Practices

How To Determine Nutrient Needs

Frank G. Viets, Jr., and John J. Hanway.

You can tell whether soil needs fertilizers by the health and productiveness of the plants that grow on it.

Plants in poor health may be stunted or when nutrients are critically low show signs of sickness on leaves, stems, or fruits. In some plants, however, the need for more or different nutrients is less easily seen. Often they may not appear stunted or show deficiency symptoms, but they will respond to the addition of nutrients to the soil. This hidden hunger will become more common as farmers increase their yields.

Three steps are necessary to determine the nutrient needs of soil: The problem must be diagnosed. The degree of deficiency must be determined.

The amount of fertilizer needed for the desired yield must be found.

PLANTS AND SOIL conditions must be examined in detail in the field. There is no way of getting around that. This diagnosis can then be checked by simple fertilizer tests in the field or greenhouse, by quick tests of plant tissues, and by analysis of soils and plants.

Often you can easily see that a crop is not making proper growth. Sometimes plants exhibit general or specific symptoms of poor nutrition. Too little sulfur and nitrogen, for example, produce a general chlorosis a yellow or pale-green color over the entire plant. Some deficiencies, like that of iron, show up mostly in the younger tissues; the young leaf blades are white or pale yellow but the veins may be normal.

Although symptoms are a useful guide to the need for nutrients, one has to be careful in interpreting them, particularly when two or more deficiencies exist at the same time. Climate also may affect the expression of symptoms. Therefore the diagnosis should be confirmed by chemical tests of the plant tissues or by applying the nutrient to the soil or the foliage and seeing how the plant responds.

Of particular importance in diagnosis is the influence of the soil profile. Poor drainage may induce symptoms of deficiency. An example is so-called lime-induced chlorosis, or iron deficiency, which is common on susceptible crops or poorly drained sites in the West. Plants in fields where surface soil has been removed by erosion or in preparation for irrigation often are deficient in nitrogen, phosphorus, iron, potassium, or zinc, especially if the subsoil contains lime. Overliming can induce some deficiencies on acid soils. Deficiencies of one or more elements frequently occur where sand or gravel underlies shallow soil.

It is important but not always easy to exclude other possible causes of poor growth or symptoms that look like mineral deficiencies. Some insects suck juices from plants and so reduce growth. The toxins of some insects deform plants and produce symptoms like those of mineral deficiency. Nematodes may retard development of roots.

Some plant diseases, particularly the virus diseases, produce leaf patterns that can be confused with symptoms of mineral deficiency. So do various organisms that produce dead areas in leaves. Root rot can reduce the ability of a plant to forage for nutrients.

Excess salts in the soil reduce the entry of water into plants and restrict their growth without producing specific symptoms of deficiency. This problem is common in western irrigated areas and may become so in the Eastern States. Accumulation of sodium on the clay of soils leads to an alkali condition that often is linked with poor growth or no growth of crops and the presence of deficiency symptoms of iron and sometimes zinc.

The damage done by drought may be mistaken for nitrogen deficiency in grains, corn, sorghum, and grasses.

Frost damage may also produce symptoms that may appear to be deficiencies of nutrients.

Applying fertilizers too close to the seed at planting or sidedressing fertilizers at too high rates or too close to the plants may produce injuries that reduce growth or kill plant tissues.

Improper cultivation may result in deficiency symptoms. Lightning may sear small areas in a field.

After you have made a field diagnosis, you can confirm it by pot experiments in a greenhouse or strip tests in the field. These tests are made by adding (singly or in various combinations) the fertilizer elements suspected of being deficient and observing the plant growth that results.

THE NEXT STEP is to determine the extent of the deficiency.

One way to do that is to make an experiment in the field itself. The deficiency is estimated by adding nutrients and determining what effect the additions have on the plants. The fertilizer containing the nutrient is added to the soil or sprayed on the plants at various rates of application more than one nutrient may be deficient, and several rates of each nutrient may be added in combination with different rates of the others.

Several kinds of information may be had from these experiments. The simplest is the response curve of yield in relation to the amount of nutrient supplied. It gives information about the supplying power of the unfertilized soil in terms of bushels or tons of produce. If the increase in yield is great, the experiment shows that the soil has too little of the nutrient in question. The shape of the response curve also shows how much nutrient is needed to produce the desired yield level under the existing set of conditions.

Other methods of evaluation of the available nutrients involve chemical analyses of unfertilized plants. Such estimates usually are based on only the tops of plants, because it is hard to test the roots. The chemical analysis and the yield of the plant tops allow one to calculate the amount of a nutrient element removed from the soil by the plants. The amount removed by the unfertilized plants can be used as an estimate of the power of a soil to supply nutrients.

1. A theoretical curve of yield in relation to plant composition, where only one nutrient limits growth. As the nutrient content increases above the critical level, yield does not increase.

The method has limitations, however, because factors other than the availability of the nutrient may restrict plant growth and thereby reduce the amount of the nutrient removed.

L. A. Dean, of the Department of Agriculture, in 1954 proposed a method of plotting the phosphorus uptake by plants grown with a series of applied rates of phosphate. The method gives an estimate of the available phosphate in the soil in terms of the phosphate that is applied. The nutrient uptake must be proportionate to the nutrient applied if this method is to be used conveniently.

R. D. Munson and George Stanford, of the Iowa Agricultural Experiment Station, later applied this method to nitrogen studies with German millet grown in pots.

Field studies on the applicability of this method of estimating nutrient availability have been started at several experiment stations.

Another way to estimate the nutrient availability can be used in a field or greenhouse. It makes use of an isotope of the nutrient element. An isotope used for tagging identifying--a fertilizer differs from the normal or common form of the element either in its weight or its ability to emit radiation that can be detected. These isotopes are useful in research studies because they can be detected readily, but they enter in the same kinds of chemical reactions in the soil and are absorbed by plant roots just like normal isotopes.

2. Theoretical yield of nutrient line AB, obtained from fertilizer application. Line AD is direct measure of nutrient available from unfertilized soil. Line CD is an estimate of nutrient in soil in terms of that applied obtained by extending line AB to C.

The tagging isotope is incorporated into fertilizer. Then the amount of nutrient taken up from the fertilizer can be determined accurately by calculating the proportion of normal isotope to "tagged" isotope in the plants grown on the soil.