Soil Tests

A soil test is a chemical or physical measurement of soil properties based on a sample of soil (74). Commonly, however, a soil test is considered as a rapid chemical analysis or quick test to assess the readily extractable chemical elements of a soil. Interpretations of soil tests provide assessments of the amount of available nutrients, which plants may absorb from a soil. Recommendations for fertilization may be based on the results of soil tests. Chemical soil tests may also measure salinity, pH, and presence of elements that may have inhibitory effects on plant growth.

A basic principle of soil testing is that an area can be sampled so that chemical analysis of the samples will assess the nutrient status of the entire sampled area. Methods of sampling may differ with the variability of the area being sampled and with the nutrients being tested. A larger number of samples may need to be taken from a nonuniform area than from a uniform area. Movement of nutrients into the soil, as with nitrate leaching downward, may cause the need for sampling of soil to be at a greater depth than with nutrients that do not move far from the site of application. Wide differences in test results across a field bring into question whether a single recommendation for fertilization can be made for the entire field (74,75). Fertilization of fields can increase the variability of nutrients of a field, and the assessment of the fertility level with respect to nutrients will become more difficult. Variations in patterns of applications of fertilizers, such as placement of fertilizers in bands in contrast to broadcasting of fertilizers, can affect soil samples. The proceedings of an international conference on precision agriculture addressed variability in fields, variable lime and fertilizer applications in fields, and other factors involved in site-specific collection of data, such as soil samples (76).

Results of soil tests must be calibrated to crop responses in the soil. Crop responses, such as growth and yields, are obtained through experimentation. In the calibrations, the results of soil tests are treated as independent variables affecting crop growth and yields; otherwise, all other variables such as weather, season, diseases, soil types, weeds, and other environmental factors must be known and interpreted. The consideration of results of soil test as independent variables may impart difficulties in interpreting the results, especially if the environmental factors have marked effects on crop yields.

Results of soil analysis, sometimes called total analysis, in which soil mineral and organic matter are destroyed with strong mineral acids, heat, or other agents do not correlate well with crop responses (77). Generally, soil tests involve determination of a form of a plant nutrient with which a variation in amount is correlated with crop growth and yield.
These forms of nutrients are commonly called available plant nutrients. The different forms of nutrients are extracted from the soil with some solvent. Many different methods of extraction of soil samples are being used for measurement of available nutrients in soils. Extractants are various combinations of water, acids, bases, salts, and chelating agents at different strengths. The extractants are designed to extract specific nutrients or are universal extractants (77-83). Much discussion has occurred as to whether one method of extraction is better than another. Morgan (77) noted that any chemical method of soil extraction is empirical and that the results give only an approximate quantitative expression of the various chemical constituents in soil. Morgan stated further that no one solvent acting on the soil for a period of minutes or hours will duplicate the conditions involved in provision of nutrients from soil to plants. Researchers may choose to continue to test soils with extraction procedures with which they have experience and for which they have compilations of results. Researchers who analyze only a relatively few samples may choose to use procedures for which published results are readily and commonly available. Methods of extraction and analysis for specific elements are addressed in several monographs and handbooks (84-86). Chemical analyses are the most accurate part of soil testing since they are chemically reproducible or precise measurements of the amounts of nutrients extracted from soils. Selection of the method of analysis depends largely on the facilities that are available to scientists.