Easy as Gel, A Method for Mapping Available Phosphorus in Undisturbed Soils

Phosphorus (P) availability in soil is a major limiting factor in modern agricultural production. One constraint is the spatial distribution of available P In the soil profile and along active roots. Mapping the available P in soil is essential to overcome this constraint, but current methods are either laborious (such as manual detailed separation of subsamples from the root zone) or expensive (such as Energy-dispersive X-ray spectroscopy). We devised a simple method to map soil available-P using a layer of agar gel, which is placed in direct contact with the soil. Phosphorus diffusion into this layer is controlled by the concentration gradient between the soil and the agar. The viscous agar gel retains the spatial distribution of soluble P at the time of contact. Color reagents may then be applied to stain the agar-bound P. The applicability of the method has been demonstrated to compare P movement in soils differing in texture and carbonate content. A 4-mm layer of each soil was spread evenly over a 5-mm layer of agar gel and wetted so that no loose water was present. A grain of superphosphate fertilizer was then placed on top of the soil at the center of the dish. Following 24 hours incubation the soil was rinsed off and P was stained blue using ammonium-molybdate reagent. The staining showed visually distinct patterns around the previous location of the fertilizer. The radii of the stains differed significantly as related to the P adsorption capacity and pH of the soil. Measuring the P content in representative agar subsamples by melting them enables to interpret the P concentrations throughout the map based on color intensity. Future prospects include using this method to map P distribution around plant roots and around novel P fertilizers. Method complications will be discussed.