Applying Fertilizers

R. L. Cook and Walter C. Hulburt.

Plant nutrients differ in the way they may be applied most effectively as fertilizers because of the differences in their chemical properties, the amounts plants need, the chemical and biological activity in the soil, and their solubility, which varies according to their formula and physical condition.

The nitrogen that exists in the soil as organic matter is a basic consideration. Organisms decompose it and release the nitrogen as ammonia. Other organisms change the ammonia to the nitrate form. The organisms themselves use nitrogen to build the tissues of their own bodies.

If the organic matter (such as straw and corn stover) in the soil is high in carbon, it releases too little nitrogen to supply the metabolic requirements of the organisms. Organisms then may compete with crops for soil nitrogen, including the nitrogen that is applied as fertilizer. Thus a deficiency may develop at a time when the growing crop urgently needs nitrogen.

Experiments at Michigan State University showed that 300 pounds of nitrogen an acre applied at planting time was not enough to supply the nitrogen requirement of sugar beets throughout the growing season. But plants that had received the same total amount of nitrogen fertilizer at three different times--one at planting time and two later were well supplied with nitrogen the whole season. The weight of the crop and an analysis of the beets indicated that only 50 percent of the nitrogen that was applied at planting time could be accounted for. About 70 percent of the nitrogen applied in three parts was accounted for, however, and soil and tissue tests indicated some was still available to the crop at harvesttime.

R. L. Carolus found that potatoes in Virginia required 7 pounds of nitrogen an acre during the first 7 weeks of their growth period. During the next 5 weeks the crop used 53 pounds to the acre.

W. R. Jones and H. A. Huston recorded similar data for corn in Indiana.

During such periods of lesser need, fertilizer nitrogen may be used up by soil organisms or may be leached out of the root zone of the crop.

Rates, methods, and time of application of nitrogen fertilizers should be varied with different soils and crops so that an adequate supply is present at all times for the growing crop.

Usually some nitrogen should be applied in the starter fertilizer. A. C. Caldwell, of the University of Minnesota) learned that ammonium nitrogen in contact with phosphate caused Young corn plants to make better use of starter phosphate. Sometimes it is wise to make applications primarily to feed the organisms that decompose the organic matter.

PHOSPHORUS exists in soil as primary and secondary minerals and as a constituent of organic matter. A small amount is held by clay in an adsorbed state and an extremely small amount is in solution.

Phosphorus combines readily with other elements. It is applied as a constituent of various carriers, the simplest of which is phosphoric acid (H₃PO₄). This acid and several phosphate-carrying salts ionize to form H₂PO₄-, HPO₄--, and PO₄--- ions, depending on the soil pH.

PO₄--- ions are present only in alkali soils and are not important in plant nutrition. Iron and aluminum hydrated oxides combine readily with H₂PO₄ in medium to strongly acid soils. The resulting phosphate compounds are so insoluble that they cannot be used readily by soil organisms or higher plants. This fixation is more effective in the more acid soils.

Efficient management of fertilizers is the art of feeding the plant rather than the soil: It is getting phosphorus into the plant without allowing the reactions between soil minerals and phosphorus to take place.

The nature of the plant, the characteristics of the soil, the chemistry of the phosphate carrier, and the amount of capital to be invested are factors to be considered in deciding on methods of using phosphorus fertilizers.

Experiments by Kirk Lawton, James Vomocil, and Lowell Owens and his coworkers at Michigan State University show that fertilizers containing a high percentage of their phosphorus in water-soluble form should be applied differently than those that contain all or most of their phosphorus in insoluble forms.

Broadcasting and mixing soluble phosphates with soil (as with a harrow) or spraying solutions on the soil surface result in maximum fixation and least efficiency as far as the immediate crop is concerned. That is because maximum contact is provided between the particles of fertilizer and the hydrated oxides, which are a part of the soil and with which the phosphate ions readily combine.

Placing the phosphates in bands beside the rows of seeds is a more efficient way to apply them both as to crop yields and the percentage of the phosphorus the plants take up.

Dr. Lawton and Dr. Vomocil showed that soluble phosphorus moves rapidly out of fertilizer granules (much fertilizer is now sold in a granular or pelleted rather than powdered form), but moves slowly away from the granules.

The result is a highly concentrated spherical zone of soluble phosphorus around a granule as long as the soil is not disturbed. Roots may then penetrate that sphere and feed readily on the phosphorus in soluble form.

Dr. Owens and his coworkers found that banding and granulating caused an increase in the intake of fertilizer phosphorus by wheat and sugar beet seedlings but that both banding and granulating were not necessary. In other words, the banding of granulated fertilizers did not result in greater use of fertilizer phosphorus than resulted from the banding of pulverant (powdered) fertilizers.

Fertilizers containing only citrate-insoluble and perhaps citrate-soluble (but water-insoluble) forms of phosphate should be applied in finely powdered form and should not be banded.

Rock phosphate is an example of the materials that should be finely ground and mixed with soil to obtain maximum solution effect. That may be true also of highly ammoniated superphosphates, calcium metaphosphate, and fused tricalcium phosphate.

Producers of high-value crops and greenhouse operators should concern themselves with raising the available phosphorus levels of their soils to a point where this fixing power has been satisfied and there are no longer appreciable quantities of "hungry" minerals to combine with the phosphate ions. Phosphate fertilization then can be a simple matter of maintenance by any convenient method.

The specific recommendations listed later are primarily for those who wish to fertilize their current crops at the least cost to insure top yields.

POTASSIUM exists in soil as primary and secondary minerals, as an exchangeable cation attached to soil colloids, as a constituent of fresh organic matter, and in solution. It leaches from soil rather readily when a large amount is present in exchangeable or soluble form. An equilibrium among mineral, exchangeable, and soluble forms of potassium exists in normal soils.

Plants take in more potassium than they need when it is present in excess of their normal needs. Such luxury consumption is wasteful of soil potassium. Excessive applications of potash salts tend furthermore to drive the potassium equilibrium to the direction of unavailable mineral forms.

The farmer should consider these reactions of potassium in soil and the tendency toward luxury consumption by plants when he chooses the method and time of application of potash fertilizer. Because potash salts are somewhat toxic to germinating seeds and young seedlings, excessive amounts should not be applied with seeds or too close to seeds or young transplants.

Broadcast applications of potash are agronomically sounder than are such applications of phosphorus fertilizers because reactions with soil minerals are unimportant except when the rates are excessive. Small, frequent applications are preferred.

Band applications of potash are better for some crops that need a plentiful supply of potassium from the earliest seedling stage. This is probably true of alfalfa, which has responded well to band seeding.

MINOR AND SECONDARY ELEMENTS generally are applied most economically and effectively as constituents of mixed fertilizers. One reason is that the amounts needed are generally so small that separate applications are difficult, except as they may be applied in solution as foliage sprays. Such methods, though, are more costly unless the crop is to be sprayed for other reasons and the minor element is compatible with the spray material.

Several minor elements (among them manganese and boron) are more efficient when they are applied in bands with complete fertilizers. They are usually needed in alkaline soils, in which they are quickly rendered unavailable when mixed with the soil.

Copper and zinc may be mixed effectively with complete fertilizers or may be applied through spray equipment. This latter method is used on vegetable and fruit crops that are sprayed to control diseases and insects.

LIQUID AND GASEOUS FORMS Of fertilizer compete with the solid forms.

The use of liquid and ammonia fertilizers has grown steadily.

Applicator equipment, metering devices, and storage facilities have been perfected and make direct applications of anhydrous ammonia practical. In fact, on many farms anhydrous ammonia may be the most economical form of nitrogen.

Anhydrous ammonia and all nitrogen solutions containing free ammonia must be stored in pressure tanks and be applied by equipment that immediately covers them with sufficient soil to prevent volatilization usually 5 to 6 inches deep with the anhydrous form. Soil should be friable and neither too dry nor too wet. Dry soil may not close in readily over the ammonia, and there may not be sufficient soil water to effect adsorption. Wet soil may contain so small a volume of air-filled pores that the ammonia gas cannot penetrate enough soil to make adsorption complete.

Nonpressure nitrogen solutions may be effectively applied on the soil surface. Wide-boom equipment can be used; it saves time and costs.

The wide use of phosphoric acid and ammonium phosphates as carriers of fertilizer phosphorus has stimulated the practice of applying complete and mixed fertilizers in solution. Precipitation salting out--limits the concentration of such fertilizers, especially in regions where temperatures may be low. The formula determines the actual concentration; however, (as an average) 30 units is the top grade that may be used with assurance of a stable solution in Northern States.