Soil Testing

Lee Ann Fields  |  3/6/2018 6:46:01 PM

This winter, after deciding to make a new perennial bed and rework an old bed, I thought it important to have the soil tested.

I picked up a soil test kit from the Lincoln Parish LSU AgCenter. One soil test is $16. Three tests are $36, a savings of $12. In about 10 days the results arrived.

The first results were pH or how acid or basic the soil is. Its range is 1-14; 1 being the most acid. Many plants prefer a pH of about 6.5, the pH which makes the most nutrients available. This is a logarithmic scale, not linear. My smallest bed had a pH of 7.62. Three years ago it was 6.60, meaning it has become 10 times more basic. This sent me to our local Ag Agent, Gary Stockton, who explained I had likely used an animal manure to fertilize (I had). My 7.62 pH was too high. The solution is relatively simple: to lower pH, add sulfur. In my case, LSU recommended

7.20 oz. of ammonium sulfate/100 square feet or 3.30 oz. urea/100 square feet. This bed is small – only 13’ X 4’ or 52 square feet. That means I only need to work about 3.6 ounces of ammonium sulfate into the area. The ammonium sulfate will also add nitrogen to the soil. If the pH had been lower than 5.5, the recommendation would have been to add a form of lime to the area.

It is well known that nitrogen is a required element for plants. It is responsible for the formation of proteins, nucleic acids and chlorophyll. We have about 78% nitrogen in our atmosphere, but this nitrogen is not available to plants. It is not reported in the soil test because it varies widely throughout the year. We add nitrogen to the soil in the form of ammonium nitrate, ammonium sulfate, or urea. It is converted to a plant usable form by microbes in the soil, particularly soils high in organic matter. Thus, soil high in organic matter benefits plants because more nitrogen is available to them.

The first element reported is phosphorus. Phosphorus, a macronutrient, is the second listed element on fertilizer packages (the P in N-P-K).Insufficient amounts of phosphorus results in plants with weak roots, poor flowering and fruiting. Availability of phosphorous to plants is strongly related to pH (5.5 – 7.0 is best). Plants need only small amounts of phosphorus, a good thing since it is not readily available in the soil. Phosphorus plays a huge role in cell division, respiration and in photosynthesis. To correct phosphorus deficiencies, add super phosphate or ammonium phosphate.

Potassium is also reported. It has the chemical symbol K and is the last macronutrient listed on a fertilizer package. LSU recommended that muriate of potash (potassium chloride) be applied to my soil sample with a medium amount of potassium. Potassium promotes vigorous growth (as a result of improved photosynthesis), helps plants resist disease and promotes tolerance to cold. With sufficient potassium plants grow faster, transport water efficiently and produce larger flowers and fruits.

These three elements are called the macronutrients and are commonly considered to be the most important in soil chemistry. There are three secondary macronutrients: calcium, magnesium and sulfur. These are usually present in sufficient quantities.

The interaction between all of the soil elements is a complex balance. To make the soil work to your best advantage carefully follow the recommendations provided by your soil test results. If you need help interpreting soil test results, call Gary Stockton at the LSU AgCenter.

This article is written by Donna White, Lincoln Parish Master Gardener.

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