Great interest has been shown in rapid methods for estimating the amount of nitrogen that a soil can furnish a crop. No method has been proposed that is entirely satisfactory. The most used method, and probably the best, is to determine nitrate formation in a few grams of soil kept in the laboratory at constant temperature for about 2 weeks. The amount of nitrate formed is then compared with that formed in similar soils where crop yields have been determined.

LOSSES OF NITROGEN from soils may occur through crop removal, erosion, and leaching, and as gases. The comparative importance of the four channels varies with the soil, crop, and soil-management practices.

Losses through crop removal are proportional to the size of the crop. Such losses can be lowered only by changing the farming system. Grain and hay crops, for example, may be fed to animals on the farm rather than be sold.

Erosion control is especially important because erosion tends to loosen and float away much organic matter. The nitrogen in such eroded material may be several times higher than in the soil left behind.

Leaching removes large amounts of nitrates from soils but no more than traces of other forms of nitrogen. The losses occur most readily from the more sandy cultivated soils, especially in warmer climates and in places where the rainfall is enough for the surface water to penetrate to the ground water several times a year.

Losses of gaseous nitrogen from soils occur chiefly as ammonia, free nitrogen gas, and nitrous oxide. Traces of certain substances, such as amines, hydrocyanic acid, and nicotine, may also escape from growing plants.

Ammonia, applied as a fertilizer, often is rapidly lost from soils to the extent of 25 percent or more. Such volatilization is largely limited to alkaline soils, although they need not be much above pH 7. It is accelerated by drying, especially if the temperature is high and the ammonia is near the surface. It occurs most readily from sandy soils and also from decaying masses of nitrogenous organic materials at or near the soil surface even if the soil is slightly acid, since the ammonia formed may raise the pH locally. Failure to incorporate manure with soil may help explain the low recovery of its nitrogen in the crop.

Losses of nitrogen from soils in the form of free nitrogen gas and oxides of nitrogen are chiefly the result of bacterial action. Such losses are of considerable economic importance. Denitrifying bacteria that produce these gases are widely distributed. Ordinarily they use atmospheric oxygen for growth, but if the supply is deficient they can obtain oxygen from nitrates. In doing so they release gaseous nitrogen. The losses are greatest if soil aeration is poor and if nitrate fertilizers are applied in the presence of masses of plant materials that are undergoing decomposition.

The efficient use of the nitrogen sources available to the farmer involves the prevention of unnecessary losses, the return of manures and crop residues, use of Nature's methods of fixation, and the addition of commercial sources of nitrogen to bring the yield up to a satisfactory level.

Attention should be given more to the use of good farm-management practices than to just the maintenance or building up of soil nitrogen.

The level at which soil nitrogen can be held varies for each soil and for each climatic zone and is affected by the cropping system. With good farm management, including adequate use of fertilizers, good crops can be produced on soils of any nitrogen level if the environment can be made suitable for the crop. This does not imply, though, that abundant soil nitrogen is not highly desirable and favorable to larger and more profitable crops.

In the control of soil erosion and leaching, a greater use of sod crops, catch crops, and cover crops, especially on rolling or hilly land, is needed. Application of soluble forms of nitrogen as short a time as possible before needed by cultivated crops will greatly increase the proportion that is utilized. This statement applies chiefly to the more sandy soils in regions where much leaching occurs just before and during the growing season.

All crop residues should be returned to the soil unless insect infestation, diseases, or the crop to be grown make this impractical. The residues supply some nitrogen, help to hold soluble soil nitrogen, and replenish the supply of active organic matter. Special attention needs to be given to the preservation and use of animal manures.

Gaseous losses of ammonia can be minimized by not applying anhydrous ammonia or ammonium salts to alkaline soils; if they are used, one has to make certain that they are well mixed with the soil. Highly nitrogenous organic matter added to all soils should be handled in the same way.

Losses of nitrogen gas and oxides of nitrogen can be reduced by avoiding the application of nitrates to soils that are poorly aerated and drained. Large applications of undecomposed plant materials also may favor such losses by reducing the oxygen in the soil.

More legumes should be grown in many areas, especially where moisture is not limiting and there are enough livestock to utilize them. The deep-rooted perennial or biennial ones that fix the most nitrogen and require the least cultivation are most beneficial to the soil. Because the amount of nitrogen fixed on nitrogen-deficient soils is likely to parallel closely the total dry weight of the legume, it is important that growth conditions, apart from nitrogen, be made satisfactory.

Nitrogen fixation by free-living soil micro-organisms can probably be increased by the addition of crop residues, such as cornstalks and straw, that are low in nitrogen.

Commercial nitrogen should be used as a supplement to the other forms of nitrogen to the extent needed. Other elements should be supplied in such quantities as to keep the proper nutrient balance.