SOIL-PHOSPHORUS FRACTION DYNAMICS DURING 18 YEARS OF CULTIVATION ON ATYPIC PALEUDULT

Traditional soil P test methods estimate plant available inorganic P b ut ignore the less available inorganic and organic P pools. In low-inp ut systems where fertilizer P additions are very low to nil, these les s available P pools may be a better measure of potential plant availab le P, particularly in highly weathered soils. The objectives of this s tudy were to determine the size and changes in soil P pools in the non fertilized and fertilized treatments of a long-term continuous cultiva tion experiment established on a Typic Paleudult in Yurimaguas, Fern. A modified version of the Hedley et al. procedure was used to sequenti ally fractionate soil P into increasingly recalcitrant organic and ino rganic pools. The use of path analysis highlights the interactions amo ng P pools and the different roles of the pools in P cycling between t he nonfertilized and fertilized system. For the fertilized system, the NaOH-extractable inorganic P pool acts as a sink for fertilizer P but desorption is rapid enough to maintain high levels of plant available P. For this system, inorganic P pools explain 96% of the variation in the level of available P. Organic P is the primary source of plant-av ailable P in the nonfertilized system and explains 44% of the variatio n in available P. Available P, measured by anion exchange resin, is de pendent on crop residue (b values +/- standard error = 4.98 +/- 3.57), whereas yield depends most strongly on the available P (0.16 +/- 0.11 ) and on NaOH-extractable organic P (-0.17 +/- 0.11). The lack of stab ility in organic P levels in the first 10 yr illustrates the need for long-term experiments. The presented results support the notion that t raditional soil P tests are inadequate for low- to no-input systems.