be rolled, or otherwise firmed, so as to renew capillarity as far as possible. Green Manuring Not Advisable on Stock Farms. -Green manuring as a general practice is not to be recommended in any style of stock farming. The Crimson clover as an orchard cover-crop. The cover-crop is one method of green-manuring, and the crimsonclover is a good example of the nitrogen-gathering plants used for this purpose. crops which are most valuable as green manures are also of great value as feeds, and it will be found more profitable to feed them to the animals and return the manure to the field, as will be shown later. On the whole, it may be said that green manuring will prove desirable in any system of farming (including truck farming) where the crops are sold from the farm, and especially if all the crops produced are much alike in food requirements. On the other hand, if the farmer is engaged in animal husbandry the crops are of such great value as feeds that turning them under must be considered a wasteful practice. CHAPTER X ROTATION OF CROPS Origin of Rotations.-It is the common experience of farmers in those parts of the world where the land has been cultivated for a long time, that the fertility of the soil is maintained for a much longer time by growing a variety of crops instead of producing one crop continuously. The adoption of a system of rotation of crops has been the outgrowth of accident rather than the result of an understanding of its underlying principles. The system of alternating years of barefallow and wheat may be said to be a two year rotation and was the first to be adopted. History teaches us that this was later followed by a three year rotation consisting of fallow, wheat, beans or oats; and still later, when the value of clover and fallow crops became evident, this rotation gave way to the now famous Norfolk rotation of turnips, barley, clover and wheat, the typical English rotation. The Norfolk four year course represents the more common type the world over, consisting as it does of cereals alternating with hoed crops and leguminous crops. Plants Differ in Food Requirements.-There are many arguments to be advanced in favor of growing a variety of crops on the soil. The different crops vary in their food requirements and in their ability to procure this food from the soil. Where one crop is grown continuously on the same field nearly all of the plant food available to that crop may become exhausted, while the soil would contain large quantities of food. in forms that could be assimilated by plants of another class. Some crops evidently require the mineral matter to be in a readily soluble form, while others can use "tougher" forms of plant-food. The early writers on agricultural chemistry supposed that the crop during its growth excreted substances that were injurious to itself, while they were at least harmless and perhaps beneficial to plants of a different class. This view is not now accepted, but it is believed that the failure to produce profitable results where one crop is grown continuously is due to the exhaustion of the forms of plant food available to that particular crop. Some crops make an especial drain on one element of plant food. By growing crops with different food requirements there is less likelihood of any one element becoming exhausted and the different elements are more evenly used. Plants differ in Manner of Growth.-The various crops differ widely in their systems of root growth. Some plants like wheat are comparatively shallow rooted, and must obtain their food from the surface soil, others, as the clovers are very deep rooted, and are able to use food that would not be within reach of the more shallow rooted plants. The deep rooted plants can not only procure the low lying food, but probably bring a part of it to the surface where it remains upon their decay for the use of the succeeding crop. It is well known that the shallow rooted plants do better when preceded by a deep rooted crop. Rotation Improves Soil and Economizes Labor.— When a variety of plants is grown the soil receives different treatment for each crop, so that the faults of one year are likely to be corrected the next, and for this reason, the soil is kept in much better physical condition. As a general rule the ground can be better prepared for the succeeding crop if a judicious rotation is practiced than if the same crop is grown continuously. The roots and stubble of clover and the grasses are also factors of some importance in improving the texture of the soil. Taken altogether the texture or tilth of the soil will be found to be much improved by rotation of crops. Where a variety of crops is grown on the farm it results in economy of labor, for the work of caring for them is distributed throughout the season instead of all coming at one time. In this way it makes it possible to secure cheaper and better help than where only a few kinds of plants are produced. Rotation Aids in Controlling Diseases, Insects, and Weeds.—Rotation also enables the farmer to control plant diseases and to head off the injurious insects. Most of the piant diseases are caused by bacteria or other fungi which live only on one genus of plants, or at any rate, are more or less restricted in the number of crops that they can use as host plants. Where one crop is grown continuously these disease-producing fungi are given every opportunity to be carried over from one year to another. Most of these germs are comparatively short lived, so that if three or four years of crops that are not suitable host plants intervene the germs are likely to be destroyed. In the same way |