The Economics of Fertilizers

L. B. Nelson and D. B. Ibach.

The wise use of fertilizers can increase farm returns and help maintain or improve the soil. But potential gains from them are not realized on many farms.

Agronomic, economic, and managerial factors influence the use of fertilizer and must be considered together in relation to the crop to be fertilized and in relation to succeeding crops and the farm as a whole.

Some of these points were discussed in previous chapters. Some we repeat here by way of summary and to emphasize how the factors mesh together.

The starting point in a fertilizer program is to take inventory of the existing nutrient supplies, particularly those obtained directly from the soil.

Some soils may be deficient in one nutrient. Others may lack two or more. Only a few have all nutrients in proper balance for the best plant growth. The nutrient status of the soil differs between fields and within the same field. It changes with time. It reflects the farm cropping practices.

You can estimate the soil-nutrient status in several ways, although none will give a direct estimate of the exact amount of nutrient to apply to get the best balance.

Soil tests may provide the most useful estimate, but they are subject to error unless samples are properly taken and proved tests are made and interpreted by technicians. Information on the characteristics of the soil, the cropping history of the field, and the results of fertilizer trials conducted in the locality increase the chances that the tests are interpreted properly.

The kind of crop and how fast and how big it grows determine how much nutrients it uses.

A 40-bushel corn crop has one requirement for nutrients. A 100-bushel crop has another. The problem is to estimate the optimum economic level of yield to be reached. Some of the controlling factors are discussed later. But data on the amounts of different nutrients used in producing specified yields of crops can help the farmer select the kind and rate of fertilization.

If one crop uses more of a nutrient than another in producing the yield desired on the same soil, more of that nutrient must be applied for the one than for the other. For example, potatoes, tobacco, and alfalfa use large amounts of potassium. Comparable yields of corn, wheat, and soybeans use less potassium.

The quantities of nitrogen, phosphorus, potassium, and sulfur taken up by a crop are especially useful in estimating the kind and rate of fertilizer to apply. We need not consider the amounts of nitrogen that legumes use because they obtain most of their nitrogen through the activity of bacteria in their root nodules.

Crops recover and utilize in any year only a portion of the nutrients applied. Nonleguminous crops frequently recover 20 to 70 percent of the nitrogen applied. Recovery of phosphorus is lower; usually it may be 5 to 15 percent. Potassium is intermediate.

Phosphorus and potassium not recovered during the year of application may be recovered partly by future crops. At the Rothamsted Experimental Station in England, crops are still utilizing phosphorus from large applications made more than 50 years ago. Nitrogen left over from the year of application except in dry climates or during dry seasons may be lost from overwinter leaching.

The higher the rate of fertilizer application, the lower is the percentage recovered during the year of application. Recovery is less when fertilizers are applied on soils that are well supplied with a particular nutrient. Crop residues also may tie up nitrogen and phosphorus. Some soils more than others hold elements in unavailable forms. Leaching and erosion reduce recovery, as does lack of moisture. Liming acid soils favors recovery of soluble phosphates but lowers recovery of the insoluble forms of that element, such as phosphate rock. Placement of fertilizer markedly affects recovery of the nutrients applied.

TOO LITTLE OR TOO MUCH of a nutrient may affect adversely the yield and quality of a crop.

We could give many examples. Adding certain nutrients sometimes accentuates the deficiency of another nutrient and results in either no increase in yield or a lower yield. Excessive nitrogen may overstimulate the growth of leaves and stems and interfere with the formation of seed.

Plants can grow well under a rather wide range of nutrient conditions, if no one nutrient is in short supply. Thus a farmer has some leeway in achieving nutrient balance and can compensate somewhat for a lack of it by applying fertilizer generously. But he will get better and more economical results if he considers the supplies of nutrients and the need for them before he applies fertilizer.

If soils are seriously deficient in two nutrients, adding one of them affects yields little or not at all.

Results from a field experiment conducted on soil that was very low in nitrogen and phosphorus illustrate this point. With no application of either nitrogen or phosphorus, the yield of corn was only 4 bushels. Adding 40 pounds of nitrogen but no phosphorus resulted in a yield of 12 bushels. Adding 40 pounds of phosphoric oxide but no nitrogen gave a 21-bushel yield. In one instance the increase was 8 bushels; in the other, 17 bushels. But when 40 pounds of each nutrient were applied, the yield was 58 bushels. The gain from the combination was 54 bushels, or 29 bushels more than the sum of the two increases. Thus the two nutrients complement each other. The response from the combination is much greater than from either alone.

OTHER FACTORS affect the response to fertilization. Different combinations of such things as density of stand, water supply, variety of crop, or degree of control of insects and diseases affect the results a farmer hopes to get from fertilizers. Any factors that limit yield must be given attention if potential response from fertilizer is to be realized.

It often happens that a factor that does not limit yields becomes a problem when nutrients are plentiful. An example: A low level of nitrogen may be adequate if a lack of moisture limits the yield of corn to, say, 30 bushels an acre. But when the field is irrigated, the rapidly growing corn might encounter a nitrogen deficiency and the yield might be only 25 bushels. If both nitrogen and water are adequate, however, the yield might be 90 bushels an acre.

Too little soil moisture is probably the greatest handicap to effective results from fertilizer. Plants depend largely on the nutrients in the plow layer. When this layer dries out, the uptake of nutrients is impeded even though there is ample moisture deeper in the root zone. Thus irrigation water applied at the proper time and in the right amounts can greatly increase returns when fertilizer is applied. But care must be taken not to over-irrigate.

Excess water leaches much nitrogen and some potassium from the root zone.

A particular variety of crop may not respond as well to fertilizer as another. Some of the old varieties of open-pollinated corn, for example, cannot utilize large applications of nitrogen as can some of the better hybrids. Some hybrids respond more to fertilizer than others do. Crops that are not adapted to a region may not respond profitably to fertilization.

Too few plants to the acre often limit returns from fertilizers on row crops. Some common planting practices were developed under conditions of low soil fertility, so that the number of plants per acre is too low for the higher rates of application of fertilizer now used.

In an experiment with hybrid corn in North Carolina, the yield was 39 bushels when there were 4 thousand plants and 20 pounds of nitrogen were applied per acre. Increasing the nitrogen to 120 pounds an acre resulted in a yield of 63 bushels, a gain of 24 bushels. With 10 thousand plants and 20 pounds of nitrogen an acre, the yield was only 41 bushels, a gain of 2 bushels over the combination of 4 thousand plants and 20 pounds of nitrogen. But when plant population and nitrogen were stepped up to 10 thousand plants and 120 pounds an acre, respectively, the yield was 92 bushels--a gain of 53 bushels over the low plant population and the low nitrogen application. This is an example of complementary effect or of two yield factors working together.

THE RESPONSE greater yields is the basis for determining the most profitable use of fertilizer.

Yields of irrigated corn at different rates of nitrogen applied in an experiment at Prosser, Wash., show how information on response is used in estimating the most profitable rate of application. When 12 rates of nitrogen ranging from o to 520 pounds an acre were applied, yields ranged from 12 bushels to about 150 bushels an acre. Additional nitrogen increases yields.

1. The yield response of irrigated corn to nitrogen at Prosser, Wash., shows how the response diminishes with increasing quantities of fertilizer. Such curves are used to estimate most profitable rates.