In analyzing these graphs, scientists aren't looking for the point of maximum yield, but a point of maximum economic return, that is, the point on this graph at which further increases in yield don't justify the increased cost of the fertilizers. Of course, these recommendations are only general guides, every field and cropping history is different, so the grower must use several other tools to apply the right amount of fertilizer to his crop. Most fertilizer guides relate their recommendations to a soil test, in which the soil from the first 2 feet or more is analyzed for nutrient content. If there is already a high level of a nutrient in the soil, the grower accounts for this and adds less of this nutrient to the crop. During the growing season, samples of the plant tissues can be collected and analyzed for nutrient content.
Tissue sampling allow farmers to fine tune the fertilizer recommendations for a crop.
For example if a weekly tissue sample shows unusually high levels of nitrogen, he may be able to lower his nitrogen applications through the pivot that week. Tissue samples are also commonly used to diagnose micronutrient deficiency problems. Nothing quite beats experience. A good grower develops a knowledge for what amount of fertilizer was needed on a specific field to obtain maximum economic returns in the past. A record of the cropping history, especially what the previous crop was, is also an essential guide for how much fertilizer to add.