Use of Ion Exchange Resins to Evaluate Soil Nutrient Availability for Commercial Crops.

Soil testing represents a link between a remarkable amount of research information on one side and the possibility to solve many plant nutrition problems for specific farmer sites on the other. In Brazil, ion exchange resin has been used very effectively since 1983. This method uses a mixture of cation and anion resins that enhances P extraction and also permits the determination of exchangeable Ca, Mg, and K in the same extract. Inexpensive time saving equipament was developed to adjust the method to routine laboratory practices, making it quick and operational. Many agronomic trials show advantages of resin in evaluating phosphorus (P) soil availability for commercial crops. For example, in field trials, with beans, sunflower and soybean, having the goal of evaluating the effect of liming, it was demonstrated that resin was the only method showing an increase in soil P availability, which was in close agreement with an increase in plant leaf P. Other methods (Mehlich 1, Bray 1 and Olsen) failed to show the trend of increase P availability to crops by liming, and Olsen even showed a decrease in soil P availability, totally opposite to plants behavior.  In short, P extracted by ion exchange resin has been shown to be a sensitive index of P availability in soils. Having such a good alternative for P determination in soils can improve P fertilizer recommendations and advance fertilization technologies, such as precision agriculture. Although the reliable agronomic results showing the ion exchange resins as a good alternative to evaluate soil nutrient availability, in special P, adoption in routine soil testing is still limited worldwide. It is considered that problems recently noticed in terms of lack of consistency in the evaluation of P and K soil availability, specially in temperate regions, may lead to higher interest in this method.