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What Can Soil Testing Do Well?

Posted Apr. 1, 2002

Soil testing is a wonderful tool that can help you make money. It can save you money on unnecessary nutrients if your soil is already sufficient in nutrient(s). It can also increase yield by showing you that a nutrient is deficient and applying that nutrient will increase yield.

While soil testing is a good tool, it's not perfect. As Clint Eastwood once said in a movie, "A man's gotta know his limitations." It's equally important to know the limitations of soil testing, as well as its strengths.

First, let's examine some of the strengths of soil testing. To me, the most important piece of information on a soil test report is the soil pH. The pH can tell you whether or not you need lime. If lime is needed, it should be applied as soon as possible. The acidity in the soil will limit plant growth to such an extent that plants will be unable to utilize the fertilizer you apply. Acid soils also make phosphorus unavailable to plants. On the other end of the pH scale, a high pH can cause micronutrients to become unavailable to plants. The first number you should look at on your soil test report is the pH level. Labs can do a very accurate measurement of soil pH.

Soil nitrate levels can be useful if used correctly. The farther east you are (higher rainfall), the less useful soil nitrate concentrations are. In high rainfall areas, nitrogen can be lost to leaching or denitrification. Also, rain can move the nitrate below the area you are sampling. If soil nitrate levels are important to you, it is a good idea to take both a 0- to 6-inch and 6- to 12-inch sample. If soil nitrate levels are high and the time of peak N use by plants is near, fertilizer nitrogen use can be reduced and money can be saved. A weakness of the soil nitrate test is that it will not fully detect recent manure or ammonium nitrogen applications, and because of potential losses, nitrogen detected well ahead of the peak use period may not be available when needed by plants.

Soil phosphorus (P) and potassium (K) levels are also very important on a soil test report. There are some cautions to consider when interpreting these levels, though. First, these are not the total levels of these nutrients in the soil. They represent a portion of the total that is extracted from the soil by chemical solutions. The amounts extracted from the soil must be calibrated with field research trials to determine how much to fertilize at each nutrient level. Keep in mind that different labs often use different extracting chemicals. The raw numbers of P and K from one lab often cannot be compared to the raw numbers from another lab because they use different chemical extractants. It is more appropriate to look at the interpretation of that number (sufficient, deficient, high medium, low, etc.) than to look at the raw numbers.

Another factor to consider when examining P and K levels on soil test reports is that the levels will change slowly after applications of fertilizer. Phosphorus bonds with elements in the soil and forms insoluble compounds that are not extracted with soil lab extractants. It may take as much as 20 pounds of P2O5 fertilizer to raise the soil test P level one pound per acre due to chemical tie-up in the soil of P. Potassium levels may drop quickly if you are producing hay in sandy soils. Sandy soils, as a rule, do not contain or retain large amounts of potassium, and hay removes a lot of K.

The sodium and soluble salt levels on the report can identify problem areas. High sodium and/or salts can cause greatly reduced plant growth. Different plant species tolerate salts differently. Established bermudagrass is very salt tolerant and can thrive in a high salt concentration. Most clovers and alfalfa do not tolerate salts well and would either grow poorly or die in high salt areas. If you suspect salt problems, sample the soil when it is very dry, and sample 0 to 4 inches deep. The salts will migrate up to the surface of dry soil and concentrate there.

Soil testing is only fair, at best, for detecting crop response to micronutrients. Micronutrients occur in very small concentrations in the soil, and field trial data correlating crop response to soil test levels of micronutrients are lacking. If you suspect a micronutrient deficiency, it is usually better to analyze the plant tissue than the soil.

The organic matter concentration listed on the soil test report is of great importance. Often, soils that are unproductive have a very low organic matter concentration. One reason is that organic matter greatly increases the water holding capacity of the soil. Another reason is that organic matter releases nutrients to plants over the growing season. A typical soil organic matter content in our area is 1 percent. I have seen levels in fields as high as 6 percent and as low as 0.1 percent. Other factors being equal, a soil with a high organic matter content is almost always more productive than one with a lower organic matter content.

If I were to grade the dependability of labs to analyze certain aspects of soils and then have good interpretative data for making recommendations based on those analyses, I'd grade like this: soil pH gets an A. Labs can do a good job of analyzing pH and a lot of data exist showing the response to liming crops at certain pH levels.

Soil nitrate gets a B in drier areas and a D in wetter areas, based on its usefulness. It can save fertilizer money in more arid areas when soil test nitrate levels are high, and can cause problems in high rainfall areas where nitrogen can be lost to excessive water and the results can be confusing.

Soil test P and K analyses together get a B. They are accurate in most situations. There are isolated soil conditions where the extractants for phosphorus do not work well, but overall soil test P and K can be very helpful in determining whether these nutrients are needed.

Soil micronutrient analyses get a D. Plant tissue sampling is a better tool for determining micronutrient problems, but soil testing can occasionally be used with success.

Soil testing for sodium and salts gets a grade of B. That improves to an A if the sampling is done in dry soil. Soil tests can accurately show whether salts are a problem.

Overall, soil testing is the best tool we presently have for determining the probability of getting a yield response to fertilizer. The use of this tool improves as we better learn its strengths and weaknesses.

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