of the soil, as well as for many other oxidation processes. The formation of nitric acid in the soil, which is the immediate form in which nitrogen is supplied to most higher plants, can only take place in the presence of an abundant supply of oxygen. Indeed, if the oxygen is not present in the soil in sufficient quantity, processes are very likely to be set up which will destroy the nitric acid present in the soil in the form of nitrates of lime or other bases, setting the nitrogen free in the ordinary air form.

Ventilation is necessary in the soil to supply air nitrogen for the use of those forms of life which appropriate it and fix it in organic form in the roots of alfalfa, clover and other allied plants, and for other organisms which fix it in the soil. It is necessary to ventilate the soil for the purpose of removing the carbon dioxide which forms in large quantities and tends to dilute the air by its presence, and to change its composition otherwise by the removal of oxygen from the soil air which in part has contributed to its formation.

PROCESSES OF SOIL VENTILATION

The most general and effective process of soil ventilation is that due to the action of wind pressure and wind suction, which drives air into the soil and draws it out by suction in the same manner that it does in

dwellings and stables. It is easily seen through the waving of grain and the grass of fields, when the wind is blowing, that the pressure of the wind is never steady, and that it travels in waves of high pressure and waves of low pressure. Under the highpressure waves air is forced into the soil; under the low-pressure waves it is withdrawn; and in this way, if the soil is sufficiently open, the air of the soil is withdrawn and a fresh supply takes its place.

In all sections where there is a deep, porous substratum, like beds of gravel, sand or dry soil, such as are so extensive in all arid regions, whenever the barometric pressure changes there is a material movement of air into and out of the soil wherever the surface soil is particularly open, the air enter

ing or leaving such soils and spreading laterally so as to enter or leave those portions which are covered with less pervious layers.

Then there is the general but slow process of diffusion which tends to produce a movement of oxygen into the soil and an escape of carbon dioxide from it so long as there is any difference in composition between the inside and outside air. The effectiveness of this process of diffusion is greatest wherever the soil porosity is greatest and when the temperature is highest. The same process of diffusion tends to produce a movement of the vapor of water into and out of the soil in the same manner that it does in the case of gases, but, just as the loss of water through a mulch, by this process, is relatively small, so is its efficiency in ventilation relatively small.

The entrance of water into the soil, as in the case of rains or irrigation, and the escape of the water by drainage or evaporation, always results in moving like volumes of

air out of or into the soil, thus bringing about soil ventilation.

So long as the temperature of the soil is greater than that of the air outside, it maintains a difference of pressure, operating in exactly the same way to produce ventilation that the heat in a chimney does, and this may have considerable effect, in the hot sunshine, upon the upper layers of the soil.

In order that wind action (which is the strongest factor in soil ventilation) within the root zone may be most effective, it is important, not only that the pore space shall be large in itself, but that the passageways themselves shall be relatively large; otherwise the amount of flow must be small. It must be clear, therefore, that the influence of the physical condition of the soil as a home for the roots of plants, and for the microscopic life which functions there in the interest of crops, is very great, and hence that it is very important to develop these conditions where they are imperfect and to be able to maintain them when once established. These conditions are thorough and deep

granulation and the existence of other passageways leading from the surface through the root zone, such as are left by the decay of roots and by the action of earthworms, and such as are sometimes formed by the shrinking and cracking of the subsoil under excessive drying.

CONDITIONS WHICH FAVOR STRONG GRANULATION OF THE SOIL

1. Thorough Drainage

Thorough drainage is one of the prime requisites for deep, strong and permanent granulation. The soil moisture itself is one of the chief factors which tend to bunch the small particles into compound clusters, acting in the same manner that it does in shaping itself into spheres in the raindrop and in the drops of dew on the blades of grass, or as it rolls itself into spheres with the dust about it when sprinkled on a dusty floor. As the soil dries and the space occupied by air enlarges, the soil moisture pinches and draws itself about the soil particles like an