You have taken a soil sample and the soil testing laboratory has just sent you the results. Wonderful. But what does it mean? There is no reason to get your soil tested if you do nothing with the results. There is valuable information in there. Today’s article will help you understand how to read the testing report in order to make the right preparations for our garden:

The standard soil test from most laboratories measures soil pH (acidity), organic matter, phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sodium (Na). Certain plants will have different requirements for specific nutrients.

Soil pH

pH is the measure of acidity or alkalinity of a solution. It is formally a measure of the activity of dissolved hydrogen ions. A solution is considered to be neutral when the concentration of hydrogen ion is equal to the concentration of hydroxide ions. A neutral solution has a pH of 7. The pH scale ranges from 0 to 14. pH values less than the neutral 7 indicate acidity, while numbers greater than 7 indicate an alkalinity. In soil the pH directly affects nutrient availability. Plants thrive best in different soil pH ranges. Azaleas, rhododendrons, blueberries and conifers thrive best in acid soils (pH 5.0 to 5.5). Vegetables, grasses and most ornamentals do best in slightly acidic soils (pH 5.8 to 6.5). Soil pH values above or below these ranges may result in less vigorous growth or symptoms of nutrient deficiencies.

pH & Lime. Lime is commonly used to correct a soil’s acidity. The pH value is used in conjunction with exchangeable acidity (Ac) in determining lime rates. Once lime is applied pH can be used to check if correct measures were met.

Nitrate-Nitrogen (NO₃-N) and Sulfate-Sulfur (SO₄-S)

Nitrogen is normally the most limiting nutrient for optimum plant growth. Soil tests that estimate soil N availability are not currently used because soil N exists in many forms which may change with time and influence plant availability. Soil nitrogen (N) and sulfur (S) are measured in nitrate-nitrogen (NO₃-N) and sulfate-sulfur (SO₄-S) forms. For most crops grown in Arkansas, nitrogen fertilizer recommendations are developed from research trials and are based on previous crop, soil texture, yield goal and sometimes cultivar. Analysis for soil nitrate-N, however, is done routinely only for a few selected crops, and it is used to refine their N-fertilizer recommendations. Soil samples can be analyzed for nitrate-N if requested for other crops, but fertilizer-N recommendations, particularly for lawns and forages, are not adjusted.

Sulfate-sulfur and nitrate-nitrogen can leach in sandy soils and typically accumulate above the dense or clayey soil horizons. For this reason, positive crop responses to sulfur fertilization are not common in clayey soils. As organic matter decomposes, sulfur and nitrogen are released into the soil solution. As a consequence of these complex reactions, the concentrations of these nutrients may vary considerably with time, environmental conditions and soil depth. Recommendations for sulfur fertilization are based on cropping history and, to some extent, on soil test level, especially for corn, cotton, wheat and forages.

Phosphorus (P) and Potassium (K)

Phosphorus and potassium are two of the three macronutrients (the other being nitrogen) required by plants for optimum growth. They are required in larger amounts compared to the micronutrients.

Phosphorus. Fertilization is not likely when the soil tests:

Under 36ppm (72lb/acre) row and forage crops,

Above 25 ppm (50 lb/acre) for fruit crops

Above 75 ppm (150 lb/acre) for vegetable production

Potassium. Fertilization is not likely when the soil tests:

Above 175 ppm (350 lb/acre) for vegetables and row and forage crops

Above 90 ppm (180 lb/acre) for fruit crops

Calcium (Ca) and Magnesium (Mg)

Most sandy soils have calcium concentrations below 400 to 500 parts per million (800 to 1,000 lb/acre), while clayey soils usually test above 2,500 ppm. Normally, the higher the calcium level, the greater the soil clay content. Recent limestone applications may result in higher calcium levels. If the soil pH is maintained in the recommended range for the crop grown, calcium deficiency is very unlikely. In general, the higher the clay content, the more lime will be required to raise soil pH to the desired level.

Any good soil test laboratory will provided an explanation of each element tested for. Recommendations for correcting problem issues may also be made. Added soil amendments can be done during site preparations when till and cultivating. Before planting be sure to correct any imperfections in your soil quality, your plants will thank you by granting you a generous harvest.

Best Regards,

Jimi Demetriou

References:

North Carolina. Department of Agriculture & Consumer Services. Agronomic Services Division.

http://www.ncagr.com/agronomi/

University of Arkansas. Division of Agriculture.

http://www.uaex.edu/

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