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Crop Response to Phosphate Fertilization and to Residual Phosphate Levels: II. Evaluation of Residual Phosphorus "Availability" by Chemical and Plant Tests in a Greenhouse¹

Yield response in two field experiments was found to be a poor indicator of phosphorous "availability". This greenhouse experiment was performed in order to obtain an indicator of "availability" based on plant response in addition to those based on soil extraction methods.

Bulk samples of soil were taken from each of 16 treatments of two field experimental sites (Neve Yaar and Lachish) on which eight consecutive crops had been grown and differentially fertilized. The residual P levels (NaHCO₃-soluble P) ranged from 5.5 to 24 ppm P. The greenhouse treatments consisted of four levels of phosphate applied as triple superphosphate in amounts of O, 40, 100, and 250 ppm P to pots containing 3 kg of soil.

Within greenhouse treatments, at any one residual level, the absolute yield on the Lachish soil increased with increasing rates of P application. For the Neve Yaar soil this held only up to a residual level of 11 ppm P. In both soils, with few exceptions, the larger the residual P level (field treatments), the smaller the yield increase due to the fertilizer treatments given in the greenhouse (i.e., at residual level of 5.6 and 24 ppm P the step wise dry matter yield increases were from 1.29 to 6.25 and 4.40 and 5.31 g/pot respectively).

Relative yields, chosen as an indicator of P "availability" in soil, of the nonfertilized pots of the greenhouse experiment were correlated to amounts of P applied in the field. Relative efficiencies of the residual phosphate accumulated in the field experiments, as described by the slopes of the regression lines, showed an inverse dependence on the heavy superphosphate applications (48–432 kg P/ha) applied to the field experiments. This suggests a higher efficiency of fertilizer P, can be obtained on calcareous soils, when applied in small annual amounts.

Key Words: NaHCO₃ extraction

² Technion-Israel Institute of Technology (present address: Volcani Institute of Agricultural Research, Neve Yaar, Israel).

³ Technion-Israel Institute of Technology, Haifa, Israel.

⁴ Volcani Institute of Agricultural Research, Beth Dagan, Israel.

Received for publication September 21, 1971.