From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Winter 2002, No. 6


New crop varieties and hybrids, pest management, precision agriculture technologies, the new farm bill, crop harvest pressures, marketing, equipment maintenance, and many other challenges can make it difficult for farmers to keep pace with change. Many are often caught up in the details and may forget some basic field and forage crop production principles.

One of the most basic principles in soil fertility and plant nutrition management is maintenance of an optimum soil pH. Everything else can be managed right, but if this one basic step is neglected, crop yields and profits can suffer. Soil pH affects the availability of phosphorus and several other nutrients. All ammonium-based nitrogen sources, including anhydrous ammonia, urea, and manure…contribute to soil acidity. The higher the nitrogen rate applied, the greater the residual acidity can be as the ammonium naturally converts to nitrate and hydrogen ions (acidity) are left behind.

If soil samples have not been collected within the last two to three years from each field, or each management zone within a field, do so before spring planting, or before established crops begin spring growth. The PPI summary of North American soil test results from the fall of 2000 and the spring of 2001 show that low soil pH limits crop production in many fields. Some crops such as alfalfa have a relatively high optimum pH range (6.8 to 7.2), while other crops, including potatoes, have a lower optimum pH (5.5 to 6.0) range. For most field and forage crops, it is desirable to maintain soil pH between about 5.8 and 6.2. Low pH (acid) soils:

Liming to raise soil pH to optimum levels reduces the potential for aluminum and manganese toxicity. It also increases cation exchange capacity in variable charge soils, increases the availability of several nutrients, supplies calcium, and it improves nitrogen fixation by legumes.

All lime needs should be based on a representative soil sample. If soil samples have been taken to the tillage depth (0 to 6, or 0 to 8 inches) in the past, and fields are currently managed under no-till conditions, it is wise to consider separate sampling at two depth increments: typical tillage depth and also a shallower sample at 0 to 2 inches or 0 to 3 inches. In no-till systems, or strip till systems, nitrogen is often surface-applied. As a consequence, the upper few inches of soil can become strongly acidic. The traditional 0 to 6 or 0 to 8-inch soil sample may not detect strong acidity in the upper few inches. Strong acidity in the top few inches of soil can slow early root development and also interfere with nutrient and moisture uptake.

Think about how long has it has been since soil samples were collected to determine if your soil pH is being properly maintained. Next, consider how long has it been since lime was applied on your fields and farm. Agricultural it time?


For more information, contact Dr. Cliff S. Snyder, Midsouth Director, PPI, P.O. Drawer 2440, Conway, AR 72033-2440. Phone: (501) 336-8110. E-mail:

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