Effect of Composted Organic Matter on Boron Uptake by Plants

B is one of the seven essential micronutrients Boron uptake by plants is controlled by the B level in soil solution required for the normal growth of most plants. rather than the total B content in soil. The affinity of organic matter for However, the range of B concentrations in the soil soluB can affect B uptake by plants because of changing B concentration in tion causing neither deficiency nor toxicity symptoms soil solution. The role of soil organic matter content on B soil solution in plants is narrow. The amount of B adsorbed by soils concentration and uptake by bell pepper (Capsicum frutescens L.) varies greatly with the contents of various soil constitwere studied. The organic matter used was mature compost (COM), uents, mostly clay minerals, sesquioxides, and organic produced from the solid fraction of separated straw-containing cattle matter (Keren and Bingham, 1985). The factors influmanure. Plants were grown in five soil–sand–COM mixtures conencing B adsorption and desorption from soil constittaining 0, 1, 3, 6, or 10% COM by weight. Four levels of B were uents are: (i) B concentration in the soil solution, (ii) pH, applied. The soil was analyzed for B content at the beginning of the (iii) type of exchangeable ions, (iv) ionic composition of experiment and at harvest. Boron concentration in the leaves was determined 45 d from planting. Boron concentration in the soil soluthe soil solution, and (v) wetting and drying cycles tion at the beginning of the experiment decreased with increasing (Keren and Bingham, 1985). Boron adsorption and delevels of COM. This decrease was most prominent at high levels of sorption from soil adsorption sites regulate the B conB application. The effect of the COM level on leaf B concentration centration in the soil solution. This regulation depends was also prominent at high B application rates, with increasing levels on the changes in solution B concentration and on the of COM resulting in less B in the leaf tissues. Boron concentration affinity of the soil constituents for B (Mezuman and in the leaves was highly significantly correlated (r 2 0.88) with the B Keren, 1981). Thus, adsorbed B may buffer fluctuations concentration adjusted in the soil solution. This correlation coefficient in solution B concentration, and B concentration in soil was further improved (r 2 0.98) when B concentration in the soil solutions may vary only slightly with changes in soil solution was calculated using a B adsorption model. The results prewater content. sented herein indicate that organic matter plays an important role in controlling B concentration in the soil solution, and that it has a The role of organic matter in B distribution between prominent effect on reducing B uptake by plants. the liquid and solid phases of soils is not yet fully understood. Boron deficiency has been observed in soils with high organic matter contents (Hue et al., 1988; MascaU. Yermiyahu, Gilat Experimental Station, Agricultural Research renhas et al., 1988; Liu et al., 1989; Valk et al., 1989). Organization Mobile Post Negev, Israel; R. Keren, Institute of Soil, This deficiency has been shown to be related to the high Water and Environmental Sciences, Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel; and Y. Chen, The Seaaffinity of organic matter to B (Berger and Pratt, 1963; gram Center for Soil and Water Sciences, Faculty of Agricultural, Yermiyahu et al., 1988, 1995; Liu et al., 1989). The posiFood and Environmental Quality Sciences, P.O. Box 12, The Hebrew tive correlations between soil organic matter content Univ. of Jerusalem, Rehovot 76100, Israel. Contribution from the and B adsorption (Elrashidi and O’Connor, 1982; Hue Institute of Soil, Water, and Environmental Sciences, Agricultural et al., 1988) support this hypothesis. Boron adsorption Research Organization, The Volcani Center, Bet Dagan, Israel, no. Series 607/99, and from The Hebrew Univ. of Jerusalem, Rehovot, by soils has been observed to be minor in acidic to near Israel. Received 19 Jan. 2000. *Corresponding author (uri4@ neutral pH, but may be of greater significance in high netvision.net.il). Abbreviations: COM, compost organic matter. Published in Soil Sci. Soc. Am. J. 65:1436–1441 (2001). YERMIYAHU ET AL.: EFFECT OF COMPOSTED ORGANIC MATTER ON BORON UPTAKE 1437 adsorbed B associated with the added B. Therefore, the pH soils in the presence of organic matter (Gu and initial adsorbed B was ignored in the calculations (see beLowe, 1990). Garate and Meyer (1983) concluded that low), assuming zero B adsorption. The solution electrical conthe main factors affecting B retention by organic matter ductance was 0.01 dS m . Ash content of the compost was were pH, Ca, and fulvic acid contents, and the humic:ful29%. Humic and fulvic acid fractions were extracted from vic acid ratio. In contrast, Marzadori et al. (1991) rethe compost by sequential extractions using 0.1 mM NaOH, ported that the amount of B adsorbed by soil is considerfollowing the procedures recommended by the International ably greater after the organic matter has been removed Humic Substances Society (St. Paul, MN). The concentration and that hysteresis is observed. Adding organic matter of the humic acid based on total dry weight and on organic to soil has also been reported to increase B content and matter dry weight was 112 g kg 1 and 158 g kg , respectively. The concentration of the extracted fulvic acid fraction based its availability to plants (Blagojevic and Zarkovic, 1990; on total dry weight and on organic matter dry weight was 40 g Pakrashi and Haldar, 1992). These observations are in kg 1 and 56 g kg , respectively. The total concentration of contradiction with the well-known findings that boron humic substances amounted to 214 g kg 1 of the organic matadsorption by organic matter is substantially higher than ter. Further chemical details on the compost have been dethat adsorbed by clays (Yermiyahu et al., 1988). scribed by Inbar et al. (1986). Boron adsorption by organic matter and soils has been described by a competitive adsorption model (MePlant Growth Experiment zuman and Keren, 1981; Yermiyahu et al., 1988). This model allows for the fact that two aqueous B species, The soil was sieved through a 2-mm sieve and mixed thoroughly with washed COM, and pure sand was added as an B(OH)3 and B(OH) 4 , having different affinities to the inert substance with respect to B adsorption. The total amount adsorbent, are involved and that their proportions in the of added COM in the mixture (soil–sand–COM on a dryequilibrium solution vary with pH. With this adsorption weight basis) was 0, 10, 30, 60, or 100 g kg 1 of mixture and model, the B adsorption capacity of the soil-COM mixthe added sand content was 100, 90, 70, 40, or 0 g kg 1 of ture was seen to increase with COM content (Yermimixture, respectively. The loess soil content was constant in yahu et al., 1995). all mixtures. Each soil–sand–COM mixture was tested at four The role of soil organic matter content on B soil levels of added B (0, 0.1, 0.5, or 1.0 mmol kg 1 ). Boron was solution concentration and B uptake by plants is not added as boric acid (B(OH)3 ) in solution. Each treatment fully understood. Boron uptake by plants is, generally, consisted of five pots, filled with 2 kg of soil–sand–COM mixcontrolled by the B level in the soil solution rather ture, and arranged in the randomized complete block experiment design. Two days after B addition, 2-wk-old pepper than total B content in the soil (Keren et al., 1985a,b). seedlings were planted in each pot. The water contents of the Because the affinity of organic matter for B is higher soil mixtures in the pots were maintained between 25 to 30% than that of clay minerals, at a given total B content, B (w/w) by adding distilled water at least daily. A mulch of uptake by plants is expected to decrease as the organic polystyrene spheres was used to reduce evaporation during matter content in the soil increases. This hypothesis is the growing season. Boron-free half-Hoagland solution of 100 tested in the current study, the objective of which to mL per pot was added weekly (Hoagland and Arnon, 1950). determine the effects of organic matter in soil on B The plants were harvested 45 d after planting by removal of availability to the test plant bell pepper. the entire plant from the pot. Leaves were separated from stems, rinsed three times with deionized water, and dried at 65 C. The dried leaves were weighed, ground, and stored in MATERIALS AND METHODS a desiccator. Leaf samples were analyzed by ashing 0.25-g Soil and Composted Organic Matter samples in a furnace at 550 C for 4 h. Five milliliters of 1 M HCl were added to the cooled ash, and the solution was filtered Soil (Loess, calcic Haploxeralf) was collected from an unculafter 15 min. A 3-mL aliquot was taken for B analysis using tivated field in the southern coastal plain of Israel. This soil the azomethine-H procedure (Gupta and Stewart, 1975). After developed from loessial windblown material originating from plant harvest, the moist soil from each pot was well-mixed the Sinai Desert. Soil characteristics obtained by routine proand a saturated paste was prepared and allowed to reach cedures were: contents of clay, silt, and sand: 16, 43, and 41%, an equilibrium. Boron in the saturated paste solution was respectively; CaCO3, 45 g kg 1 (Nelson, 1982); organic matter, determined by the azomethine-H procedure. The electrical 11 g kg 1 (Nelson and Sommers, 1982); cation exchange capacconductance of all saturated paste extracts was below 1.5 dS ity, 168 mmolc kg 1 (Rhoades, 1982); pH of saturated paste, m , considered the value above which pepper growth is im7.8 (McLean, 1982); and B concentration in saturated soil paired by salinity (Maas, 1977). extract, 0.028 mmol L 1 (Gupta and Stewart, 1975). Mature compost produced from the solid fraction of sepaComputations rated straw-containing cattle manure was used in this study. The COM was leached in a column with deionized water Boron distribution between the solid and liquid phases at at a solid:liquid ratio of 1:60 and then dried in an oven at a water content of 30% (w/w) for different soil–sand–COM 30 C. Thirty mL of deionized water were added to 2 g of COM mixtures was calculated using the adsorption equations (Eq. and shaken for 4 h; the solution was then decanted using high[1] and [2] proposed by Keren et al. (1981): speed centrifuge. Water-extractable Na, K, Mg, Cl, SO4 and NO3 in the dried COM were determined. The concentraQBT T[KHB(HB) KB(B)] 1 KHB(HB) KB(B) KOH(OH) [1] tion of those ions was 0.05 mmol L . The B concentration in the extraction solution was 0.04 mmol L . Although the total B content in the washed COM was not determined, where QBT is the total amount of sorbed B; T is the maximum possible B adsorption capacity (mol kg 1 ); KHB, KB, and KOH the estimated adsorption value (calculated using the sorption model, Yermiyahu et al., 1988) was small compared with the are adsorption coefficients; (HB), (B), and (OH) are solution 1438 SOIL SCI. SOC. AM. J., VOL. 65, SEPTEMBER–OCTOBER 2001 Table 1. Maximum boron adsorption and adsorption coefficients KHB, KB, and KOH for the various soil–sand–composted organic matter (COM) mixtures. Adsorption coefficients COM Maximum B System Adsorbent content adsorption KHB KB KOH