The purpose in subjecting a soil sample to analysis for a given nutrient is not to determine the total amounts of the nutrient in question in the soil. In fact, the total amount of a given nutrient in a soil seldom has much relationship to what amount is available to a cotton crop. A key challenge in conducting a soil analysis is to use an extraction procedure that removes a portion of the nutrient from the soil that relates to the plant-available form and amount. For example, if one is analyzing a soil for P levels relative to cotton needs, a chemical extract is commonly used that measures a portion of the total soil P which is available to the cotton plant (phosphate-P, PO4-P).
The soil tests and extracting procedures can vary a great deal across the cotton belt due to differences in soils, climates, and production conditions. One may also find a considerable amount of variation among soil testing labs within a given state concerning soil testing methods due to differences in philosophy, experience, and technique. The differences in soil testing methods, and the assertions and allegations that can go along with them, can be confusing and tiring to even the best and most patient of experts. It is no wonder that farmers can sometimes become disillusioned with the value in investing in a soil testing program. The fundamental key to look for in the abilities of any lab to analyze your soils and to make reliable recommendations for fertilizing a cotton crop is that of having a satisfactory database relating the following factors: 1) soil test results, 2) fertilization rates, and 3) crop yields. Unfortunately, some labs do not make a successful connection among these three points. Some labs analyze a lot of soils and make a lot of recommendations for fertilization, which are often followed diligently, but they are not able to follow through with connecting the resultant crop yields to the soil test levels or the fertilization rates. Collecting and developing a database inclusive of each of these factors for a large number of fields and seasons is referred to as soil test calibration and correlation, and is critical to the development of a reliable and successful soil test system. Essentially, this means it is important for a lab to be able to show that for a given soil test value and corresponding amount of fertilizer recommended (and applied), a corresponding yield can be produced. This represents a time consuming and expensive process, but it is absolutely critical to developing a truly functional soil test. It is not absolutely necessary for every lab to develop this type of a soil test calibration system, but it is important that labs use soil test procedures that have been calibrated and correlated sufficiently to crop yields. This is why different soil test procedures can be used in a given region with satisfactory results, providing that the soil tests in question were properly developed (calibrated and correlated) to crop response for that region. Therefore, it would be advisable for one to inquire into the background and support a given lab has for their soil tests and the fertilizer and soil management recommendations they offer from the soil test results. Generally, the farther away a lab is, the less likely they are to use locally applicable soil test procedures and interpretations that are appropriate.
