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.