A few general rules may be made use of in arranging the order of the crops in the rotation though they cannot always be strictly followed.
Crops that require the elements of plant food in the same proportion should not follow each other.
Deep-rooted crops should alternate with shallow-rooted crops.
Humus makers should alternate with humus wasters.
Every well arranged rotation should have at least one crop grown for its manurial effect on the soil, as a crop of cowpeas, or one of clover, to be turned under.
The objection often made to this last rule is that, aside from the increase in fertility, there is no direct return for the time, labor and seed, and the land brings no crop for a year. It is not necessary to use the entire crop for green manuring—a part of it may be used for hay or for pasture with little loss of the manurial value of the crop, provided the manure from that part of the crop taken off is returned and the part of the crop not removed is turned under.
LENGTH OF THE ROTATION
The length of the rotation may vary from a two-course or two crop rotation to one of several courses. Crimson clover may be alternated with corn, both crops being grown within a year.
A three-course rotation, popular in some parts of the country, is wheat, clover, and potatoes; potatoes being the money crop and cleansing crop, wheat a secondary money crop or feeding crop, and clover the manurial and feeding crop.
A popular four-course rotation is corn, potatoes or truck, small grain, clover; the potatoes being the chief money crop, corn the feeding crop, the small grain the secondary money or feeding crop, and clover the manurial and feeding crop.