will be much more depleted than is possible for the films within thoroughly granulated soil, where the surface is enormously greater, and at the same time where the films are chiefly within the granules, removed from the water flowing by.

POROSITY OF SOIL NOT DUE TO STRUCTURE

In nearly all soils, except the very coarse, sandy types, there is a porosity not due to structure, but resulting from the borings of ants, earthworms and other animals and from the deep penetration and decay of roots. Such passageways, in all closetextured soils, are extremely important and advantageous because they greatly facilitate the rapid, deep distribution of water through the soil, allowing the soluble plant food to be swept more readily into the soil granules, while any excess of water is permitted to pass quickly into the underdrainage, leaving less time for such water to become saturated with salts from the interior of the granules. It is extremely important, from the standpoint of conserving soil fer

tility, that when the soils of a field are saturated with water the distribution should be rapid and uniform and that all excess water should pass out of the root zone as quickly as possible.

To fill the root zone with water, and hold it completely saturated for a number of days, gives the best opportunity for the excess water to extract from the soil all of the plant food it is able to carry, so that, when it passes out, the soil is depleted unnecessarily, unless it should be one which carries an excess of alkalies. There is great danger also, in leaving soils long over-saturated, of the structure being greatly injured by the breaking down of the soil granules. Wherever a soil is thoroughly ramified by deep passageways such as roots and earthworms form, both rain and irrigation water sink deeply and quickly into the whole soil through such passageways and then spread laterally from them by capillarity, thus saturating the soil granules in the shortest possible time, the excess water passing quickly into the underdrainage.

CHAPTER VIII

DEVELOPMENT AND MAINTENANCE OF GOOD PHYSICAL SOIL CONDITIONS

THE

HE growing of any crop must be regarded as essentially a feeding enterprise or problem. As such it differs in no fundamental way from the feeding and maturing of an animal for the purpose of marketing its carcass or the products that may be matured by it. Just as with the animal so the crop to be grown must be placed under those conditions of temperature, sunshine and humidity which through its long evolution have become indispensable to its normal development.

As in the feeding of live stock, it is necessary to supply crops continuously, and in abundance, with a well-balanced ration of food materials that are highly assimilable. It is not unusual to regard a fertile field, in

its power to produce, as analogous to a wellappointed feeding barn fully stored with an over-abundance of food materials of all essential kinds. Such a conception, however, is far from expressing the true relation of a rich soil to the crop maturing on it. The barn is a lifeless storehouse to which animals are brought for a time to mature and be fattened. Not so a fertile soil in which a field of wheat is ripening. Much more nearly does the analogy hold when the comparison is made with animals grazing in a luxuriant pasture of mixed nutritious herbage which grows day by day, now faster and then not so fast, as it is fed away. Within the body of the soil, if it has a high producing power, there are countless millions of living forms, invisible to the unaided eye, which spring into being, pass through life's phases, reproduce their kind and then die. As these flourish on the organic matter, the moisture and the mineral food materials carried in solution and in the air of the soil, they produce, through their growth and through their death, through their interac

tion among themselves and on the soil, its moisture and its air, food materials in soluble form which are essential to the life of higher plants and which determine in a high degree the immediate productive capacity of soils.

In this light it will be seen that in a very important and a very fundamental sense the soil is a home or habitation in which dwell, in close association, not only multitudes of microscopic life, but the roots of higher plants as well. For the accommodation and the sanitary housing of such communities it is clear that there must be adequate room, abundant water supply and ample ventilation and drainage.

NEEDS OF SOIL VENTILATION

The needs of ample soil ventilation are of the same type and quite as urgent as are those for dwellings and stables where people and animals are housed. It is necessary to supply free oxygen for many of the activities of both roots and the microscopic life