What ultimately matters to growers and farmers is what is the current nutrient status of the soil, and what is the safest and most economical way to supply nutrients

One surprising fact is that of soil testing laboratories generally don't actually test soil. They test extracts of soils at least when performing chemical tests. Soil samples are generally mixed with an extracting solution of some kind. The mixture is then shaken and then filtered resulting in a clear liquid extract which is then subjected to various chemical tests. The extracting solution may be distilled water, although with most laboratories one or more dilute acids are used to extract nutrients from soil samples. The theory is you want your extracting solution to remove from the soil those nutrients that are reasonably available to plants. Nutrients that are in chemical forms that will never be available to a plant are not really of any use to the soil testing lab or the farmer. At the same time, distilled water as an extracting solution only tells you what is available today, and doesn't give you much of an idea of what the reserve fertility levels are. Therefore, using dilute, weak acids as extractants will generally reveal levels of nutrients in forms that will likely be available to the plant.

Numerous soil extraction methods exist, though there are six or eight extracting methods that are the most popular throughout the world. The trouble is, these different extracting solutions vary in the amount of nutrients they pull out of a soil, and therefore if you send the same sample to different laboratories that are using different procedures, the test reports will reveal different numbers. Another source of variability in the numbers may be in the units that the laboratory is reporting. Some laboratories report in parts- per million, others report in pounds per acre, others in milli-equivalence and still others in micromoles. All this variability certainly generates confusion.

An obvious question that growers frequently ask is "Why don't laboratories just use one standard testing method and report format". There are three principle reasons for this. One is that different soil testing procedures work better in different types of soils in predicting crop response. A testing method for one type of soil might work well, but it may perform poorly in completely different type of soil in terms of predicting crop response. Secondly, many laboratories which have been in business for a number of years have accumulated a great deal of data with one extracting method, and they are reluctant to change methods once an extensive data base has been established. The third and more complex reason is the soil testing values should really be considered as a relative index, than as an absolute value. The Fahrenheit and Celsius temperature scales are two examples of indices used to measure temperature. Forty degrees might be cold under one scale, and quite warm on the other scale.

What ultimately matters to growers and farmers is what is the current nutrient status of the soil, and what is the safest and most economical way of supplying nutrients to the plants. Growers who jump around and use different laboratories to do their soil testing generally become confused by the differences in methods and units. My suggestion to growers in general is to find a laboratory that you are comfortable using and stay with them. Some important factors in choosing a laboratory are quality of data, price, speed, testing method, and ease of understanding the report. In general, growers are urged to find a laboratory they are comfortable using and generally stick with them.

Let us then get back to the original question in the title of this article: Why test soil? A major use of soil testing is to evaluate the suitability of soil before planting a crop, and to use soil testing to help plan the fertilizer program. Soil testing can also be used in order to select what crops should be grown on a particular piece of ground. Secondly, soil testing can be used to monitor the growth and quality of a crop. Soil test levels can often be correlated with plant growth, yield and quality. Soil testing can also be utilized to fertilize efficiently in order to help minimize environmental concerns.

A final reason for utilizing soil testing is that growers generally don't like surprises. When crop difficulties occur, soil testing is often a key factor in the diagnostic process. Soils can be tested for numerous things today including nutrients, heavy metals, pesticides, nematodes, physical structure, microbes, etc. The range of services from public and private laboratories is often extensive. While soil testing is not a substitute for good varieties, good weather, and good culture, it remains a useful tool for growers in many types of industries in producing and maintaining quality plants.

Contamination is a very serious environmental problem, whether the issue is lead-contaminated soil or, if there is significant run off, widespread water contamination. Chemical contamination can cause serious diseases, like mesothelioma, so cases should be reported to a doctor right away.

Lynn Griffith is President of A&L Southern Laboratories, and for more information about their total crop production services, call 954-972-3255

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