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it may be said that the injurious insects are limited to certain plants for their food supply, and if these plants are not grown on the field for a number of years the insects may die from starvation. These remarks do not apply, of course, to those insects which have migratory powers. There is no doubt, however, that both diseases and insects can be more easily suppressed if rotation is practiced. Where one crop is grown continuously the soil becomes infested with certain weeds which are not destroyed by the system of tillage necessary for that crop. The varying treatment to which a soil is subjected in a well planned rotation makes this condition impossible so that the destruction of weeds may be considered as one of the very desirable results of a rotation of crops. In lands badly infested with particular weeds it may even be desirable to omit from the rotation for a while the crop whose growth presents the best condition for their propagation.
Effect of Rotation on Crop Production.—The effect of rotation on crop production is strikingly shown in the following table compiled from data furnished by the Rothamsted Experiment Station.
EFFECT OF ROTATION ON CROP PRODUCTION—AVERAGE OF EIGHT COURSES (32 YEARS)
Bushels per acre
The rotation was the Norfolk rotation consisting of turnips, barley, clover and wheat, each grown one year, and the figures given are the average of eight crops
each of barley and wheat, representing a period of 32 years. The experiments with wheat and barley grown continuously on the same plot for 50 years have previously been mentioned. For the sake of comparison the table gives the averages for the same eight years in which these crops were grown in rotation. All the crops were harvested and removed from the field, and as no manure of any kind was used it will be seen that the increased production of barley and wheat is a result of rotation solely. No stronger argument in favor of rotation of crops is necessary.
Planning a Rotation.—Rotations are in use that cover periods of from two to seven years. In planning a rotation the farmer must be guided by his own conditions and requirements in the way of crops. A few general rules may be laid down, however. Every rotation should include at least one cultivated or hoed crop, such as corn, potatoes, etc., in order to receive the benefits of such a crop in the way of destroying weeds, improving tilth and setting free potential plant food. At least one leguminous crop should be included. The legumes are generally deep rooted crops, and in addition to increasing the nitrogen supply of the soil, bring up plant food from the subsoil and leave it where it will be available to the succeeding crop. These deep rooted plants render the subsoil more porous and hasten its disintegration. A crop that is exacting in its food requirements should follow one that is less exacting, or in general terms, the crops should vary as much as possible in their food requirements, manner of growth, root system and the season of the year in which they occupy the ground. Whatever fertilizers are used should be applied to the particular crop which will give the most profitable returns for their use.
Résumé.-In Part I the reader was reminded that continuous cropping without the use of fertilizers finally results in practical exhaustion of the soil. The food of the plant, and the history of the formation of the soil were briefly considered, and the conclusion was evolved that to maintain the fertility of the land two things were necessary; first, to make more of the potential food available; second, to add something to take the place of the materials removed in the crop.
Part II has been devoted to a discussion of the first proposition. Tillage, drainage, irrigation, fallowing, green manuring and rotation are distinctly methods of changing potential plant food into available forms and, with the exception of the nitrogen gathered by the legumes, add no plant food whatever to the soil. Although, as has been previously stated, it is claimed by some that by an intelligent use of these processes alone a profitable yield can be obtained indefinitely, it is the common experience that even with the use of the best methods of culture known in the past, it is impossible to maintain the fertility of the land without the use of some form of fertilizers. As it is obviously impossible to return the crop to the soil, the next thing that suggests itself is to feed the crop to the farm animals and use their excrement as a fertilizer. The subject of barnyard manures is of sufficient importance to justify its discussion at some length as Part III of this treatise.
BARNYARD MANURE Effect of stable manure on growth of corn. The plot on the left was manured with stable manure, while the one on