1. Data are presented on the influence of various levels levels of application of ammonium sulfate, calcium superphosphate, potassium chloride calcium carbonate and magnesium sulfate on the sugarcane and corn leaf contents of nitrogen, phosphorus, potassium, calcium and magnesium, at each of two stages of growth. 2. The relation between the quantity of a nutrient available in the soil and the leaf content of the nutrient is expressible by a smooth curve that approaches a value asymptotically. Mitscherlich's equation may be used satisfactorily for this purpose. 3. The capacity of a soil to provide a crop with a given nutrient may be estimated by using the following equation:
M = log (A – Y) – log A,
where M = available quantity of a nutrient per unit of area expressed in units of the corresponding fertilizer material; Y = percentage dry-matter content of the respective nutrient by the plant leaves; and A and C are values of parameters previously estimated from the results of a calibrating field experiment. 4. The precision of the equation to be used for this estimation is higher when the calibrating experiment is performed in soils lower in available content of the nutrient than otherwise, and when the leaves are sampled at an early, rather than at a late stage of growth.