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Range in Soil Phosphorus Critical Levels with Time

Soil Science Dep., North Carolina State Univ., Raleigh, NC 27695-7619

* Corresponding author.

ABSTRACT

Soil test interpretations differ according to the mathematical function used to determine the critical level, and they also differ because the critical level may not be the same from year to year. Data from a long-term P study, which had excellent yield responses to soil P, were used to calculate Mehlich-3 extractable P (M3P) critical levels with linear-plateau and exponential functions. Five crops of corn (Zea mays L.), four of soybean [Glycine max (L.) Me^, and three of wheat (Triticum aestivum L.) were grown in a 9-yr period on a Portsmouth soil, a fine-loamy over sandy or sandy-skeletal, mixed, thermic Typic Umbraquult. The M3P critical levels calculated with an exponential function at 95% of maximum yield averaged 67% greater than those obtained with a linear-plateau function. Economic analyses were conducted with the exponential function, considering the value of the crop and the cost of the fertilizer to maintain the soil test at the critical level for a year; this resulted in critical levels that were only about 26% greater than those obtained with the linear-plateau function. The average M3P critical level for these three crops with the linear-plateau function was 30 mg/L on this soil, whereas with the economic exponential function it was 38 mg/L. The critical levels varied markedly among years and crops. The sample standard deviation was 15 mg/L for the 95% maximum yield, 10 mg/L for the economic optimum, and 7 mg/L for the linear-plateau M3P critical level interpretations. For these crops grown on this low P-fixing soil, the M3P critical level from the economic optimum interpretation would be between 18 and 58 mg/L based on the observed variation. There was no relationship between critical level and yield potential for the three crops, especially corn an soybean.

The research reported in this publication was funded by North Carolina Agric. Res. Service.

Received for publication September 3, 1991.

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