Crop Fertility Concepts

Three main interpretations have developed as a basis for making fertilizer recommendations. These have developed slowly, but became important during the 1950s and 1960s when soil testing research was conducted. Because our research database is limited, general principles are developed so that decisions can be made in areas where all the desired information is not known. Since agricultural production always includes unknowns, crop fertilization recommendations are based on interpretation of data and experience. Reasonable scientists have come to different conclusions on what these general fertility principles are. Some of the differences are due to geographic location; some are due to the specific nutrient in question; and other differences are due to the value placed on the many possible objectives.

These three crop nutrition concepts are called: the deficiency correction approach, the maintenance approach, and nutrient removal. These approaches recognize that:

1. Only a fraction of a given plant nutrient in the soil is measured by a soil test.
2. It is impossible to measure how much of a nutrient will be readily available to plants. This means that a soil test value is an index of the soil’s fertility status, not a quantitative measure of the total amount of nutrient in the soil or its availability.
3. Early researchers recognized that the level of available nutrients measured may range from low to high in a given field. Early recommendations were intended to be applied on a field basis.

The deficiency correction concept states that a nutrient should be applied only if there is a reasonable expectation of a crop response. The idea of a limiting factor resulted (Bray, 1944, 1945). This approach is the basis for the correlation and calibration process discussed in Soils - Part 9. A soil test is developed that indicates when a specific nutrient is yield-limiting in a field. Research is conducted to determine crop yields at different soil test levels for a given nutrient (correlation). The next step determines how much fertilizer is required for optimum yields at different soil test levels (calibration).

This approach requires the most intensive research because the soil test needs to be responsive to changes in soil levels and correlated with crop response. The two essential questions to be answered are:

1. Will the crop respond to fertilization?
2. How much fertilizer is needed?

In addition, the soil test should be broadly applicable to various crops and across geographic regions. The database should be large enough that a probability statement can be made with each fertilizer recommendation. For example, “When soil tests for phosphorus are at 10 ppm (Bray and Kurtz #1), there is a 0-20 percent probability of a yield response to applied phosphorus.” (NebGuide G859, Fertilizer Recommendations for Soybeans)

The advantage of this method is that the only fertilizers applied will be those that increase yields, and these will be applied at optimum rates. This has been called “fertilizing the crop,” since emphasis is placed on achieving crop response. The University of Nebraska soils faculty prefers this method because it is based on UNL research and has been proven over many years. This method is both economical and environmentally sound.

The approach is illustrated graphically in Figure 10.1. The deficiency correction approach to fertilizing the crop only recommends fertilizer to the point of economic optimum yield. (Fertilizer is recommended until soil test reaches Point A on Figure 10.1). Experience has shown that fertilizer recommendations to correct deficiencies increase the soil test level for most non-mobile nutrients.

The point called “economic maximum yield” is the yield at which a farmer makes the most profit from fertilizer (Fig. 10.1). If a farmer applies less fertilizer than this, he will save money on his fertilizer bill, but the money lost from decreased yields will be larger than the money he saved on fertilizer. If a farmer applies more fertilizer than needed to reach the economic yield, he may increase his yield somewhat, but his fertilizer costs will increase more than the increase in crop value.

Yield increase x price of corn > lbs of fertilizer x cost of fertilizer

The maintenance approach sets a soil test level goal (Point B on Figure 10.1), and recommends fertilizer to build the soil to the specific nutrient level that has been determined to be ideal. This approach uses soil test levels, as does the deficiency correction approach, to determine when to fertilize. Soil tests for this approach still have to be correlated, as with the deficiency correction approach. The difference is that emphasis is placed on maintaining the soil fertility level at or above the point of the economic maximum yield. This has been called “fertilizing the soil,” since emphasis is placed on achieving a specific nutrient level in the soil. Those who recommend this approach have interpreted the research data to conclude that this approach benefits the producer over time. Generally, the maintenance approach uses a higher soil test level than the critical level used for the deficiency correction approach.

This approach is used by several midwestern universities. Some soil testing laboratories split their recommendations into a fertilizer recommendation and a “build” recommendation. The “build” recommendation is designed to speed the increase in soil test level to the chosen optimum value. Whether it applies to soils of a given region must be tested to confirm its validity in terms of crop response and farm profitability.

A specific example of the maintenance idea is the nutrient balance concept. This concept states that for optimum crop growth there is a “best ratio” of basic cations (positively charged ions) and a best total base saturation for a given soil. There has been little information published that confirms that a best cation saturation ratio really exists for all soils or that it should vary from one soil to another (Leibhardt, 1981; McLean, et. al. 1983). Because the balance concept includes only calcium, magnesium and potassium, using an extrapolation of the balance concept by applying a ratio approach for making recommendations of micronutrient elements and sulfur is not valid. This has brought about criticism of laboratories using the balance concept. Others have extended this balance concept beyond soil fertility concerns, and claim that the proper soil cation balance produces disease and insect resistance.

The balance concept resulted from research on soils where cation saturations varied widely. The initial work (Bear, et al., 1945) was done with alfalfa on one New Jersey soil having uniform amounts of exchangeable magnesium and hydrogen and variable amounts of exchangeable potassium and calcium. From this study, a “best ratio” for the cation composition of the cation exchange capacity (CEC) was proposed — 65 percent calcium, 10 percent magnesium, 5 percent potassium, and 20 percent hydrogen. Later work by Graham in Missouri (1959) suggested that the percentages of calcium, magnesium and potassium be 75, 10 and 2.5, but could vary around these values. The saturation ranges were: 65-85 percent calcium, 6-12 percent magnesium, and 1-5 percent potassium. There is general agreement that variation of the cation composition in these ranges will not likely affect yield appreciably.

While the idea of keeping a soil in “balance” is appealing, it should not be taken so far as to demand remedial treatment in most cases. If each nutrient is non-limiting and extreme excess is not apparent, the relative relationship between nutrients will be acceptable.