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Crop Response to Phosphate Fertilization and to Residual Phosphate Levels: I. Field Experiments¹

Intensive phosphate fertilization practices over the last two decades have resulted in large quantities of phosphates accumulating in soils. The objective of two field experiments was to evaluate yield response to various levels of residual phosphorus.

The experiments were carried out within an irrigated rotation having two crops per year. The initial experimental treatments consisted of four rates of superphosphate equivalent to O, 48, 144, and 432 kg P/ha, applied prior to each of three crops. Following the harvest of the third crop the original plots were quartered and six additional crops were grown on each of the two soils. Superphosphate was added before the seeding of each crop in amounts equal to O, 13, 26, and 52 kg P/ha.

Out of the 18 crops harvested, there were only four yield responses to P fertilization significant at either the 0.05 or 0.01 level. The high initial residual P level (10-11 ppm NaHCO₃-soluble P) and the low P fixing power of the soils (68 and 75% of the added P remained exchangeable) could explain the low frequency of response to phosphate fertilization. It was only after five to six successive crops were grown, that the bicarbonate-soluble P in the control plots dropped below 8 ppm. In these cases plant response to residual P was found. Accepted agronomic fertilizer rates (13 to 52 kg P/ha) applied to crops 4 to 9 were capable of maintaining soil residual P levels between 6.3 and 17.5 ppm P (NaHCO₃-soluble). At residual P levels of 26.2 to 97.5 ppm P (NaHCO₃-soluble) similar fertilizer rates were not capable of maintaining or recharging the soil "phosphate pool" which had been achieved by the heavy fertilizations (48 to 432 kg P/ha) applied to crops 1 to 3.

Key Words: Phosphate availability

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.