Another explanation was found in the theory that roots excrete certain sub stances that are noxious to the plants excreting them and innocuous or even beneficial to other plants. The excretory theory was taught early in the past century by the renowned Swiss botanist, Pyramus de Candolle. It was no doubt a suggestion from the animal kingdom. This theory was practically given up before the middle of the past century. Yet it is most interesting to find recent experiments in England on the growing of grass in orchards leading to the suggestion that one plant may exert some influ ence on the soil deleterious to another plant. It is suggested that this influence, however, is biological rather than chemical—in some way, per haps, concerned with the little-understood germ life of the soil. Recent publications by the United States Department of Agriculture (Bureau of Soils) state that root excretions are probably very intimately associated with soil productivity, that much of the value of manurial substances lies in the cleansing of the soil of these toxic excreta, and that the value of rotation of plants is determined largely by the presence or absence of such excreta.
Some of the reasons why rotation-farming is considered to be advan tageous (under present teaching) may now be mentioned.
(1) One crop tends to correct the faults of another crop. The contin uous growing of one crop usually results in the injuring of the soil in some respect ; a rotation tends to overcome and eliminate such effects. It evens up and works out the inequalities. The general average of many or several kinds of treatment is better than the effects of one treat ment.
(2) Plants differ considerably in the proportions of the kinds of foods that they take from the soil. In rotations, the different plants make the maximum of their draft on the soil at different times in the year, thereby allowing the progress of the seasons to even up the inequalities.
(3) By a judicious choice of crops, different plant-food materials may be incorporated in the soil in available condition, through the decay of the parts plowed under or left in the ground. The most marked benefit of this kind probably comes from incorporation of nitrogen Com pounds through the use of leguminous plants. These plants have the power, by means of their root nodules, of fixing the free atmospheric nitrogen of the soil ; and the new compounds are turned back to the soil in condition to be utilized by plants that do not have the power to appropriate the nitrogen of the air. Since nitro gen is the most expensive and usually the most easily lost of the plant-food elements that the farmer has to buy, this role of the leguminous plants is most important. It is significant that most of the
early rotations, developing before rational expla nations of them could be given, comprised some legume.
(4) Some plants have the power, more than others, to utilize the content of the subsoil. Such plants may not only make less proportionate draft on the upper soil, but by their decay may add to the richness of such soil. It has been determined, for example, that lupines are able to take more food from the subsoil than oats. Most of the legumes have similar power, largely because of their deep-rooting habit ; and this affords additional explanation of the good results accruing from the use of such plants in the rotation.
(5) A rotation of crops can be so planned as to maintain the supply of humus in the soil. This humus, coming from the decay of organic matter, adds to the plant-food content of the soil and, what is usually more important, exerts a great influence in securing a proper physical texture of the ]and. The Bureau of Soils recently asserts that the chief value of humus is to cleanse the soil of toxic excreta. The humus is chiefly supplied by the grass crops and clover crops in the rotation. The practice of "green-manuring" rests chiefly on the need of supplying humus. Green-manure crops are those that are grown for the special purpose of being turned under, root and top, and are not usually a definite part of the rotation ; but, so far as it goes, the root-and-stubble part of similar crops employed in the rotation answers the same purpose.
(6) Well-considered rotation schemes reduce the necessity of excessive use of concentrated or chemi cal fertilizers. On the other hand, they may utilize such fertilizers to greater advantage than do the con tinuous-cropping schemes, as has been shown by the Ohio Experiment Station.
(7) A good rotation provides for the making of farm manures, because it grows crops for the feed ing of live-stock. As a general practice, it is better to market the hay and straw crops in the form of ani mals or animal products than to put them on the mar ket directly ; for the farmer not only has the opportunity to make an extra profit by an extra process, but he gains the manure with which to maintain the fertility of his lands. He raises the crop to feed his stock to secure manure to raise a better crop. In Q the maintaining of fertility, the live stock farmer hasthe great advantage of the horticul- turist or other special farmer, for the latter must resort to special practices or special pur- chases in order to maintain the produ cing power of his land.