The entire nitrogen requirement of the nonlegume crops must be met with nitrogen from fertilizers or farmyard manure when monoculture is used. Because the nitrogen in fertilizers is subject to loss from the soil through leaching, careful attention must be paid to the timing of applications of nitrogen fertilizers in order to meet crop requirements.

Successful monoculture requires close attention to erosion control, except for the special case of perennial sod.

Problems of soil structure may become severe when one grows crops that require frequent tillage operations and return little organic residue to the soil. The exact kinds of soil that will stay in satisfactory physical condition under continuous cropping with each of the different crops are not definitely known. The best course for a farmer to follow is probably to watch the water relationships of the soil. If he sees a growing tendency for water to stay ponded on the surface or to run off more readily from sloping areas, the soil-management system should be changed to provide greater returns of organic residues, less frequent tillage, and perhaps more deep-rooted crops. Poor physical conditions in the soil will be more evident in seasons that are too wet or too dry.

Users of monoculture will be completely dependent on chemical insecticides, disease-resistant varieties of plants, soil fumigation, and similar methods of controlling insects, plant diseases, and other pests. The severity of this problem varies from crop to crop, from one locality to another, and even from time to time.

Chances for the successful monoculture of different crops vary widely with the characteristics of the crop and the diseases and insects that attack it. On one end of the scale are the continuously grown perennial sod crops, which are successful almost everywhere. On the other end are the intertilled crops, such as peanuts, potatoes, and bright leaf tobacco, which rarely are grown successfully by monoculture and then only on soils of naturally good physical properties. Continuous wheat or alternate wheat and fallow has been successful over relatively large areas in the Western States and Canada. Soybeans have been grown continuously on some of the flat claypan soils of the southern Corn Belt. Continuous sugarcane is the rule rather than the exception in areas where it is grown.

The use of the newer insecticides and tillage methods and nitrogen fertilizers has opened up new possibilities for successful production of continuous corn. Continuous cotton has been successful in some of the nearly level soils of the South and on irrigated areas in the Southwest, although special subsoil tillage operations may be required to break up compacted soil layers.

One-year cropping systems, in which a winter crop alternates each year with a summer crop, tend to resemble monoculture systems in their advantages and requirements. Among these systems are those in which a close-growing winter cover crop (such as rye, ryegrass, crimson clover, vetch or a mixture like rye and vetch) alternates with a summer row crop (like corn, cotton, or tobacco). The winter cover crop is not ordinarily harvested but is turned under as a green manure crop. The use of the close-growing winter cover crop protects the land from erosion for at least part of the year and tends to decrease problems of soil structure maintenance. The range of soil conditions upon which intertilled crops can be successfully grown each year is increased by adding a close-growing winter cover crop to the monoculture systems.

Another 1-year system with two crops consists of winter grain followed by annual lespedeza. Here the winter grain is the harvested crop, and the lespedeza contributes to the system primarily through its effect in maintaining available nitrogen in the soil.

Systems where two harvested crops are grown each year are found in some of the vegetable producing areas, where the growing season required for each vegetable crop in the system is short.

In the Southern States, winter grain and soybeans, both crops being harvested for seed, is a promising i-year, 2-crop system. Similar systems found in irrigated regions of the Southwest include winter grain and cotton.

An important fact to keep in mind concerning these 2-crop annual sequences is that there must be enough water for both crops. The use of these systems therefore is confined to the more humid States and to irrigated areas with long warm seasons.

Crop sequences without a legume or other sod crop are used on cash-crop farms, especially in subhumid sections. The sequence in which the crops follow one another frequently varies from cycle to cycle, depending on conditions of soil moisture and the demand for the different crops. As far as maintenance of nitrogen and soil structure are concerned, these nonsod rotations resemble monoculture systems.

THE CHOICE of a cropping system that will provide for the longtime maintenance of soil productivity involves many factors.

The pattern of different kinds of soil on the farm must be considered. If the soil on the farm is quite uniform and is subject to hazard from erosion, a cropping system that involves fairly regular use of grass-legume sods, rotated over all the cropland on the farm, should be given serious consideration.

On the other hand, if the farmer has several different kinds of soil, a monoculture system with each crop grown continuously on the soil where it is best adapted may be most satisfactory.

The agronomic characteristics of the forage crops to be grown are also important in determining the cropping system. Forage crops which are relatively easy to establish by seeding but which tend to decline in yield as time goes on are best used as a component of fairly short sod-based rotations. Red clover and timothy mixtures are a good example. If the preferred forage crop is rather slow or difficult to establish, but when once established remains productive over a long period, continuous use of the same field for this crop may be desirable. Birdsfoot trefoil is an example of a desirable forage crop that is well suited to continuous production on the same soil.

Every plan of a cropping system should provide for flexibility, in order to permit changing the acreage of crops from year to year to adjust to changes in demand and farm organization. Provision for alternate crops for use on areas where the intended crop is destroyed by weather, insects, or diseases early in its growing season is also needed. As a general rule, regular rotations are best adapted to regions where annual rainfall is less subject to extreme variations.