Rapid whole-plant bioassay for phosphorus phytoavailability in soils

Chemical P extraction from soils is an indirect and frequently questionable index for P availability. To monitor the dynamics of P availability in soils more directly following the application of P fertilizer, manure or sludge, a rapid, whole-plant bioassay was developed using tomato (Lycopersicon esculentum Mill.), Chinese cabbage (Brassica rapa L. var.pekinensis) and wheat (Triticum aestivum L.). Plant P extracted in 0.1 M H₂SO₄ (P _i ) and total P (P _t ) concentration or content in stem, leaves or whole shoots were highly correlated (P < 0.01) with P fertilizer rates or water-soluble (WSP) or Olsen P in various soils, over wide ranges of soil P status. The whole-plant P _i content was found to be as informative as the more complicated indices of P _t or P _i concentration. The assay was used to compare availability of fertilizer-P and sewage-sludge-P after incorporation into alluvial soil during 1-100 days of incubation. While both soil and plant indices had shown that fertilizer-P was more highly available than sewage-sludge-P in each period, the bioassay was much more sensitive than the Olsen-P or WSP soil indices in showing P fixation and decrease of availability during incubation time. The bioassay is sufficiently rapid (5-12 days) to allow a study of short-term changes in soil-P availability following incorporation of various P additives, and it is applicable to a very wide range of P availability values (6-535 mg Olsen-P kg^-1), extending from lower than desired for crop production to higher than permitted from an environmental standpoint.