| Selected factors proposed to be limiting the effectiveness of the assessment of plant and soil P status by current testing methods were studied, to provide information for use in improving the processes and interpretations.; Growth and P status of field-grown spring wheat was monitored under a wide range of conditions. Plant P deficiency restricted growth early in the season, beginning one to two weeks after crop emergence, even where seed-placed P fertilizer was applied. However, within two to five weeks from emergence, shoot relative growth rates in the P-deficient treatments were at least as high as those in the high-P treatments. Plant analysis was most successful for predicting yield-limiting P deficiency if conducted during the first several weeks of growth. Determination of tissue total P concentration in the leaf or whole shoot, and of extractable inorganic leaf P concentration, could provide a good basis for assessing plant P status; comprehensive interpretive criteria were developed for each test. Leaf P concentrations generally declined until the stem extension stage, but shoot P concentrations declined throughout the growing season. For each test, concentrations tended to converge with time among sites and treatments.; Examination of spatial variability of extractable P in four field soils revealed a high degree of variability over very short distances (1–2 cm), even where fertilizer had not recently been applied. Persistence of small localized zones high in P availability may enhance residual fertilizer P uptake by plants.; Extractability of fertilizer residual P compounds by the Olsen NaHCO 3 soil testing procedure was studied. Although the most highly soluble compounds dissolved rapidly, added octacalcium phosphate did not dissolve with soil present. Soil solutes, including Mg2+, Fe 2+, and others, greatly retarded the dissolution. Octacalcium phosphate is an important P fertilizer reaction product in Manitoba soils and is moderately available to plants; its failure to dissolve may limit the effectiveness of the Olsen test in fully reflecting the residual availability of recently applied P fertilizer to crops. The simple solubility of a compound in a pure soil test extractant may not be a good indicator of the degree to which it will be recovered by the test in the presence of soil. |
