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Nature's Methods Contrasted with Man's.-The important lesson to be learned from a study of the origin of the soil is, that Nature undisturbed has many ways of adding to the supply of available plant food in the soil. The various forces that have been under discussion have all tended to change more and more the potential food into forms that can be assimilated

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Nature undisturbed returns all the plant food removed from the soil, by the decay of the plants which it has produced

by the plants, so that the amount of vegetation which the soil can produce has been constantly increasing. Under natural conditions this growth is not removed from the ground, but is again made available, so that the land is constantly increasing in fertility. Thus it will be seen that the fertility of the virgin soils is the result of accumulations due to a variety of forces acting doubtless through countless ages, a period during which practically nothing has been removed from the soil while much has been added thereto.

Man, on the contrary, has reversed this process and while adding little to the soil has removed much therefrom. Through the constant harvesting of crops and leaving the ground bare and exposed to the action of the elements, he is rapidly depleting Nature's store of food, and the yield steadily becomes smaller. The effect upon the physical condition of the soil due to the

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Who can tell how long it has taken this stream to cut this deep gorge in the limestone

removal of all vegetation is serious, for in this way the soil is deprived of its humus-making materials, which is unquestionably quite as important as the actual loss of the chemical elements of fertility.

How to Prevent Exhaustion of the Soil.--Although Nature's method of maintaining the fertility of the soil is without doubt the most effective, it is of course im

practicable for the farmer, for he must remove most of his crops from the field in order that they may be put to the various uses for which they are raised. A study of the formation of the soil, however, suggests two things that he can do to prevent the exhaustion of the fertility. The first is so to treat the soil as to assist and hasten Nature in the process of converting potential plant food into available forms; and to guard against a too complete destruction of the organic matter in the soil. The second is to return to the soil an amount of nitrogen, phosphoric acid and potash equivalent to that removed by the crop.

PART II

MAKING POTENTIAL PLANT FOOD

AVAILABLE

The effect of early spring plowing on the conservation of moisture. The darker plots were plowed earlier than the lighter ones and contained several tons more water per acre.

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