Nutrient recommendations are determined by soil test results; however, even when two labs use the same methods and generate equivalent results, their nutrient recommendations may differ. These disparities arise due to differences in soil test interpretation and recommendation philosophies. Over the years, three basic philosophies have emerged: the sufficiency approach, the build and maintain approach, and the base cation saturation ratio (BCSR) theory. Both the sufficiency approach and the build and maintain approach follow the general concept that there are definable critical levels of nutrients in soil, and that below this level crops are likely to respond to additional nutrients applied. When nutrient concentrations are in the optimum range, just above the critical level, there is a low probability of crop response to the addition of that nutrient. With the build and maintain approach, fertilizer recommendations are made with the goal of building the soil's nutrient levels into the optimum range, then maintaining these levels by applying nutrients at rates that approximate crop removal. The sufficiency approach is a more conservative philosophy where nutrient recommendations are intended to meet crop needs, not build soil fertility. No nutrients are recommended above the critical soil test level. The sufficiency approach is designed to “feed the crop” while the build and maintain approach is designed more to “feed the soil.” In theory, the sufficiency approach is a more profitable system since fertilizer is only applied when there is likely to be an economic return. However, in practice the sufficiency approach is also more risky due to the inherent uncertainty associated with soil testing.
The third philosophy, the BCSR theory, promotes the idea that maximum yields can only be achieved by creating a balanced ratio of calcium (Ca), magnesium (Mg), and potassium (K) in the soil. At one time, many private labs and a few public labs used the BCSR concept to interpret soil test results and make nutrient recommendations. Over time, as the body of research evidence illustrating the flaws of the BCSR concept grew, leading most private labs and essentially all of the public labs to abandon the system.
Most of the guidelines developed by land grant universities and used by both public and private soil test labs follow a combination of the sufficiency and build and maintain approaches, the goal being to provide adequate, but not excessive, levels of essential nutrients to promote healthy plant growth. Nutrient recommendations provided in this Guide are a reflection of this compromise. The nutrient guidelines are intended to help growers optimize crop yield and quality, maximize return on fertilizer investment, and minimize nutrient losses to the environment.
The nutrient recommendation tables in this Guide are applicable to the New England soil test results given as very low, low, optimum (medium or high), and above optimum (very high or excessive). Table 5 provides a brief interpretation of each of these categories. In general, the goal should be to maintain nutrient elements within the optimum range as reported on a soil test. When nutrient levels are within this range, the needs of most crops will be met, and crop response to application of a given nutrient is unlikely. Some amount of the nutrient may still be recommended to replace that which is removed by the crop and maintain soil test levels. If levels are below optimum (very low or low), increasing levels to optimum will usually improve production and provide a return on fertilizer investment. If levels are above optimum, application of that nutrient is very unlikely to result in a crop response and may attract pests and/or cause environmental harm.
Nutrient recommendations in this Guide are intended to meet crop needs and provide enough to build soil test levels to the optimum range over several years. Crops and even cultivars of the same species vary in their uptake and removal of nutrients and this is accounted for in the nutrient recommendations. Keep in mind that factors other than nutrients may limit crop growth, and simply adding more nutrients will not improve yield. To optimize yield and maximize response to fertilizer, sound agronomic practices must be used (e.g., crop rotation, timely planting and harvest, pest control, soil health, and water management).
Table 5: Interpretation of Soil Test Level Categories
| Category | Interpretation |
|---|---|
| Very Low | Soil test level is well below optimum. Very high probability of crop response to additional nutrients. Substantial amounts of additional nutrients required to achieve optimum yield. Recommendations are based on crop response and are designed to gradually increase soil nutrient levels to the optimum range over a period of several years. |
| Low | Soil test level is below optimum. High probability of crop response to addition of nutrients. Moderate amounts of additional nutrients needed to achieve optimum yield. Recommendations are based on crop response and are intended to gradually increase soil nutrient levels to the optimum range. |
| Optimum | Most desirable soil test range on economic and environmental bases. For most crops, low probability of crop response to additional nutrients. To maintain this range for successive years, the nutrients removed by crops must be replaced. |
| Above Optimum | The nutrient is considered more than adequate and will not limit crop yield. At the top end of this range, there is the possibility of a negative impact on the crop if nutrients are added. |
| Environmental Critical Level | This soil test level is independent of crop response and, due to environmental concerns, is only defined for soil test P. This P concentration is associated with elevated risk of P loss in leachate and runoff at concentrations high enough to impair surface water quality. No P should be applied and steps should be taken to minimize losses from leaching and runoff. |