We take about twenty soil samples on our farm each year – ideally in late summer or early fall. In some ways soil samples are even more important for organic farmers because our supplements tend to release nutrients slowly. If we get behind, it is harder to catch up than on conventional farms where fast-release supplements are typical.
I use soil test results mainly to keep track of multiyear trends in our soils. I particularly focus on pH, phosphorus, and potassium. I track those levels in each field and add supplements in the fall if they seem to be dropping outside acceptable ranges. My favorite supplements are high calcium limestone for pH, colloidal clay phosphate, and potassium sulfate. These are all mined, naturally occurring materials. Decaying geologic deposits will eventually provide P and K but they rarely keep up with intensive production and the nutrients that we haul off in the crops that we sell. Most mountain soils are acidic so limestone is critical for pH adjustment and as a calcium source. Acid rain, mostly from coal burning, also makes limestone addition needed more often than might otherwise be required.
North Carolina farmers have a real deal on soil tests – they are provided at no cost by the NC Department of Agriculture (NCDA). If you avoid spring when every gardener in the state is sending in tests, results are usually back in a week or two. Results are also available on-line if you don’t want to wait for the mail. The computer printout looks as though no humans are involved but expert agronomists manage the process. I usually mention that we prefer organic recommendations when we send in the samples and NCDA will make suggestions for organic supplements. They have also run special tests to address particular needs. One year our certifier decided they wanted proof that our tomatoes needed boron supplements. Between soil tests and tissue tests we were able to make the case for continuing to add pre-plant boron.
You did not ask about tissue tests but they are another tool that comes in handy in crop nutrient management. If soil tests are for ‘steering the ocean liner,” tissue tests are more like guiding a speed boat. Tissue samples are from the most recent fully expanded leaf (in tomatoes for example) so they tell us what nutrients the crop is using right now. Most farmers can look at a crop and tell that something is not right but the tissue test will help narrow down the problems. For indeterminate (long season) tomatoes we normally need to gradually add nitrogen through the season. Leaf yellowing and wimpy stems are one way to decide when to side-dress your tomatoes but tissue tests help to know when those symptoms are about to appear. Hydroponic growers often do tissue tests weekly but we find that samples every few weeks work well with organic supplements. There is a small charge (less than $10) for tissue analyses but they are well worth the price if you have a problem to solve.
While I am on the topic of nitrogen, the soil sample test results for nitrogen come from literature for your intended next crop and not from your soil sample. Since nitrogen changes form easily and can move out of the root zone between when you take the soil sample and when you plant, the NCDA agronomists decided to provide a book value. I wish they would note that policy on report, but I would rather have the information than not have it. Another good source for nitrogen recommendations is Knott’s Handbook for Vegetable Growers that lists the NPK content of most crops.
To understand more uses for your soil sample report, check with your Extension agent or the NCDA agronomist for your area. The Organic Growers School each spring and fall often offers classes on soils so check the web site for upcoming classes or to review soils presentations from the archives.
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Ask Tom © 2013 Tom Elmore & Organic Growers School
Tom Elmore is co-owner and operator of Thatchmore Farm in Leicester NC. He has grown certified organic fruits and vegetables for 25 years and serves on the Boards of the NC Greenhouse Vegetable Growers Association and the Organic Growers School.