The key is to find one that will extract an amount of a nutrient proportional to what a plant extracts. To determine this, experiments are carried out in the greenhouse where plants are grown in pots on soils typical of those to be submitted for fertilizer recommendations. Following the desired growth period, the plants are harvested and analyzed. The amount of nutrient extracted by the plants is then compared with that extracted by the chemical solutions. This involves growing Crops on fields treated with the nutrient being evaluated.
Nutrient rates range from zero to more than adequate for maximum yields.
By conducting a series of experiments, usually over several years, on a wide range of soils, it is possible to relate the laboratory soil test value to actual field yields and rates of fertilizer necessary to achieve maximum yields. Because Crops have different growth habits and nutrient requirements, the same procedure must be followed for each important crop in an area. Such work requires considerable time and effort. However, it is essential if a laboratory is to provide sound fertilizer recommendations. Once the relationship between soil test values, fertilizer rates and yield of a given crop is determined, it is possible to determine the most economical rate of fertilizer application for a given crop.