brought about under conditions of moisture, warmth, a limited amount of air and in the presence of lime. Until humification of the leaves, roots and other plant débris takes place the beneficial effects of humus itself are not obtained, and the material, while loosening the soil (sometimes injuriously), is practically inert as far as enriching the soil with its nitrogen and mineral elements is concerned. This was shown by the experiments of Professor Hilgard, carried on through a period of two years. (Soils, page 359.)

Humus contains all of the decomposition products of both vegetable and animal matters, and therefore includes at least minute quantities of amido, amin, and purin compounds, most of which are beneficial and some are injurious to vegetation.

Humus is so thoroughly disseminated in the soil that its presence is noticeable only in the dark color it gives when in large amounts. It is black or brown in color, resembling charcoal in its lightness and porosity as well as in its destructibility by fire. It is colloidal in nature and though derived from vegetable matter, it has lost all of the fibrous and cellular structure of plants. It is highly absorbent of gases and of moisture in the latter case, swelling to eight to fourteen times its dry volume.

Humus has no definite composition beyond the fact that it is made up of the same elements that enter into the composition of plants-carbon, oxygen, hydrogen, nitrogen, and some mineral elements. The proportion of carbon is, however, greater than in plants because of the partial decay. Humus is really a mixture of compounds in proportions that vary with the nature of the substance from which it was obtained, and from a mixture of different substances in the soil. This is especially true with regard to the nitrogen-the most valuable element in humus. Some organic substances are almost lacking in nitrogen and yield a humus containing but little of it, while, on the other hand, a humus may be extremely rich in nitrogen because derived from substances very rich in that element.

In a soil humus is usually combined with lime and other elements as humates, and is then not soluble in water, but is dissolved by a solution of caustic alkali and of carbonate of soda, thus giving rise to the name "black alkali." In soils that are weak in lime it often occurs as free humic acid, and is then soluble in water and causes injurious acidity in soils, which must be counteracted by the application of lime. By the action of air and bacteria it is slowly destroyed and carbonic acid, water and ammonia are produced. Free humic acid is frequently produced by the growing of lime-loving plants, such as alfalfa and other legumes, the lime being absorbed by many successive crops and the humus left behind as humic acid. This has taken place in the Eel River Valley soils of Humboldt County, California, and only the application of lime has restored the land to its former fertility.

Amount of Humus Formed from Vegetable Matter. With regard to the amount of humus formed from the decay of vegetable matter, Professor Hilgard says (Soils, page 128): "Only very general and indefinite estimates can be given of the amount of humus formed from a given amount of vegetable matter, since these must vary according to the conditions under which the transformation occurs. The greater or less access of air and of moisture, the temperature and pressure under which the process occurs, will modify very materially the quantitative as well as the qualitative result. In the hot, arid regions the fallen leaves may wholly disappear by oxidation on the surface of the ground, while under humid conditions they are mostly incorporated with the surface soil. If we assume that in the humification of plant débris no nitrogen is lost, it would seem that in the humid region one part of normal soil humus might be formed from five to six parts of dry plant débris; while in the extreme régime of the arid regions from eighteen to twenty parts of the same would be required. But as most probably some nitrogen is lost in the process of humification, a considerably larger proportion of original substance may be actually required."

Professor Snyder of the Minnesota Experiment Station by his experiments (Bulletin 53) obtained the following results: One part of humus to 32.7 parts of fresh cow manure, or 24.5 parts of green clover, or 10.8 of meat scraps; the humus of the cow manure contained 6.16 per cent of nitrogen, that of the clover 8.24 per cent and that of meat scraps 10.96 per cent. Oat straw and sawdust remained in the ground apparently unchanged for more than a year, but finally gave one part of humus for 5.5 parts of straw and for 9.5 of sawdust, respectively; the humus of the straw had 2.5 per cent of nitrogen and that of the sawdust only 0.3 per cent. In the drier soils of California the straw and sawdust would doubtless remain unchanged for several years.

Value of Humus.

The great value of humus to the soils of this State is not as well recognized generally by farmers as it should be, and they probably do not fully realize that by maintaining a proper supply in the soil through a careful system of green-manuring they will avoid the expense of hundreds of dollars worth of nitrogen, that most expensive of fertilizers, and at the same time keep the soil in a splendid physical condition not produced by fertilizers alone.

Potash, phosphoric acid and nitrogen have their beneficial effect only in maintaining the growth and productiveness of plants and do not affect the soil itself materially; lime influences the structural formation of the plant, the character of the fruit, the physical condition of soils, and neutralizes any acid condition. But humus goes still farther by not only supplying important and expensive elements of food to the

plant and producing excellent mechanical conditions in the soil, but by influencing the life and activity of important bacteria. The following is a brief summary of the value of humus:

Humus Improves the Texture of Soils. It does this in clay soils by causing aggregation or flocculation of the particles of clay into a granular condition, thus producing looseness and easier tillage. The soil is thus better aerated and ventilated, and is made warmer for plant development. By this condition it is made more receptive for rainfall, permits greater freedom in the downward percolation of water, and prevents its evaporation by the sun's heat. When incorporated with loose sandy soils it occupies the spaces between the grains, thus binding the sand together, and giving better texture. By this the soil is made to hold moisture which would otherwise be lost by percolation and by evaporation; and it is made cooler in summer because of absorption and radiation of the heat from the black particles. It decreases the conductivity of a sandy soil for heat, thus preventing overheating of surface plant roots in such soils.

Humus Largely Prevents the Crusting of Soil Surfaces. One of the troubles that California farmers have to contend with is the tendency of the soil to become crusted over or compacted after a rainfall or after irrigation. This seems to be quite general throughout the State, and we believe is in part due to insufficiency of humus and of other vegetable matter which would maintain a granular structure in the soil. An intermixture of, well-rotted stable or farmyard manure would also tend to prevent the crusting, but the turning under and humification of suitable green-manure crops until the amount of humus produced reached several per cent would be better and more permanent.

Humus in the Soil Absorbs Water and Water Vapor. This is due to its porosity, which gives to it a very high absorptive power. But it does this only in its diffused colloidal condition in the soil. When separated from the soil and dried at a low temperature it loses its colloidal nature and acts only as so much fine silt.

Humus Promotes Chemical Action in the Soil. The humus acts upon the inert and insoluble minerals in the soil and slowly renders their elements of plant-food available for plant use.

According to Prof. Harry Snyder of Minnesota,* it is only the humus that contains a high percentage of nitrogen that acts most energetically upon the inert mineral matter, and he designates this as "active humus," while that with low content of nitrogen he classes as "inactive humus."

Humus Contains Nitrogen and is a Nitrogen Fertilizer. Nitrogen exists in the soil partly in the free state in the air that permeates the soil mass; partly in the vegetable and animal material that has not

*See Chemistry of Soils and Fertilizers, page 100.

undergone humification; partly in the humified vegetable and animal matter in water, and hence very variable in amount from day to day and liable to be lost by drainage. That the unhumified material in the soil does not yield its nitrogen to plants until after complete humification has been shown by the experiments of Professor Hilgard, whose conclusions are as follows: "It thus appears that although the nitrogen of the unhumified organic matter constituted about 40 per cent of the total in the original soil, it would during the entire year have contributed only to an insignificant extent to the available nitrate supply; while the fully humified 'matiere noire' contributed fourteen times as much. During the growing season of four or five months the unhumified organic matter would have yielded practically nothing to the crop." (Soils, page 360.)

The humus itself then is the most reliable source of nitrogen, keeping it in reserve, to be given to the roots of plants by degrees, by ammonifying and nitrifying bacteria and at the same time when most needed, viz, in the growing season. The fertilizing value of humus depends, as has already been remarked, upon the amount of nitrogen that it contains and which may be changed to ammonia and nitrates through the agency of bacteria and given to the soil and plant. The nitrogen content naturally varies according to the nature of the materials from which the humus was formed and to any diminution that may have occurred from bacterial action or other causes, and we therefore find great differences in one and the same column of soil. Sometimes the difference is very great between one foot and the next below, for which it is difficult to account.

This humus nitrogen is not, so far as we know, directly available to plants, except in minute quantities of soluble amido compounds formed by the decay of nitrogenous matter, but becomes so through the action of ammonifying and nitrifying bacteria in the soil, which convert it into nitrates.

The humus, therefore, is a valuable source of nitrogen for plant use, and at the same time is a cheap source, because a crop of legumes can be grown at but the small expense of growing, cutting, and plowing under. The percentage of nitrogen that humus contains varies in California from a little more than one per cent in the desert sagebrush lands to as much as twenty per cent in better lands; the average is from five to six per cent.

Humus Contains Soluble Mineral Plant-food. When humus is separated from the soil and burned it leaves an ash which contains large percentages of potash, soda, phosphoric acid and lime, which while in combination with humus are regarded as being immediately available for plant use.

While the amount thus contained in the soil is not large,

varying from .01 to .05 per cent of potash and of phosphoric acid, it adds very materially to the fertility of the soil, the latter percentage being equivalent to about two thousand pounds per acre-foot.

Destruction of Humus.

Humus, like all other organic substances, may be destroyed by dry decay through exposure to hot dry air, either in the immediate surface of the soil or in the portion that is stirred by the plow or cultivator. The loss each year is imperceptible and the effect is seen only after many years of cultivation, except in soils naturally weak in humus, when there is a decrease in productiveness and a tendency to form a hard crust on the surface. In Minnesota* it was found that cultivation of wheat for eight years caused a loss of 17,000 pounds of humus and 1,700 pounds of nitrogen per acre. The crop itself used 350 pounds of the nitrogen, thus leaving 1,350 as the loss due to destruction of the humus by cultivation. It was also found that summer-fallow was more destructive of humus and its nitrogen than was continuous cultivation in grain; the loss in two years by summer-fallow was 600 pounds of nitrogen or twice the amount used for two crops of wheat.

Humus in California Soils.

In California the general practice of continuous grain-growing on the same land, and especially of summer-fallow, for the past many years has resulted in the lessening of the humus content and its nitrogen in the soil by the cultivation required, and as a consequence the yields of grain have fallen off year by year. An instance of this destruction by cultivation was shown in the examination of the soil of the University Farm at Davis, which had been under continuous grain culture for thirty or more years when purchased by the University. The humus percentage of the first foot was found to be .79, with 4.55 per cent of nitrogen. The humus in the virgin soil just outside of the wheat field was 1.25 per cent and its nitrogen 5.90 per cent. This difference of .46 per cent of humus and 1.35 per cent of nitrogen, between the cultivated and uncultivated soils, indicates a loss of more than one third of the nitrogen, caused by wheat culture. This is equivalent to 18,400 pounds of humus and 1,520 pounds of nitrogen per acre. The effect. of this loss of humus and its nitrogen has been seen in the falling off in productiveness and in an increased tendency of the soil to form a thin surface crust, and in other ways to so affect the physical properties of the soil as to interfere with the proper development and functions of the grain roots. The soil becomes lighter in color, more compact, poorly aerated and less retentive of moisture. Percolation of water from rainfall or irrigation is slower and evaporation from the surface more rapid, the soil becoming dry to quite a depth.

This loss of humus and its nitrogen in the surface soil is a necessary

*Bulletin No. 70, Minn. Expt. Station.