Soil Testing Time

It's a good idea to get a late summer soil test, and apply appropriate fertilizers with the fall/winter cover crops. Over the past 5 years I have looked at hundreds of Arkansas Soil Test Reports for gardens. I am aware that many gardeners, who get the soil test, don't fully understand the report. And further, many gardeners have a still harder time making the necessary calculations to figure out what organic fertilizers to use and how much to apply on their beds. It's not my goal to try to explain all the details in this article, but as some of you move to organics, and are using the Arkansas Soil Test, here are some guidelines.

Taking Soil Samples: First, get some soil boxes from the Extension Service. If your garden is small and fairly uniform, one sample will be sufficient. That pint box should be filled with soil from subsamples taken from 5-10 different places in the garden. Don't include the coarse material on the soil surface. Take those subsamples from the 2-6 inch layer of soil. For larger and/or more variable gardens, include more samples. When submitting the samples, explain specifically that you want a test for Nitrogen. The test results will take 2-3 weeks, so plan ahead.

Interpreting the Soil Test Report: Compared with some other states, I really like the Arkansas test results. They do the analysis of 11 different elements and give the results for each element in actual pounds per acre. I have also checked and the Lab results are consistent, i.e. we get the same results from duplicate samples from the same soil. They also provide the soil pH and the estimated base saturation (i.e. the soil Cation Exchange Capacity). The CEC is a measure of the positive minerals (Ca, Mg, K, and Na) in the soil that are attached to the clay and humus particles. For growing nutrient dense produce, you should have those elements in specific ratios.

The soil pH measures the acidity or the alkalinity (i.e. sweet or basic) of the soil. Soil pH of 7.0 is neutral. Lower numbers are acidic and higher numbers are alkaline. For most vegetables, strive for a pH of 6.4 (slightly acidic). Some plants like blueberries, azaleas, and rhododendrons like a more acid soil. Lime raises the pH. Sulfur, sulfates and acetic acid (vinegar) will lower the pH.

Caution: There are some things in the report that are superfluous: Under the Nutrient Availability Index section you will see concentrations in ppm. That means parts per million, and is the number that the Lab determines from their chemical analysis. If you multiply the ppm by 2 you get the pounds per acre. It is based on the assumption that an acre of soil (7 inches deep) weighs 2 million pounds. As a gardener, you have no need to use the ppm values.

You will also have an Estimated Soil Texture name, which is not a real soil texture measurement. It comes from looking at the CEC, which comes from the amount of clay and humus in the soil. The higher the CEC, the more likely you will get a soil texture classification of "clay." You can get the same classification with highly organic soils without any clay in the soil. Just ignore the soil texture name provided.

Be alert that the recommendations for application of Nitrogen are not specific to your soil. I have 6 recent Soil Reports with corresponding rates for Nitrogen of 0, 54, 62, 214, 262 and 596 pounds per acre, and the recommendation for adding nitrogen is identical in all of them. The fact that you get a recommendation for nitrogen, even when they do not do a nitrogen test, should put you on high alert that this recommendation is not meaningful.

Critical Numbers: More than anything else, I look at the pounds per acre for each of the 11 elements. How do you know what the "right" pounds per acre numbers should be? I have spent a lot of time analyzing that tough question. Because I am most interested in recommendations that are developed for growing nutrient dense produce, I have had to go to another source outside of Arkansas for the answers. Pounds per acre numbers for nutrient dense produce have been developed, originally by Dr. Cary Reams, and subsequently by many others. Unfortunately, the Arkansas soil test methodology (the Mehlich 3 system) is different than the soil tests used by Reams and others (the Morgan soil test system). In essence, a stronger acid is used for extraction of the elements in the Mehlick 3 system.

To figure out the corresponding pounds per acre numbers for nutrient dense produce for the Arkansas test, I have collected soil samples, mixed them very well, split them in two parts, and sent each part of the sample to the Arkansas Soil Laboratory and a private laboratory (International Ag Labs) that does the Morgan test and has the pound per acre numbers for growing nutrient dense produce. With the pounds per acre numbers from 17 split samples from both labs, I have now derived estimates of pounds per acre needed for each element that are most appropriate for the Arkansas test. This project is continuing.

As a rule, the soil tests will reveal 3 to 5 elements that need adjusting. I have seen the test results and there are big differences in pounds per acre for elements among different garden soils. Without the soil test, making fertilizer recommendations is simply a guess. Don't gamble with your garden. Above all, don't start adding fertilizers indiscriminately to new garden areas with native soils. I have seen several examples where the gardener "ruined" the soil by over fertilizing. As a rule, new soils, even if they are low in nutrients, can be easily adjusted. Get the soil test, and if you want help in deciding what organic fertilizers to apply, please contact me.