On the other hand, different soils with a low available potassium content do not give equally good responses to applications of potash. Although the available potassium supply in the soil at any one time, as determined by the chemical test, may be relatively low, many investigators have found that the amount of available potassium in certain soils remains remarkably constant, even upon intensive cropping. This would indicate that as the plant removes the available potassium from the soil, some of the difficultly soluble forms of potassium are converted into the available form. Thus when the supply of available potassium in the soil is replenished from the difficultly soluble forms rapidly enough to keep pace with the removal of potassium by the crop, the potassium-supplying power of the soil may be considered adequate even though the amount of available potassium at any one time, as revealed by the chemical test, may be low.

Similar conditions apply to the test for determining available phosphorus.

Many mineral soils having a total phosphorus content of more than 1,000 pounds to the acre in the surface layer respond well to as little as 50 pounds of phosphorus an acre applied in the form of superphosphate, especially if no phosphatic fertilizer has been previously used. This apparent discrepancy is due to the fact that most of the native soil phosphorus is tied up in the form that is not available to plant roots. Even the applied phosphorus is tied up so strongly by some soils that, unless a sufficient amount is added to satisfy at least partly the phosphorus-fixing capacity of the soil, little or no crop response can be observed even though the soil is deficient in phosphorus.

Soil chemists have been searching for a chemical phosphorus test that would be sufficiently specific to differentiate the available forms of phosphorus from the difficultly available forms and that would thus predict accurately responses of different crops to phosphorus fertilization on different soils. Such an ideal phosphorus test is yet to be found. Some of the chemical soil tests for phosphorus in current use, however, give fairly reliable results when properly correlated with crop responses on different soil types.

The tests for some of the constituents may be considered quite satisfactory. The soil acidity test, for example, which can be readily made even in the field, is now extensively employed as a basis for lime recommendations. But, here again, the translation of the results of the soil acidity test (pH of the soil) into the amount of lime that needs to be applied to grow a certain crop requires some knowledge of soil characteristics and plant requirements.

Despite these limitations, the chemical soil tests have been shown to be of great value as a guide to fertilizer recommendations, especially in detecting extreme deficiencies and toxicities of plant nutrient elements in the soil.

The chemical soil tests, for example, can show that continuous, heavy fertilization over a period of years may result in accumulation of one or another plant nutrient element in sufficient amount to warrant the reduction or even omission of that constituent in the fertilizer without sacrifice of yield or quality of the crop. The soil problems in commercial greenhouses are often associated with accumulation of excessive and injurious amounts of fertilizer salts. Such a condition can be readily detected by chemical soil tests.

Chemical soil tests are also useful as an aid to diagnosing crop failures. Admittedly, crop failures may be due to many different factors other than lack of adequate supply of plant nutrients. Poor drainage, lack of moisture, and insect or plant-disease damage may be the cause of the trouble. Diagnosis from the results of chemical soil tests is often difficult, but some clues as to possible causes of the trouble may be obtained largely by the process of elimination.

These tests are now used extensively by many State agricultural experiment stations, principally in the central-western area, Corn Belt sections, and eastern Atlantic region, as an aid in furnishing advice to farmers and growers regarding fertilizer use and soil management. Many States render this service free of charge to resident farmers, growers, and public agencies of the State. Special soil containers with detailed instructions for collecting the soil sample printed on the container are provided for this purpose by some State soil-testing laboratories. It is imperative, of course, that the soil sample be properly taken in the field. When submitting the soil samples for testing, the farmer is also requested to fill out special blank forms, giving pertinent information regarding the field and the soil problem. This is necessary for proper interpretation of the results of the tests and for making the final recommendations.