UNUSED FERTILITY.

New Zealand Herald, Volume LXVIII, Issue 20827, 20 March 1931, Page 16

UNUSED FERTILITY.

VALUE OF HUMUS.

It has been pointed out by scientific writers, time after time, that practically all ordinary soils contain huge reserves of fcrtilisng material, but that a great deal of this is not in ft readily-available condition. It has been said that there is sufficient nitrogen present in practically every aero of ordinary soil to supply the needs of farm crops for at least 100 years, and sufficient phosphoric acid and potash to supply the needs for four or five hundred years; but the point has always been, and always will be ; that these huge reserves are mostly in an unavailable condition.

It is a very big subject- and one wo can only touch on here, but it may bo said that the lack of lime and the lack of humus are among tho principal causes of sterility in soils. Sir A. D. Hall shows tho analysis of tho soil of Broadbalk Field, Rothamsted, which has had no manures for fifty years, as und^er: — Nitrogen, 0.100 per cent., equal to 25001b. per acre; phosphoric aced, 0.110 per cent., equal to 2,7501b. per acre; potash, 0.270 per cent., equal to 6,7501b. per acre.

These figures deal only with tho first nine inches of soil. If lower soils into which tho plants' roots penetrato were included, the weights per ac.ro would be considerably increased. When we recollect that tho application of, say, lewt. of a nitrogenous manure to a corn crop will often have a wonderful effect, we can see how different is tho case of a soil that contains nitrogen in an avuilablo condition and ono that does not. Tho plants are unable to mako use of the, reserves that arc there. It is for this reason that from time immemorial tho application of farmyard manure, or green manuro, has often been attended with good result's far beyond what one might have expected from their analytical contents. Tho reason is that, as the manure decomposes in tho soil, tho provision of humus to tho soil exerts a beneficial effect, and tho plants are liable to mako use of much of what is already in the soil. It is also well to point out that humus makes light soils more cohesivo and more retentive of moisture but, on tho other hand, humus makes clay soils looser and more easily worked.

Another point that is worthy of atten J tion is that there is in all soils the property of capillary attraction, that is, as the roots of the plant draw moisture from the surfaco soil in which they are growing, there is always an upward flow of moisture to their roots, provided the soil is suitable. • The application of any decomposing organic matter, which we term humus, facilitates tho rising of water, and hence wo find that the root crop to which farmyard manure has been applied stands tho drought better. Humus generally contains a large percentage of-carbon, and when ,this is taken into tho surface soil, tho carbon combines with oxygen and produces carbonic acid gas, which sols free and makes available plant foods in the soil, and so brings about greater fertility. Wherever lime, or manures which contain lime, are applied to 6oils, it is considered that the availability of both the potash and the phosphoric acid present is increajftf.