GREEN MANURING.

Auckland Star, Volume XLIV, Issue 180, 30 July 1913, Page 8

GREEN MANURING.

AS APPLIED TO NEWT ZEALAND CONDITIONS. THE NECESSITY FOR HUMUS. (By W.R.) How few farmers there are who realise the great value of green manuring, and the prominent part it plays in successful farming. Ploughing in green crops not only returns to the land. in a "soluble"' form. tbe inanurial constituent the crops take up from the soil, but it forms humus, which in itself is as valuable in its action on the soil as the soluble plant food it contains is to the crop.

There ar? many crops which may be turned under for greeu manure. Foremost among them in a country where the rainfall is copious, say, 35in. and upward, are the legumee. Vetches are, perhaps, the chief of these in clay soils, or soils which could not be classed as sandy. Clovers, such as red or crimson clover, are useful i:i almost any soil except, possibly. light, sandy soils which appear to derive the most benefit from the ploughing in of lupine. On the barren, sandy heath lands in Ea?t Prussia lupins manured with basic slag and potash salts (probably kainit or SO per cent pota«*sic manure) were repeatedly ploughed in. nith the result that a useful soil was built up by this method.

On soils where the rainfall is light. say. under 30in.. mustard and oats are. perhaps, the best for the purpose, as they are quirk growers, and do not take as much moisture from the land as vetches, which are apt to leave the ground too dry for the following crop, unless there is a good rainfall. The value of a leguminous prrern crop is greatly enhanced by the amount of nitrogen which these plants absorb from the atmosphere by means of the bacteria which find their lodgment in the nodules on their roots, and which is returned to the soil in the tissues of the plant.-; when ploughed in. Nitrogen is returned to the ,-oil by any green crop, but it is generally accepted that it is found in greater quantities in leguminous than in other plants; hene'e the greater value of the former for manurial purposes where the conditions of moisture are favourable. Transpiration (or the giving off of moisture through tbe leaves) is greater in the case of vetches than in clovers; the latter, therefore, will not leave the soil quite so dry for the following crop. A very high value is justly placed on farmyard manure, but as this is not obtainable in any quantity in this country, a very good substitute may be found in green crops, ploughed in, and supplemented with artificial fertilisers. "W hen sowing the crop to be ploughed in. it is advisable to use manures to stimulate the crop into luxuriant growth; there is no fear of their being lost by ploughing under the resultant crop, as what tbe plants have taken up iis returned to the land in a quickly soluble form, and which in turn is used by the succeeding crop. HUMVS AND ITS VAL.LE. Not the least valuable of the effects of farmyard manure is the amount of humus it adds to the soil; the same may also be said of the effects of green manuring. Indeed, it may be claimed that the principal reason "for this form of manuring is the fact that it adds so largely to the store of humus. A. D. Hall says of green manuring that the practice has three objects: — (1) The improvement of the texture of the land by increasing the store of humus; this is particularly valuable on heavy clays and on the light, sandy soils at the other end of the scale." (2) The raving of the store of nitrates, which, on light, warm soils, form with great rapidity after harvest, and which may then easily be washed away. If some catch crop, like mustard, is sown immediately, the stubbles are clear, it will grow with great rapidity after the first rain, and wiil rjather up these nitrates, converting them into proteins, which become more slowly available on the decay of the plant material. (3) For cleansing purposes. When the land is in foul condition, a good many weeds can be got rid of by growing a smothering crop.

It will be noticed that Uall places first the increase of the store of humus. He stresses the effect it has on heavy claye. and also on light, sandy soils, the effect on the former being to disintegrate it, making it more frep and open, letting in the oxygen of the air. raising its temperature and increasing its moisture-holding capacity. On light soils the humus derived from green manuring compacts it. making it firmer, and enabling it to retain far more moisture than it otherwise would. Humus darkens the colour of the soil, thus raising its temperature: for it has been proved by experiment that even a light dusting of soot, so as to darken the surface of the soil, has raised the temperature two degrees above the soil alongside, which had a very light, dusting of lime, enough to give a light colour to the surface. If such was the effect from surface colouring, what may bo expected wiien the soil is dark all through?

1 know an instance of the effect ol humus so ameliorating the texture of a .stiff, cold and in tract a/b!e soil, that, after three crops of vetches were turned under, the land became as friable as old garden soil. It will be readily seen how amenable even a very unpromising, soil may become by the action of humus as a result of green manuring. Deplete the store of humus in any soil, and you immediately enormously reduce its fertility and its power to retain moisture. The capacity for holding moisture is of the utmost importance in successful farming, for it is hardly necessary to tell farmers that all plant food reaches the plant through the medium of water, in which it must first be dissolved before the plant can assimilate it. A soil, therefore, that has the property of storing moisture in abundance will yield the maximum return of crop. At chc other end of the scale is a soil which has a superabundance of water and is sodden: such a soil is far more detrimental to a crop than that which does not retain a sufficiency for a good average crop, for the latter will yield just in proportion to the amount of water it will hold, while the former will only carry vegetation that in the majority of cases is of little value. In a water-logged soil there can be no ingress of oxygen, nor is there

any in the case of a stiff clay soil; the supplying of humus through the medium of green manuring will remedy the latter by opening it up, while thorough draining stall £h» SvQ in £hs. former.

I_have stated that all plant food is conveyed to the crop by soil water, and that humus, supplied by green manuring, greatly increases the water storage capacity of the land. All water is stored in films, which coat tbe soil particles; anything, therefore, which opens up the compacted clay by separating the closelyadhering particles increases the surface area to which the water film adheres, and so gives more moisture for the crop to draw upon. There are two agents which have this effect, namely, humus and lime, but as I am treating of the former only. I need not refer to the effect of lime. Of course, it is possible to have an excess of humus, as in the case of peat swamps, or land to which farmyard manure has been applied in such excessive quantities that it finally becomes so sour that a heavy application of lime becomes necessary. That, how ever, is not likely to be the case in this country. Excessive cropping, without green manuring or other application of organic matter, will in time deplete the store of humus in the soil, thus lessening its water-holding capacity, and. as a consequence, decreasing the crop, for however rich the soil may be. or however much the application of artificial fertilisers may be. the return in crop will be in direct ratio to the amount of water stored in the land. Say that a given amount of water is required for a fortybushel crop, and the soil holda that amount, and no more, all the artificials one may apply will not increase the yield, because more moisture than the land holds is required. Many a man finds his returns steadily decreasing, notwithstanding the fact that be increases the application of fertilisers; he little knows that by constant cropping he has se> exhausted the supply of humus that the capacity for holding moisture is lessened, and so tbe return of crop diminishes each successive year. Before an increase ran be secured the store of moisture must be increased, and this can only be done by restoring tbe depleted humus: then the artificial fertilisers will help to increase the yield, for it is manifest that if there is only sufficient moisture for a 30 or 40-bus'nel crop that quantity must be increased if a bipper yield is to be secured. These remarks apply more particularly to land that is frequently broken up for cropping; hut even when old pasture is to be ploughed for turnips or any other crop, if the grass is allowed to grow to a considerable length before ploughing it will add materially to the yield of the succeeding crop, whatever it may be. THE TIME TO SOW. As a general rule, the best time to sow for a green crop to plough in is the autumn, but under certain circumstances a summer seeding is necessary, as. for instance, if winter wheat is to be sown the green crop must, be ploughed in sufficiently early to allow of its being weil decayed before sowing the wheat. For spring crops, autumn sowing will be the most suitable: the crop -will be fit for ploughing in about June or .July; by August, or early in September, this will have sufficiently decayed to admit of the crop being put in. As nitrification, or the conversion of the. organic nitrogen into the nitrate form, sets in rapidly when the ground gets warm, the main crop should be sufficiently advanced in growth Lo absorb the nitrates before they are washed away by heavy rains. Autumn-sown crops serve another purpose than for turning under; they afford a cover lo the surface of the soil, and prevent the heavy rains of winter from beating it hard; such a cover crop also helps to keep down weeds, and also keeps the ground porous by the action of its roots. QUANTITIES TO SOW. If vetches are to be sown, a bushel, with the addition of one to one and ahalf bushels of oats, barley, or rye corn (as the latter seed is small, a bushel should be sufficient!, will be a fair seeding per acre. If oats alone are to be sown. 2J bushels, preferably Algerian, will be sufficient, but if the mixture is to consist of oals and mustard, li bushels of oats and S or 101b of mustard will be a fair seeding.

If the farmer elects to sow clover for turning under, crimson clover at the rate of about 8 or 101b per acre will be a liberal quantity to sow. This should be sown about March, and should be ploughed in just as it is showing bloom. This applies more particularly where green manuring is desired for an orchard. If such is the case, the clover may be ploughed in early in spring, and as the warm weather advances decay and nitrification proceed rapidly, and the active tree roots take up the nitrates as fast as they are formed. In addition to. or in place of, mustard, rape may be sown at the ratt. of 2 or .lib; this is also a very good crop to plough in. COST OF SEEDS. Tares and vetches, from 10/6 to 12/6 per bushel: oats. 3/9 to 4/; barley, about ihe same as oats: mustard. 5d lb: rape varies from 3d to as high as (id. according to supply and demand; crimson clover. 0d; rye corn, about 4/3 per bushel. Tbe quickest known grower of any grass or other crop suitable for ploughing in is probably Western Wolths: but as it is so valuable a fodder plant it seems a pity to use it for that purpose, especially as other but slower growing crops, costing far less, will answer the purpose. But if rapid growth is desired, Wolths is much the quickest of all—price, lOd lb; quantity, 201b per acre.