Soil Test Recommendations

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. These include 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. The concept is rooted in work conducted by Dr. Firman Bear and his colleagues at the New Jersey Agricultural Experiment Station in the 1940’s. Dr. Bear’s work showed that the luxury consumption of costly fertilizer K by alfalfa could be reduced by applying high rates of relatively low cost Ca (as ground gypsum or calcitic lime). Later publications by Bear and his colleagues suggested that there were ideal ratios of Ca, Mg, and K; however, they provided no data to support these claims so it is unclear how they were developed.

Around the same time, Dr. William Albrecht and his colleagues at the University of Missouri also began to investigate the BCSR theory. Based on this work, Albrecht suggested a balanced soil should have a cation saturation of 60%-75% Ca, 10%-20% Mg, and 2%-5% K. It is unclear how Albrecht derived these values, and further careful evaluation of the work a number of years later revealed several fundamental flaws in the methods used. A number of soil scientists have evaluated the BCSR theory over the years and their work has failed to support claims of BCSR theory and has illustrated that using it to manage soil fertility results in inefficient use of both capital and natural resources. Despite this fact, a number of unsubstantiated claims regarding the importance of attaining an ideal BCSR, especially pertaining to the importance of very specific Ca: Mg ratios, continue to be promoted. There are no research data that support these claims. 

At one time, many private labs and a few public labs used the BCSR concept to interpret soil test results and make nutrient recommendations. As the body of research evidence illustrating the flaws of the BCSR concept grew, most private labs and essentially all of the public labs abandoned 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. Generally, nutrients should be in the optimum range for good yield and quality. When levels are below the optimum range (very low or low), the addition of more of the nutrient will usually improve production and provide a return on fertilizer investment. Nutrient recommendations are intended to meet crop needs and provide enough to slowly (several years) build soil test levels to the optimum range. When soil test levels are in the optimum range, crop response to application of that nutrient is unlikely, but some amount may be recommended to maintain soil tests levels by replacing a portion of crop removal. In the nutrient recommendation tables for each crop listed in the guide, this is indicated by a range such as 0-50 lb per acre. 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. If a nutrient is in the Above optimum range, crop response is very unlikely and application of that nutrient is generally unjustified. It is important to 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 nutrients, sound agronomic practices must be used (e.g., crop rotation, timely planting and harvest, pest control, and irrigation).  

Table 5: Interpretation of Soil Test 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.  Fertilizer rates 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 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 basis. For most crops, low probability of crop response to addition of 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.