CHEMICAL MANURES.

Auckland Star, Volume LXI, Issue 66, 19 March 1930, Page 23

CHEMICAL MANURES.

COMMERCIAL FERTILISERS. POTASH, SALT, LIME AND /I GYPSUM. (By "SUNDOWNER.") A review of the so-called chemical or commercial fertilisers would not be complete without a discussion of potasn manures, to which for the sake of convenience I will add in this article a discussion of various substances which, while not actually manures,. haA*e the property of changing the chemical relationships of ''certain plant foods., s converting them from an unavailable to an available form. The chief of these substances are lime, , saltpetre, and gypsum.

The Effects of Potash. Potash is used largely by all plants in producing the such starch and sugar. Further, it has a verv vital effect- on • the ability of the plant to absorb nourishment from the sunlight and air, though how this process is effected by potash is not yet scientifically clear. Gertain it is, however, that where potash is entirely absent in the soil the life of the plant comes to an abrupt end. Where potash, though not totally absent, is deficient, the crops and pasture grass are poor, dull green in colour, with a tendency to wither at the tops of the leaves as though they had been frosted. Clovers and other legumes require considerable potash and it is probable that the improvement in the growth and prominence of these plants following liming is in part due to the fact that lime liberates a certain amount of potash which previously was insoluble in the soil. Commercial Forma of Potash. Most farmers and gardeners ale acquainted with carbonate of potash, the form in which potash occurs wood ashes, but from a farmers's point of view he is interested in oxide of potassium, in which form he purchases it in the commercial potash manures. Vast Stores of Potash. According to King, a reliable authority, average sandy soils contain from 2 tons per acre up, while clay soils contain 8 tons or more of potash per acre. Although apparently abundant, unfortunately this is largely in insoluble forms, but in so far as New Zealand is concerned our soil as yet contains sufficient potash for most crops, the only crops which might require extra supplies being potatoes, mangels, turnips, beet and similar root crops which produce a large amount of sugar and starch. Kainit, a product of the Strassfurt deposits in Germany, is a form in-winch potash, when necessary, can be applied in New Zealand.

As a result of forest and other growth during thousands of years past, and also in mineral form, potash is fairly abundant in most of our soils, but much of this supply, as previously stated, is unavailable,'unless it is acted upon by the various "amendments" previously referred to. As potash manures are expensive, this method of increasing the available supply—i.e., by means of amendments —where necessary is by far the cheapest, and it will therefore be of most practical interest to the farmer if we proceed to discuss these. "Amendments." Lime, in its native form limestone, is fairly well known to all .New Zealand farmers, and it is very often assumed that/ because outcrops of limestone rock occur on country, the soil of that locality must necessarily contain abundant lime. This has been proved to be an entirely erroneous conclusion, because carbonate tof lime is so readily dissolved by the carbon dioxide contained in soil water, that even in soil lying on a limestone subsoil'it is quite probable that the lime has been leached therefrom to a level below the reach of grass plant roots'; Testing for Lime Requirement. " A sure test for acidity, or lack of lime, which any farmer can make, is with litmus paper, which can be purchased cheaply from most chemists in little booklets. Open the soil sufficiently with the blade of a pocket knife to insert a small strip of litmus paper, then press the soil close about the paper. Repeat this process at various points about the area, to be tested, marking each paper with a stick. In an hour's time carefully take out the papers and note the shade of colour. If the papers still remain blue the soil is alkaline or neutral and does not require lime, while the various shades varying from pale pink to red will indicate the amount of acid in the soil at the various points sampled, and give some indication of the amount of lime required to correct the condition. Various Forms of Lime.

Lime can be applied in three forms, i.e., ground limestone, which is slow acting, but safe to apply ,to any type of soil, and especially desirable for light, sandy, gravelly, or pumaceous soils lacking in humus; quicklime which is produced by burning limestone and expelling the carbon dioxide gas which it contains j and slaked lime, which is quicklime, which, for agricultural purposes, has been slaked. by being exposed to moist air, rain, and dew, \yhen it again takes up. a considerable, amount of the carbon dioxide expelled by. burning. ' '■ i ;.;

Quicklime and slaked lime are caustic, and hence objectionable to handle, but are much quicker in their action than is ground limestone. On. heavy clay lands and on peaty, and similar _soiis containing a large amount of hunuis, quicklime and slaked lime will have more rapid effects than limestone, and will hot damage the,soil, but rather considerably improve it." '

Lime Liberates Plant Food.

Besides correcting acidity in the soil, lime acts on the inert humus, liberating the nitrogen from this, and also on the insoluble mineral potash in the soil, making this available to the plants. Further, it acts on clay soils by making them more friable and easily penetrated by water, air and plant roots. It also produces conditions favourable to the multiplication of soil bacteriaj for these cannot thrive in an acid soil, and through the work of the soil-bacteria a vast amount of plant food is liberated from otherwise insoluble matter.

Because of its effect in rapidly making humus soluble, quicklime applied to light porous soils, deficient in humus, will stimulate immediate gro>vth, but will ultimately impoverish these. In effect, 561b of quicklime is equal to 741b of slaked lime, or 1001b qf limestone, hence, apart from the cost of haulage, limestone i 3 cheapest (and safest), unless it costs more than half as much as quicklime, an unusual occurrence. The writer has always thought that much more use might be made of the outcrops of lime which occur-throughout the North Island by farmers combining to erect suitable co-operative crushing plants at centrally-situated outcrops. Salt, Saltpetre and Gypsum. Salt, saltpetre and gypsum, are "amendments" which act on the potash salts in the soil, making them more rapidly available. Salt and saltpetre have been largely used in the past on almost all crops, including grass, and where potash is lacking in an available form, have resulted in greatly-increased crops. •• By the application of acwt of salt per acre test crops of mangels have been increased from 27 to 35 tons per acre, while the results with wheat and other cereals have been equally marked. Pasture top-dressied with from 5 to lOcwt of salt has increased from 50 to 100 per cent in carrying capacity, while the stock have been much more healthy and free, from parasitic infection. It must be remembered, however, that heavy applications of salt or saltpetre cannot be continued without ultimately seriously damaging the fertility of the soil. At infrequent intervals and in moderate applications, salt is a wonderful growth stimulator.

Gypsum (calcium sulphate) may also be used as a potash liberator. Crops, such as turnips, mustard, rape, cabbages, etc., which benefit from a supply of sulphur, will secure some of this requirement from gypsum, but its main direction in which it stimulates growth is in making available the potash reserves in the soil.