6,984 pounds; phosphorus, 2,824 pounds; and potassium,

12,460 pounds.

When these amounts are considered in connection with the requirements of the four crops-corn, wheat, clover and timothy-for the fifty-year period, we get the supply value of each ingredient.

By simple calculation:

Nitrogen will last 2.30 fifty-year periods, or 115 years. Phosphorus will last 3.28 fifty-year periods, or 164

years.

Potassium will last 5.12 fifty-year periods, or 256 years.

Four important factors to take into account.-But there are four other factors that enter into this furnishingsupply consideration besides the supposedly-available stores. These factors are:

1. The constant contribution to the available store from previously unavailable material.

2. The return to the land of straw, stover and other manurial refuse.

3. The increase of the nitrogen supply through the legumes.

4. The help that comes from the subsoil.

The contribution to the store of available ingredients.-Nature is at work, constantly, changing the unavailable stores of plant-food ingredients into available forms: every sort of soil maker is at work. Hence, we may conclude that the crops (included in our rotation) have not only for use the supply that was available at the beginning, but they have, also, an additional supply that is being contributed constantly by the soil-making agents.

It is not unreasonable, then, to suppose, that when wise. farming is done, manure added to the land, thorough tillage performed and a good tilth maintained, this contribution to the store of available food ingredients will keep pace with the outgo through the crops removed.

In this connection attention should be called to the fact that soils are running out rapidly under the present system, due to loss of available food by constant removal of crops, and to the loss of humus, and the consequent injury to physical condition.

The return of ingredients to the land. There is seldom observed a system of soil cropping that removes the entire crop growth away from the land: always some part of the crop is returned to the soil, from where it originally came. With cotton, leaves, stalks, and often the seed find their way back; with corn, the entire stalk or else the main stem, with the leaves, in the resulting manure; with wheat, always the stubble and often the straw; and so with the most of our crops: some parts of them go back to the soil. In this way, the annual removal of plant food is lessened and a complete exhaustion is, in every way, quite out of the question.

The increase of nitrogen due to the legumes. In the cycle discussed previously, during the entire term, clover occupies the land, more or less, for twenty years. Considered in connection with this discussion, it is clearly evi

dent that the nitrogen supply, instead of being seriously disturbed, more likely is preserved and it may not be out of the range of possibilities to suppose that the normal average is ever increased: a feature quite opposite in effect to that of depletion. Certainly, land occupied by clover twofifths of the time during a fifty-year period preserves its producing power. Who will say it does not even increase it?

The help that comes from the subsoil.-Since roots have come into the range of observation and study, we know that they seek deeper pastures than the surface foot allows. Roots go to a depth of two, three, four and often

INCREASING THE NITROGEN WITH LEGUMES

A crop of soy beans that are bringing nitrogen into the soil and at the same time producing a high protein feed

ten or twelve feet. Consequently, the supplies that come to plants are not solely from the surface foot: for plants get food wherever their roots go, wherever the root hairs find mellow earth into which they may search.

This subsoil contribution, therefore, is a large one, and one that, in a great measure, influences the potential supply of plant food, commonly supposed belonging to the surface soil, but which is a not-inconsiderable factor of the entire food-furnishing possibilities of the land.

Potential plant food is large. We may be certain, therefore, that the potential supply of food is large, that the stores of available plant food are more or less constantly reënforced from other sources and that, this being the case, the following conclusions are correct:

1. Soils are able to produce crops indefinitely with proper treatment.

2. Soils are never exhausted of their potential plant food.

3. Soils are often depleted in producing power, but only temporarily.

4. Worn-out soils are not exhausted chemically but physically. Their humus has been used up.

5. Soils, once productive, but now unproductive, may be restored to their former state through the rejuvenation of the physical life.

6. Physical improvement is the first step necessary for restoring the producing power of soils.

7. A fertile soil, wisely managed, maintains its fertility. 8. A fertile soil, unwisely managed, loses its fertilityproducing power-but not its chemical constituents: these are present in the tightly secured storehouses of the land.

9. Crop production bears a close relation to the physical nature of the soil-the humus content, the texture, the air and water circulation, the nature of the earthy material, the previous treatment given it; but no correct estimate can be made from the chemical analysis of the soil alone.

10. Chemical analysis can be interpreted only in connection with group or type surveys, reënforced by broad observations in the field, and modified by climatic conditions, commercial opportunities, and the temperament of the operator.

CHAPTER X

THE RÔLE THAT TILLAGE PLAYS

Just when man began the improvement of soils by means of tillage tools we do not know, nor do we know the kind of tools that were first employed.

This, however, we know: the first written evidence of civilization includes in its work simple tillage operations that the fruits of the field might be increased. The first simple tool may have been a shell from the sea, or a pointed stone from the mountain side, or it may have been a sharpened stick; it matters not, for in time these and other kinds were succeeded by the crooked stick, fastened to the horn of a bull, perhaps, which in time became modified, and developed into the modern tools of tillage.

Nature tills the land.-While we often think Nature neither tills nor cultivates her fields, we forget that every force she has at work is actually performing just these very things. things. For what are the freezing and the thawing the heaving of the surface-but tillage operations; what are the crumbling and tearing and breaking by air and water but simple forms of tillage; what are the channels, made by earthworms and other animals that burrow in the earth, but plowing of another order; what are the deposits of silt, left from overflowing waters, but new earth, ready for newly laid seeds, as rich and effective as that turned by plow or any cultivating tool; what are the roots that work their way deep into the soil but vegetable tools of tillage; what are these-one and all of them-but plows and harrows and hoes, which Nature uses for preparing the land for new seeds, for new crops?