The concept that a plant having 2 sources of a nutrient will absorb its nutrients from each of these sources in direct proportion to the amts. available has the following logical consequence: The quantity of available nutrient in the soil can be detd. in terms of a standard, provided the proportion of the nutrient in the plant derived from this standard is detd. The mathematical expression of this relationship is as follows: [image] where A = amt. of nutrient available in the soil; B = amt. of nutrient in the standard; y = proportion of the nutrient in the plant derived from the standard.