1). Based on Silber (1991).
The differing values of CEC slope among the three types of tuff reflect differencesin their contents of amorphous materials with variable-charged surfaces. Silber et al.(1994) found, however, that it was not possible to determine the AEC of these tuffsbecause phosphorus released from indigenous CaP minerals during their analysis wasre-adsorbed on the tuff surfaces and thus interfered with the AEC determination. TheCEC of tuff at pH 3.5, especially that of the weathered Yellow tuff, was relativelyhigh (35 cmol kg1) and may indicate the presence of components having a permanentcharge. The pH titration curves of the tuff types differed significantly from thoseof homogenous variable-charged materials. In the cases of the less-weathered tuffs(Red tuff and Black tuff), the pH titration lines obtained with three different NaCl
214 Chapter 6 Chemical Characteristics of Soilless Media
concentrations overlapped (data not presented). Distinction of an ionic strength effectwas possible only for the Yellow tuff at a pH above 7 (Fig. 6.3A).
The overlap of potentiometric titration curves at low pHs in volcanic material fromChile was attributed to the exchange of added H+ ions with ca