The Science of the Soil.

Oamaru Mail, Volume XXXI, Issue 8830, 10 July 1905, Page 1

The Science of the Soil.

"The smallest farm." says Professor Hall in '"The Soil," perhaps tho best recent introduction to agricultural chemistry, "may present problems beyond the furthest stretch of our knowledge." The story of a soil is, in fact, exceedingly complex. It is difficult enough to understand the exact role played either by its chemical constitution or its physical condition: we have then still left the part of bacteria to explain—perhaps the most important part of all. It is natural to suppose, for instance, that clays owe their peculiar characteristics to their chemical composition. As a matter of fact, it is not easy to distinguish any chemical difference between sand and clay. If pure quartz is reduced to a very fine powder in an agate mortar, and then separated by sedimentation, the finestgrades of particles cohere strongly on drying, and show all the properties of clay. Though these, therefore, seem to arise from the mechanical condition of the substance, the addition of very small quantities of other substances, such as lime, salt, alum, causes the clay particles to aggregate loosely, and thus altogether changes the texture of the soil. This "texture" is _ equally important with chemical composition, for it implies the "pore space" through which water may circulate, and also the available "surface" of the soil particles. The surface of the particles is one cubic foot of a light soil is said to be equal to about an acre! This available surface to a large extent determines the amount of water a soil will hold, an essential point when ordinary field crops are said to transpire about 3001b of water for each pound of dry matter produced, and this enormous consumption explains the successful results of irrigation. Of tho chemical elements that go' to the structure of a plant, the carbon, hydrogen, and oxygen are drawn from the atmosphere or water, while nitrogen and the traces of metallic elements are derived from the soil. Analyses of soil, however, are not in themselves a very safe guide, for the substances may be present in such a state as to be little use to the growing plant. Even a poor soil contains enormous quantities of plantfood. It is the slowness with which this nan be utilised that- makes the difference. Mr Hall quotes the case of a plot on which wheat has been grown without- manure for 57 years. The crops have in that time removed about, 9001b per acre of nitrogen, 470 of phosphoric acid, and 760 of potash, yet the available supplies are practically unimpaired. It is now known that the nitrogen of the soil is made available to the plant through the asrency of soil bacteria. In this way a biological factor is introduced which may vitiate any attempt to predict the fertility of a soil by analvsis. Attempts made to estimate the number of bac-terin* in the soil lead to nrodisious figures, and the various organisms nlav very different parts. .As these parts Vcovne f"llv understood, artificial infection if soils by bacteria in which they hap"en to be * deficient promises good results. Th i organisms are chiefly confined _to the "nltivated surface layer of the soil. _ Resides bacteria a large number of fungi inhabit- the soil and break down the organic matter it contains. It seems that symbiosis. a -joint-partnership between the roots of : lants and the mycelia of fnnei. : s very ejirnvoTi and of great importance ; n plant- growth.