By now you have probably concluded that the authors favor the deficiency correction approach. In the discussion that follows, the approaches outlined above will be compared, and the chapter will finish with a brief review of one Nebraska experiment that examined these issues across the state. The broad approaches have been presented first so that there is a conceptual framework for the reader to approach specific recommendations. In practice, all these concepts are used to some degree.
The line between the deficiency correction concept and maintenance is vague. Even when deficiency correction is used, there can be differences between fertilizer recommendations.
The “fertilize the soil” approach continues to apply fertilizer in the area indicated as “apply maintenance fertilizer” (Figure 10.1). Critical level is the soil test value where a nutrient is no longer applied. The extra fertilizer builds the soil test level for future crops. At this soil level, yield increases from added fertilizer may not occur or may be small.
So why should a farmer ever fertilize beyond the economic maximum yield, or why would anyone recommend fertilizer beyond this point? The reason is that we cannot make a fertilizer recommendation with absolute certainty. Crop use of applied fertilizers is never 100 percent. For example, nitrogen fertilizer may be lost by leaching or denitrification, and phosphorus may be fixed or its availability greatly reduced by some soils. To reduce risk, many agronomists support the maintenance philosophy over the deficiency correction approach. They would rather a farmer not sacrifice potential yield and profit by applying beyond the soil critical level than to be somewhat below the critical level. Calculations show that profit is only marginally reduced when fertilizer is applied at slightly above the economic optimum yield point. It is usually more profitable for a farmer to slightly over-fertilize than to have yields on the low side of the economic optimum.
Another reason for using the maintenance approach is that soil sampling is variable, and a few areas of very high testing soil in a field will bring up the field average. Low testing areas may then be under-fertilized if the lower 'critical level' for the deficiency correction approach is used. This type of situation is the basis for site-specific management. Grid sampling should help locate these areas. Over-fertilizing the entire field because the areas of low soil fertility are unknown is the wrong solution. Low testing areas should be determined, and fertilized appropriately based on soil test results.
Those interested in sustainable agriculture have voiced concern about the possibility that the deficiency correction approach will “mine the soil.” Growing crops year after year without adding nutrients has historically depleted soils to the point of abandonment. This is a misinterpretation of the deficiency correction approach. Experimental evidence indicates that fertilizer rates recommended by this approach provide for a gradual buildup in soil test levels (Olson, et al., 1982). This is true, since uptake efficiency is not 100 percent, and the nonutilized nutrients remain and are found by future soil tests. Long-term use of the procedure will 'build' the soil to the critical level, and then that level will be maintained through fertilizer applications as needed (Figure 10.2).
The idealized graph of soil test values (Figure 10.2) shows the effect of different fertilization concepts on soil phosphorus. The horizontal line (A) shows the critical level at which the probability of yield increases is about 5 percent. A constant application (C) of phosphorus annually will continually build the soil test level. Depending on application rates, the two maintenance strategies (MB and MR) will increase soil test levels to whatever the build goal is and then keep the level high. Theoretically, addition of the amount removed will maintain soil test levels. Removal at a low soil test level (R1) would never have soil P levels built up, since only the removal amount was applied. In reality, soil P levels might be built up slightly since plants can use subsoil P and unavailable P, in addition to fertilizer P. The same situation holds for removal at a high soil P level (RH). Since removal and additions may not match every year, there will be some swings in the soil levels. The deficiency correction approach (D) starts out at a low level and builds gradually until the critical level is reached, and then it is maintained based on soil test values, not yields.
Soil testing was developed on the basis of taking a good sample to generate a fertilizer recommendation. The resulting recommendation was followed for three or four years, and then the soil was tested again. With the deficiency correction approach, the rate of buildup is slower than that used by the maintenance method. During the years of fertilization, however, adequate rates of fertility are supplied to attain maximum crop yields. This point is often forgotten when farmers are encouraged to take soil samples annually.
Other factors besides crop response may influence fertilizer decisions. The “fertilize the soil,” or maintenance, plus build approach may be more practical for someone who owns the land and expects to remain in business over many years; but, considering fertilizer efficiency and interest on investment, this approach is still difficult to support. The “fertilize the crop” or deficiency correction approach is more practical for the operator who must deal with short-term leases which do not provide time for recovering fertilizer investments made when applying at rates required to build the soil. This method also may apply to all farmers during periods of fertilizer shortage or extremely high costs.
Does crop need for nutrients increase above those determined by sufficiency levels if there is an exceptionally good year for growth? Bray noted that, when climatic conditions favored higher yields, plots which received fertilizer treatments and those which received no fertilizer both showed corresponding yield increases over years where conditions were not as favorable for growth. Hence, he found no evidence that the yield level should be considered when determining the level of fertilizer to be applied for immobile nutrients such as phosphorus or potassium. He recognized, however, that this is not necessarily the case in practice. He did believe that yield levels should be used as a basis for recommendations for the mobile nutrient, nitrogen.
Increased yields will increase the rate of nutrient removal, and that should affect soil test values. High yields will reduce soil test values more rapidly than low yields. When soil test levels drop below the critical level, fertilizer will be recommended. If soil test levels have not reached the critical level, the rate of increase in soil test levels will be slower under high yield conditions.
The balance-nutrient removal approach plus maintenance approach are the basic parts of many commercial laboratory fertilizer recommendations. The recommendations seem high when compared to recommendations made with the deficiency correction approach. The underlying assumption for building and maintaining higher soil fertility levels is that, long-term, high soil fertility will pay off in ever increasing yields. Because commercial laboratories now test more than 75 percent of all soil samples, the use of a given fertilizer recommendation concept becomes important in light of increasing fertilizer costs and environmental concerns.