A combined ion exchange resin and a multi-element x-ray fluorescence method was developed as an alternative to existing soil testing methods. By shaking soil and an ion exchange membrane sandwiched between permeable paper discs in an extraction cell, it was possible to extract and analyze all plant nutrient elements except Band Nin soils.
Using six soils with different physical, mineralogical and chemical properties, the effects of soil sample size, soil-water ratio and equilibration time on nutrient extraction by resin were investigated to determine optimum conditions for use in a routine extraction procedure. The results showed that increasing soil sample size and equilibration time increased the concentration of nutrients adsorbed by resin. In high base soils monovalent cation adsorption by resin decreased as polyvalent cation adsorption increased with time. This relationship was attributed to greater preference by resin for the latter. In contrast, increasing the soil-water ratio decreased the concentration of polyvalent cations and increased the monovalent cations adsorbed by resin. The capacity of the resin to adsorb nutrients, restricted by the absence of contact exchange, was dependent on the nutrient concentration in the soil solution. Thus» the quantity adsorbed was regarded as a reflection of the quantity-intensity parameter of the soil nutrient pool since the quantity of a particular nutrient removed from soil by the ion exchange resin was found to be largely determined by the nutrient concentration in the soil.
A comparison between cation exchange resin and lN ammonium acetate showed that both methods were very significantly correlated for Al, Ca, Mg, and K in a Typic Gibbsihumox (low base soil}, and very poorly correlated for Ca and Mg in a Typic Chromustert (high base soil). This difference was attributed to the selective adsorption of nutrients by ion exchange resin, in contrast to the non-selective extraction by the ammonium acetate solution. It was also pointed out that the quantity of a particular nutrient adsorbed by resin can be limited by competition among cations and the total exchange capacity of the resin. This extraction procedure using c1--saturated anion exchange membrane to estimate P, was only marginally successful. The dependence of the resin on water-soluble Pin the absence of contact exchange was believed to be responsible for this. Therefore, the use of a different and/or OH--saturated anion exchange resin was suggested for P extraction.
By multiple regression analysis of experimental data, equations containing cation concentration ratios were employed to describe yield response and nutrient uptake by sudax (Sorghum bicolor x Sorghum sudanensis). The results showed that when only nutrient cations were considered in such equations, ion exchange resin was superior to 1N ammonium acetate extraction, in predicting yield response. Similarly, legume yields obtained from a field experiment designed to study the relative response of tropical and temperate legumes to liming, were adequately predicted by resin-extractable cations.
In conclusion, the multi-element approach to soil testing presents a more sensitive method for assessing nutrient deficiencies and excesses in soils, and offers a means to study the delicate concentration dependent interactions among nutrient elements and their effects on nutrient uptake by plants
