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A pot trial was conducted in which tomatoes were grown in soils containing a range of total phosphorus depending on the amount of superphosphate which had been applied during the previous 9 years. Monocalcium phosphate was mixed with all soils to give 25, 75, or 150 p.p.m. added phosphorus. The above-ground portions of the plants were harvested after 47 days, and the pots seeded again without further additions of fertilizer. This crop was harvested 59 days later. It was shown that the response to current applications of phosphorus was dependent on the total phosphorus level. The addition of a given amount of phosphorus produced bigger yields on the high phosphorus soils. Moreover, the higher the level of applied phosphorus, the greater the difference in yield between soils of different total phosphorus levels. Yields were considerably lower in the second crop, but the interaction between total and applied phosphorus was still evident.It was concluded that the availability of the accumulated phosphorus was extremely low, and that the increased yields on the high phosphorus soils were due to a better utilization of applied phosphorus as a result of reduced fixation in these soils. The decrease in the rate of fixation was explained by blocking of fixation sites by the previously fixed phosphorus. |
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