A Biological Slope‐Ratio Method for Evaluating Nutrient Availability in Soils

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Abstract
The slope‐ratio method employed in biological assay was investigated as a means of evaluating the relative availability of a nutrient in different soils. This type of assay is used when the biological response increases linearly with the dose of the active ingredient. Under certain conditions the relative concentration of the active ingredient in two or more sources tested is equal to the ratio of the slopes of the lines representing the responses. To obtain linear responses, conditions of strong nutrient deficiency were imposed by growing the test plants in sand cultures to which increasing quantities of soil were added. Nutrients other than the one being evaluated were supplied in ample amount. In an experiment with nitrogen the results of three different types of experimental tests indicated that the ratio of the slopes of the lines representing the response functions provided a satisfactory estimate of the ratio of the availabilities. In an experiment with phosphorus, the tests indicated that for the various soils in general the ratio of the slopes was not a valid estimate of the relative phosphorus availabilities. In two instances in which there was no evidence that the slope ratios did not indicate the relative availabilities, the ratios of the availability of fertilizer phosphorus in the presence of soil to that in the absence of soil were found to be 0.2 and 0.5. These results demonstrate the invalidity of methods in which inferences about availabilities of soil phosphorus are based on the assumption that fertilizer phosphorus has the same availability in all soils.

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