The first requirement of the soil population is a supply of energy, without which it cannot live at all. All our evidence shows that the magnitude of the population is limited by the quantity of energy available. The percentage by weight of the organic matter is about two to four or five, and the percentage by volume runs about four to twelve. Not all of this, however, is of equal value as source of energy. About one-half is fairly easily soluble in alkalis, and may or may not be of special value, but about one-quarter is probably too stable to be of use to soil organisms. A second requirement is water with which in this country the soil is usually tolerably well provided. Even in prolonged dry weather the soil is moist at a depth of 3 inches below the surface. It is not uncommon to find 10 per cent. or 20 per cent. by volume of water present, spread in a thin film over all the particles, and completely saturating the soil atmosphere. TABLE II. VOLUME OF AIR, WATER AND ORGANIC MATTER IN 100 VOLUMES OF ROTHAMSTED SOIL. +---+-----------------+-----------+--------------+ | | | |In Pore Space.| | | | |Values Common-| | | Solid Matter. | | ly Obtained. | | +--------+--------+ +-------+------+ | |Mineral.|Organic.|Pore Space.| Water.| Air. | +---+--------+--------+-----------+-------+------+ |(1)| 62 | 4 | 34 | 23 | 11 | |(2)| 51 | 11 | 38 | 30 | 8 | |(3)| 41 | 12 | 47 | 40 | 7 | +---+--------+--------+-----------+-------+------+ (1) Arable, no manure applied to soil. (2) Arable, dung applied to soil. (3) Pasture. The air supply is usually adequate owing to the rapidity with which diffusion takes place. Except when the soil is water-logged, the atmosphere differs but little from that of the one we breathe. There is more CO₂, but only a little less oxygen.[8] The mean temperature is higher than one would expect, being distinctly above that of the air, while the fluctuations in temperature are less.[5] The reaction in normal soils is neutral to faintly alkaline; _p_H values of nearly 8 are not uncommon. Results from certain English soils are shown on p. 18. The soil reaction is not easily altered. A considerable amount of acid must accumulate before any marked increase in intensity of _p_H value occurs; in other words, the soil is well buffered. The same can be said of temperature, of water, and of energy supply. Like the reaction, they alter but slowly, so that organisms have considerable time in which to adapt themselves to the change. HYDROGEN ION CONCENTRATION AND SOIL FERTILITY.