Advancing the understanding and interpretation of plant and soil tests for phosphorus in Manitoba

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 NaHCO3 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+, Fe2+, 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.

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