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CHAPTER VI

TILLAGE

Tillage Increases Feeding Ground for Roots.The most efficient means of assisting nature in the conversion of unavailable food into forms that the plant

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Plowing is the most important tillage operation. It should be so done as to

leave the minimum amount of work for the harrow, etc.

can use is good tillage of the soil. Tillage, in the sense in which it is used here, signifies any operation of stirring and pulverizing the soil by means of plows, harrows, cultivators or any other implement, either before or after the seed is sown.

The most noticeable result of tillage is that the soil is made finer, the large lumps being broken up into smaller particles, and in this way Nature's work in the formation of soils is accelerated. Pulverization of the earth is beneficial in many ways. In the first place, loosening the soil makes it easier for the plant roots and root-hairs to penetrate it. Mention has been made of the fact that all soils are composed of grains of greater or less dimensions separated by air spaces. The tender root-hairs must push their way in between these soil-grains, as it is impossible for them to penetrate the solid particles themselves. It must be evident that the more the soil is pulverized the larger the number of the openings between grains, and, consequently, the greater room for root growth.

The plant is dependent upon the root-hairs for its supply of mineral food and, as these hairs grow only between and around the soil grains, it is apparent that they can feed only on the surfaces of these particles. Good tillage increases the amount of surface exposed to the roots by breaking the large lumps into small grains; and the more complete the pulverization the larger the area from which the plant can obtain its food. The rapid increase of surface due to breaking down the lumps of a soil in poor tilth seems almost unbelievable to one who has given the subject no thought. An example will serve to illustrate what is meant: A cube, 2 inches on the side, presents a surface of 24 square inches. If this cube is cut once in each direction 8 cubes are formed, each one inch on a side, giving a total of 48 square inches of surface, so that cutting only once in each direction doubles the amount of surface. Thus, theoretically, a plant should be able to derive twice as much food from the eight small cubes as from the large one.

Tillage Hastens Chemical Changes in the Soil. Stirring the soil is of great advantage in bringing together particles which have not before come into contact. In this way chemical changes may take place that render potential plant food available, for substances having different chemical properties are thus

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An incompleted soil. Good tillage will hasten the decomposition of the rocks

enabled to act upon each other. The changes whereby potash and phosphoric acid become "fixed” in the soil are reactions of this class. The changes brought about by freezing and thawing may also be accelerated by proper tillage. This is made use of by some farmers who plow heavy, lumpy land in the fall so that it may be exposed to the influence of the weather during the winter. For this purpose the land is so plowed as to leave it rough and with the largest possible area exposed to the weather. Freezing and thawing bring about disintegration of the clods in much the manner mentioned in the chapter on formation of the soil, and the resulting improvement is most remarkable in some classes of soils.

Tillage Aerates the Soil.—One of the most advantageous results to be obtained from tillage is the aeration of the soil. The introduction of the oxygen of the air into the soil is of benefit in a number of ways. In the first place, a certain amount of air in the soil is necessary for the growth of all plants usually raised on a farm. The roots can not live without air any more than those parts which grow above ground. That air is needed by the roots can easily be shown by placing a pot containing any ordinary plant in a jar of water so that the soil will always be saturated. In a short time the bad effects will be noticeable on the plant. The plant does not decline because the water is injurious but because the presence of the water excludes the air from the roots. Oxygen is also necessary to the germination of seeds, for it is a well established fact that seeds will not germinate in the absence of oxygen.

The oxygen of the air has a direct chemical action upon the mineral matter of the soil in that it tends to make the latter soluble. It also prevents the formation of certain compounds (notably the sulphides of iron) which are injurious to vegetation.

Tillage Aids Nitrification and Prevents Denitrification.—All fertile soils contain a considerable amount of organic matter, and the presence of oxygen is necessary to its decomposition. Attention has been called to the fact that the soil contains innumerable bacteria, a part, at least, of which are concerned in the decay of organic matter, and those which are beneficial to the farmer can not live without oxygen. One class of these bacteria decomposes a part of the organic matter with the formation of carbonic acid gas, and it has been shown that this gas dissolved in the soil-water is a great factor in making plant food soluble. As this decomposition goes on more rapidly in well aerated soils it will be seen that this is one reason for the increased fertility due to thorough tillage. The nitrifying bacteria previously mentioned thrive only in the presence of a sufficient supply of oxygen. Most of the nitrogen of the soil is locked up in insoluble organic compounds, and before it can be used by plants it must be converted into the form of nitrates. This process takes place only in a soil well supplied with oxygen, and experience has proven that this process is very materially hastened by frequent cultivation. The extreme importance of this process of nitrification has already been commented upon, and it remains only to say that tillage would pay for itself if it did no more than hasten nitrification.

Thorough aeration of the soil prevents the action of the denitrifying bacteria, as these bacteria thrive best in a soil devoid of oxygen. Acidity of the soil is also favorable to the growth of the denitrifying bacteria, and as the presence of sufficient oxygen in the soil tends to keep it sweet it is thus helpful in preventing denitrification.

The bacteria which enable leguminous plants to use free nitrogen are also dependent upon the air in the soil, for not only do they need oxygen, but experiments have shown that it is only from the air in the soil that they can draw their supply of nitrogen. It is necessary, therefore, in order that leguminous plants may profit by the nodule-forming bacteria, to have the soil

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