Agriculture. Exhaustion of Colonial Soils.

Waikato Times, Volume XXIII, Issue 1893, 23 August 1884, Page 6 (Supplement)

Agriculture. Exhaustion of Colonial Soils.

By E. W. Ehierson Maclyor, F. 1.0., Mem. Phys. Soo., Lon., F.C.S., Lon. ct Berlin, Ad.

Editor of Maclvor's Farmer's Annual. Lecturer on Agriculture, Technical College, Sydney. Tim ash-constituents and also the nitrogen of farm plants are obtained from the soil, whilst the carbon, oxygen and hydrogen whioh, by the way, form by far the greater part of vegetable bodies, are derived chiefly from • the atmosphere and water, and therefore have nothing whatever to do with the exhaustion of the Boil. It was formerly believed that most plants were capable of utilising the uncombined nitrogen whioh constituted about four-fifths of our atmosphere, but chemico- agricultural researches sy Lawea, Gilbert, Pugh, Boussingault, and other eminent authorities, tended to show that such was not the case. " If," says the venerable Boussing^uU, "there is one fact demonstrated in physiology, it is the nonassimilation of free nitrogen by plants, and I may add, by plants of an inferior order, Such as mycoderms and mushrooms." It must, however, be mentioned that while the majority of scientific men agree that graminacious crops and grasses cannot assimilate free nitrogen, a good riany believe that io is probable that leguminous plants are possessed of that power. The classical field and laboratory investigations of the sai'ant* just named, have \ inclined the school of agricultural chemists to which the writer belongs, to the view that, whatever differences may exist between the assimilative capabilities of the?e two families of farm plants, neither is able to directly absorb and utilise elementaiy atmospheric nitrogen. It follows, therefore, that growing vegetation must obtain their supplies of this element from the ammonia nitrates and other compounds contained in the soil, and which, together with the mineial food, enter the plants by the roots.

PLANT FOOD IN O.HE ROIL. The proportion of plant food present in every very fertile soil is small. The surface soil of the first nine inches of a good cultivation paddock may contain only 0.1 per cent. of nitrogen, and the subsoil leas than onetwentieth of that quantity. Phosphoric acid may be prenent to the extent of 0 2, but it is generally much less than this. Potash varies considerably according to the character of the rocks from which tho soil had been formed, but it is usually undev 1 0 per cent. Lime may exist in abundance, but in many AustrrHun pnils ie is rarely moie than 0.20 per cent. Magnesia is almost invariably tess abundant than lime in good soil. Tiieso proportions, sma'l though they appear, amount to laige quantities when the weight of an acre of soil is taken into calculation. Warivgton f-tates that nine inches depth of clay or loam will weigh when quite dry about 3,000,000 or 3,500,000 lbs., and that a pasture soil freed from rootss and water will weigh 2,250,000 lbs. Thus, then, a dry soil containing 0 10 per cent, of nihogen, potash, lime, or phosphoiic acid will be fiom 2250 to to 3500 lb<s. per acie. It must not, however, be supposed tiiat the whole of this plant food exists in any soil in the condition in which crops can utilise it, foi, as a matter of fact, a large quantity exists in an insoluble or " locked up " foim, and only becomes directly useful to vegetation when natural agencies at work in the soil have converted it into a soluble or available state. Nitrogen, for example, may occur in great part in combination with other elements, as humus or vegetable mould, and when in this condition°it cannot^ be utilized by cereals or grasses. By the action of the minute organisms known as Dactaium, which is present in all soils, the humus is decomposed, and its nitrogen conveitcd into nitnc acid, in which foim it is readily taken into the system of the growing plant. TLe conditions essential to this process—niti ifirat ion— are the presence in the soil of moistuie, air, and carbonate of lime, or other substance with which tho nitric acid can combine at tho moment of its formation. The more porous the soil, and tho warmer the temperature, the more rapid will be the production of the nitric acid. Much of the potash and other mineral substances required by plants occur in the soil as constituents of nndecomposed minerals, and in a condition too insoluble to be attacked by the roots of plants. Water charged with carbonic acid, humic acids, carbonate of lime, sulphate of lime, salt, &c, giadually decompose these minerals, and hheiate (heir useful elements in forms of combination in which they are available to the phnU. Air, also, plays an important pait in the bringing about thepe changes. The long-continued removal of successive crops of wheat horn soil to which no manures fcave been applied ultimately results in a reduction of the producing power or fertility below the line of piofit. Exhaustion is said to have been brought about w.ien the co&t of cultivation is moie than the crops obtained are voith. This stage has been reached in, comparatively speaking, few years, over large areas in each of the colonies. Smeaton plains^ Barrabool Hills, and other parts of Victoria, and the older wheat growing districts i South Australia and JJew South Wales, were long ago impoverished to an extent that further attempts at their cultivation had to be given up. In many localities soils are met with, which, having been yielding for thirty or forty years good crops, and which even now are yielding fair crops, must, sooner or later, give evidence of requiring manures. Indeed, it may be stated, os a truth, that fertilisers aie essential # to the maintenance of the fertility of all soils ; and that without their aid maximum ciops are impossible on all but virgin land.