The nitrogen may be supplied in a mixed fertilizer, one containing nitrogen, phosphoric acid, and potash, or as a fertilizer containing only nitrogen. One point is important. The crop should be well fertilized with phosphate and potash if they are needed. Nitrogen fertilization will not give good results if there is a deficiency of other nutrients, insufficient water, or other factors unfavorable to growth. Ordinarily when other nutrients are needed, all or part of the nitrogen required is included with them in the application of a mixed fertilizer. This is an efficient and convenient method under many conditions.

Where large quantities of nitrogen are to be applied or in regions of high rainfall and light soils, a common practice is to apply a part of the nitrogen in mixed fertilizer at planting and the rest as a side dressing along the row. This is the general practice with cotton and corn in the South. On some soils nitrogen is the only fertilizer that gives profitable returns. Examples are the soils of the Mississippi Delta and certain soils in California.

Row crops are usually fertilized somewhat as has been indicated for corn; namely, all or a part of the nitrogen is applied in a mixed fertilizer at planting. Supplementary nitrogen may be applied as a side-dressing shortly after the crop is well established and is making good growth.

Wheat in the Midwest is usually fertilized at planting in the fall with a complete fertilizer containing 2 to 4 percent nitrogen. Additional nitrogen may be applied as a top dressing in the spring, but that is not a well established practice. In the South, a spring top dressing of nitrogen fertilizer on small grains is a general practice.

Details of fertilization vary so much with soil, crop, and climatic conditions that it is advisable for farmers to consult local and State authorities for specific recommendations.

The selection of a satisfactory nitrogen fertilizer is not difficult. All of the nitrogen fertilizers on the market give good results when used properly. In general, there are no great differences in their efficiency.

A few facts should be understood regarding differences in the properties of various forms of nitrogen:

Both ammonium and nitrate nitrogen are readily absorbed by plants in the early as well as later stages of growth. Nitrate nitrogen moves with the soil water and may be leached from the soil by heavy rainfall. Ammonium and related forms of nitrogen are not readily leached from the soil.

Sodium nitrate and cyanamide are slightly basic in residual reaction. Their continued use tends to reduce slightly the soil acidity.

Ammonium sulfate is quite acid in its action on the soil. It requires about 1 pound of limestone to correct the acidity from 1 pound of ammonium sulfate. Ammonium nitrate and uramon produce only one-third as much acid as an equivalent amount of ammonium sulfate.

The physical properties of nitrogenous fertilizers, found in the market, range from fair to excellent. Granular materials free of dust are easier to handle and are generally preferred by farmers.

Since equivalent amounts of the different nitrogen fertilizers give substantially the same results for most crops, their relative cost is an important consideration. They should be purchased on the basis of cost per unit of nitrogen, with small allowances for differences in some of the properties indicated, if those properties are important considerations under the conditions of use. Comparative unit costs are calculated by dividing the ton price by the percentage of nitrogen (N) in the fertilizer. At any given location and time the price relationships shown in the table above may not hold. Buyers, therefore, should calculate unit costs from the guaranteed analysis and price data furnished by the dealer or salesman.

The final selection of a nitrogen fertilizer can therefore be made on the basis of cost per unit of nitrogen, physical properties, and their influence on the chemical and physical properties of the soil.

The crop returns from the use of nitrogen fertilizer depend on many factors other than the kind of fertilizer selected and how it is used. Nitrogen fertilizers give best results when their use is combined with other good soil and crop management practices. The soil should be limed when needed, have a good supply of available phosphate and potash, be in good tilth, and have a satisfactory supply of moisture. The crop should be an improved variety with date of planting, rate of seeding, and cultural methods best adapted for local conditions. What returns can be expected from nitrogen fertilizers when a farmer does all this?

Extensive field experiments, many of them in cooperation with farmers, indicate that the approximate increase from the use of 10 pounds of nitrogen on crops will be about as follows:

Cotton, 100 to 140 pounds of seed cotton; corn, 3 to 5 bushels; wheat ( Ohio, Indiana), 2 to 3 bushels; oats (South), 6 to 8 bushels; timothy or Sudan grass hay, 300 to 400 pounds; potatoes (Eastern States), 6 to 8 bushels; apples, 30 to 90 bushels; peaches, 20 to 60 bushels.

The data are representative of returns obtained in States east of the Mississippi River at normal to good rates of fertilization-30 to 60 pounds of nitrogen to the acre in most cases. They show that where nitrogen fertilization is needed it gives good returns.

THE AUTHOR

F. W. Parker is an assistant chief in the Bureau of Plant Industry, Soils, and Agricultural Engineering and is in charge of soil and fertilizer research in that Bureau. Before joining the Department in 1942 Dr. Parker was an agronomist in the ammonia department of E. I. du Pont de Nemours Co.