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Maintaining soil fertility in pastures

July 30, 2013

By Dr. Dirk Philipp, Department of Animal Science, University of Arkansas—Fayetteville

The soil nutrient status is one the most important and decisive factors in maintaining forage growth for supporting a viable beef industry. If forage yields are lower than expected, probable causes are nutrient removal beyond replenishing, natural soil nutrient deficiencies, and nutrient imbalances which limit nutrient availability.

Macro- and micro-nutrients are the two major groups recognized for optimum forage production. Nitrogen, phosphorus, and potassium are macro-nutrients. Micro-nutrients such as copper, molybdenum, boron, and zinc are equally crucial, but at smaller amounts. Equally important, many nutrients interact with each other at the plant-metabolic level and growth is limited if plant nutrients are not present at optimum quantities. Forages of different kind will have different nutrient requirement, too. If N is applied heavily to a mixed-species pasture for example, grasses will be favored over legumes.

Another important factor is soil pH. Values below 6.0 or above 8.0 may restrict nutrient availability and can even lead to severe effects on plants. Since pH values of soils formerly covered with forest can be relatively low—around 5 or even lower—many soils in Arkansas need pH adjustments. Liming does not immediately result in an increase of the pH; it may take one year or even longer before optimum levels of 6.5 or 7.0 are reached. A 6-months window should be reckoned with before a measurable increase of pH levels can be observed.

The key to maintaining soil fertility and pH is testing the nutrient status on a regular basis. Soil test analyses are free of cost by turning in samples to your County Extension Office, and results will be available within a couple of weeks. The UA-Division of Agriculture has published a fact sheet (FSA 2153) which describes in detail how to read these soil test reports and how to apply correct amounts of fertilizers.

Taking soil samples is considered by many a science in itself, but there are some rules which are easy to follow to obtain meaningful results. In general, areas to be sampled should not be larger than 20 acres. In Northwest Arkansas pastures might be generally smaller, but in the River Valley pastures are relatively large, so these should be subdivided for sampling purposes. Per assigned sampling area, at least 15-20 samples should be taken at a defined depth, preferably around 6 inches, in a zig-zag pattern across the field. The samples should then be thoroughly mixed in a bucket and a subsample filled into a soil box provided by the extension service. These boxes have to be labeled appropriately, and the staff at the extension offices will help with that. Too shallow sampling may skew results as fertility changes with depth. Samples are taken best with a proper sampling tool or probe, but a narrow spade can be used too. In that case it is important to be consistent and to sample the same quantity of soil at a consistent depth.

For each distinct pasture management area a separate compound soil sample should be provided to the laboratory. Required soil nutrient levels depend on forage species, history of use, yield goals, and topography. Care should be taken to sample correctly if heavy-use areas, such as watering or feeding zones, are included. Levels of some nutrients around these features, especially phosphorus, are usually elevated as animals congregate there.

Maintaining the soil nutrient status in pastures requires long-term planning and is part of the overall farm or pasture management plan. Samples should be taken at least every two years, if not more often, to stay current. With optimum nutrient and pH levels, soil organic matter can be maintained easier as well which in return has positive effects on plant growth. For more information and help with improving the soil fertility on your farm, contact your local county extension office.

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