Predicting Phosphate Adsorption‐Desorption in a Soilscape

Modeling the P availability in a soilscape requires knowledge about the three‐dimensional variability of the phosphate adsorption‐desorption (AD) curves. The high variabilities of soil parent material and of agricultural P input in a 1.5‐km 2 study area were taken into account by taking 2555 samples at the intersections of a 50‐m grid. To reduce the necessary laboratory work, pedotransfer functions (PTFs) were developed that could predict the AD curves from more easily obtainable soil characteristics. Therefore, AD curves were measured for only 40 representative samples. The AD curves were fitted with an extended and rearranged Freundlich equation with a constant adsorption energy parameter k = 0.29. The adsorption capacity parameter A of this equation was predicted from clay, dithionite‐extractable Fe (Fe d ), and pH using a nonlinear regression method. With increasing pH, A decreased for 4 < pH < 6.5 but increased for 6.5 < pH < 7.5. The Fe d could be replaced by the soil color yellowness component b *, which was easily determined with a chroma meter. The equilibrium P concentration of the soil solution, c eq , was closely related to a one‐point measurement of the AD curves, c 1.2 (the final concentration after adding 1.2 mg L −1 P). Thus, only the four soil properties clay, Fe d (or b *), pH, and c 1.2 were necessary to predict complete AD curves using the developed equations. The equations proved to be reliable when tested by an independently selected subset of the 2555 samples and by a functional validation. The developed equations can therefore be used as PTFs.