passages in the soil and subsoil, which are of great value in aeration, and render the soil more habitable for certain useful bacteria and more permeable to moisture and roots of succeeding crops.
Constant tillage of most soils may make the particles so small that they tend to run together in wet weather and bake into a hard mass upon drying. Putting land into grass for a few years permits the aggregation of soil particles and in this way a rotation corrects injuries.
Fields in grass are less expensive to work than the same area under intertilled crops as potatoes, or roots, hence a mixed farm can be managed well on less capital than one entirely under tillage. If grass can not be grown, alfalfa may be. Alfalfa is usually left undisturbed for several years and like other legumes produces marked increases in the succeeding crops. At Rothamsted Experiment Station, England, land which has been growing leguminous crops for fifty years was plowed up in 1898 and sown to wheat for the five following years with the result that the average annual yield per acre for this period was 27 bushels on the alfalfa plat, 24 bushels after white clover, 23 after red clover and sainfoin, 22 after sweet clover and 20 after peas, beans or vetches, while on the plats growing wheat and fallowed on alternate years for the same length of time, the yields averaged 7.5 bushels per acre per annum during the five years under consideration.
Grain crops as commonly grown do not permit of intertillage, hence the land is liable to become weedy. Intertilled crops can not be successively and profitably grown for a series of years unless they are specialties and bring high prices, as truck crops near towns. Under these circumstances special care is taken in manuring and fertilizing and in combatting insects and diseases. Constant intertillage depletes the soil of its organic matter, the trucker puts this back in his manure. At the Cornell University farm, which is run as a dairy farm, the four-course rotation of (1) corn (land manured about 8 to 10 tons per acre) cut for silage, (2) oats, (3) wheat (manured 8 to 10 tons per acre) and (4) clover, 10 pounds, and timothy, 15 pounds of seed (sown in the wheat), mown twice, has been quite valuable in bringing a poor unproductive farm into a high state of productivity. About 10 tons of corn silage is grown per acre, 50 bushels of oats, 30 to 40 bushels of wheat and over 5 tons of hay per acre (two cuttings). The root residues and the manures applied have been sufficient to preserve and augment the humus content of the soil.
Many plant diseases and insect attacks are easy to combat if a good rotation be adopted. These troubles have and will do more to enforce the consideration of a rotation of crops than almost any other factors. During the year 1904 in trials of mangels on the Cornell University farm the value of a rotation of crops was shown. Two plats separated by others had been growing mangels for three years. In 1903 the leaf