Depending on origin, there are several recommendation philosophies used in soil testing. However, there are some universal concepts that apply across most soil-test-based nutrient recommendations.

Crop nutrient recommendations are derived using one or all of the following: soil nutrient concentration/availability, crop grown/land use, targeted crop yield, and other soil test information that may influence nutrient availability.

Soil test recommendations can be placed into four major categories. These are: applications designed to change the soil chemical composition, applications based upon current crop needs; applications based upon current crop needs combined with increasing long-term soil nutrient levels; and applications that are based upon supplying enough of the nutrient to last until the soil is retested.

Aglime recommendations are designed to change the chemical composition of soils. Most liming equations use a combination of active soil acidity, which is represented by soil solution pH, and reserve soil acidity, which is measured using a buffering agent. Because a soil weighs several million pounds and liming requires changing both reserve acidity and active acidity, aglime is required at rates of tons per acre.

Non-aglime nutrients are needed at a fraction of the few thousand pounds that a crop weighs. Therefore, crop-specific nutrient removal rates, based upon realistic yield goals, are the backbone of most soil testing recommendations.

Several plant nutrients are considered mobile and/or extremely complex within soils. It is hard to store or build mobile nutrients within soils. Therefore, most mobile nutrient recommendations fall into the "current crop needs" category. Commonly required crop nutrients that fit into this category include nitrogen, sulfur and boron. Many soil test recommendations do not even consider soil test levels of these mobile nutrients. Other soil test factors that influence the movement and supply of these nutrients may be considered. For example, cation exchange capacity estimates a soil's texture and is often used to estimate mobile nutrient movement through soils. Percent soil organic matter is often used to estimate the quantity of mobile nutrients released from soil organic matter annually.

Equation 1:
Mobile nutrient application requirement = (yield goal x nutrient needed per yield unit) -- soil nutrient release factor -- previous crop factor

In Equation 1, yield goal represents harvestable units such as bushels per acre, tons per acre or cow days per acre. The soil nutrient release factor usually contains an organic matter and cation exchange capacity component. With nitrogen, a previous crop factor is included to estimate nitrogen additions from previous legume crops like soybeans or nitrogen immobilization (tie-up) caused by high-residue previous crops like grain sorghum and corn.

Relatively immobile soil crop nutrients like phosphorus, potassium and magnesium are recommended based upon soil test levels. Soil levels of these nutrients can be modified through long-term fertilizer management. Therefore, these nutrients fit into the "current crop needs combined with long-term soil level" recommendation category.

Equation 2:
Non-mobile nutrient application requirement = (yield goal x nutrient needed per yield unit) + [((desired soil test level --> measured soil test level) x (soil increase factor)/years to build soil to desired level)]

For non-mobile nutrients, the first part of Equation 2 (crop removal) is synonymous to that discussed with mobile nutrients. The second part of Equation 2 (build-up) determines how much of the nutrient is needed to "build" a soil to an optimum level. If the soil test level is already at optimum, then the recommendation simply becomes crop removal.

The soil increase factor from Equation 2 is nutrient specific. For example, 2 to 4 pounds of potassium plant food is required to raise the soil test level for potassium 1 pound. The "years to build soil to desired level" factor is used because it is often not economical to raise soil test levels to optimum in a single-nutrient application. The build time can range from a single application to several years depending upon cost, producer's needs and other factors. As long as the nutrient removal factor is applied to each crop, speed of building a soil to optimum will not influence current crop yield.

The fourth soil test recommendation category involves micronutrients such as zinc, iron, manganese and copper. Soil test values estimate whether these nutrients are present in sufficient amounts to produce an optimum crop. Since the micronutrients are required by crops in relatively small quantities (if present at less than optimum soil test levels) their application recommendations are based upon providing enough nutrient to last two to four cropping years.