THE SOIL

Nelson Evening Mail, Volume XLVIII, Issue XLVIII, 29 March 1915, Page 2

THE SOIL

THE VALUE OF HUMUS.

"The name humus should not be applied to all of the vegetable matter of the soil in all stages of decay, as many seem to think, but only to a peculiar product of partial decay of vegetable and animal matters brought about under conditions of moisture, warmth, a limited amount of air, and in the presence of Jime. Until humufication of the leaves, roots, and other plant debris takes place, tho 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."

The above paragraph is taken from a statement by Professor R. H. Loughridgo in California Bulletin 242, which contains a discussion as to the nature of humus, its value, and its distribution in California. We give the following short resume of the introduction to this bulletin :

is so thoroughly disseminated in the soil that its presence is noticeable only in the dark colour it gives when in large amounts, but black soil may sometimes be due to its mineral content rather than humus. Humue is black or brown in colour, resembling charcoal in its lightness arid porosity and in its destructability by fire. It is highly absorbent of gases and of moisture, i'n the latter case swelling eight to 14 times its dry volume. In chemical composition it varies with the composition of the plant from- which derived. In a soil, humus is usually combined with lime and other elements as humates, and is not then soluble in water, but is dissolved by a solution of caustic alkali and of carbonate of soda. When eoils are weak in lime it forms free lrumic acid, and this acid condition mustbe corrected by the use of lime. The unavailability of, free humic acid is one of the reasons why the lime-loving plants such as alfalfa, should have applications of limo to correct this tendency of acid formation when the lime in the soil is depleted. Professor Loughridgo says that by maintaining a proper supply 6f humus in tho soil through a careful system of green manuring the farmers of California could avoid the expense of hundreds of dollars worth of nitrogen, and at the same time keep the soil in a splendid physical condition riot produced by fertilisers alone. He then gives the following brief summary of the value of humus :

Humus Improves the Texture of Soils : It does this in clap soils by causing aggregation or floccuiation of the particles of clay into a granular condition, thus producing looseness arid 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 of the sun's beat. 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 is made cooler in sxtmmer because of absorption and radiation of the heat from the black particles. Itdecreases the conductivity ,of a sandy soil for heat, thus preventing overheating of surface plant roots in such 6oils. '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 bo 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 crusti;ng, 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 Vapour : This is due to its porosity, which gives 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 ite 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 plantfood "available for plant use. According to Professor Harry Snyder, of Minnesota, it is only the humus tl-at contains a high .percentage of nitrogen that acts most energetically upoa the inert mineral matter, and he designates this as active humus," while that «ih low content of nitrogen he classes as "inactive humus."

Humus Contains Nitrogen and is a Nitrogen Fertiliser: Nitrogen oxists in the soil partly in the free st:t;e in tie air that permeates the soil mass; in the vegetable and animal material that has not undergone lmmifioation. : partly in the humified vegetable and animal matter in water, and ham.e -very variable in amount from day to day ?r,d liable to be lost by drainage. The unhumified material in the soil doas i:<t yield its nitrogen to plants until after complete humification. The humus itself, then, is the most reliable source of nitrogen, keeping it in reserve, to be given to the roots of the plants by degrees, by the ammonifying and nitrifying bacteria, and at the same time when most needed, viz., in' the growing season. The fertilising 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 difference 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 che-ap source, because a crop of legumes' can be grown at but the small expense of growing, cutting, and ploughing under. The percentage of nitrogen that humus contains varies in California from ■& little more than 1 per cent, in the desert sagebrush lands to as much as 20 per cent, in better laiide,- the average is from 5 to 6 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'.os'per cent, of potash and of phosphoric acid, it adds very materially to the fer*«Ety of the the latter percentage being equivalent to about £2OOO per acre-foot.