FOOD FOR THE SOIL

Timaru Herald, Volume CXXXIV, Issue 18796, 7 February 1931, Page 11

FOOD FOR THE SOIL

USE OF PHOSPHATES. FACTORS GOVERNING CHOICE. In New Zealand, as in most other lands, the chief defect of the soil is a deficiency of phosphoric acid, so that in the improvement of poor grassland or the maintenance of herbage already In good heart, phosphate is the main requirement. The choice of the most suitable type, however, is an important consideration, depending as it does upon variations of soil and climate and upon new forms of fertiliser and modifications which are from time to time introduced. Conditions which govern the choice of phosphate fertilisers are dealt with by Mr H. V. Garner, of the Rothamsted Experimental Station, England, in the November issue of the British Journal of Agriculture. He says that the points of difference between phosphatic manures fall under the following heads:—(l) Availability or rapidity of action, in which superphosphate and basic slags of high citric solubility usually take the lead. (2) Provision of lime in quantities which, although small when judged by the ordinary standards of liming, are none the less appreciable. Basic slags are the best known in this respect, and on an equal basis the low-grade slags furnish more lime than the high-grade ones. (3) Unit cost, in which the finelyground mineral phosphates work out at the most favourable rates. Popularity of Basic Slag. For general purposes basic slag maintains its place in the esteem of English grassland farmers. The greater part of the home-produced article is of the same degree of solubility as the prewar Bessemer slag which was used in the early experiments at Cockle Park and elsewhere, although appreciably lower in phosphate content( 9-16 instead of 18 per cent, phosphoric acid). When dressings are adjusted to compensate for the change in grade, the same results may be expected from English high-soluble slags as from high-grade Bessemer slag, of which a relatively small amount is still imported from the Continent.

Basic slags made with fluorspor, and showing a lower degree of citric solubility than the above, have been tested against the other types at many centres and have, on the whole, been slower to act, although in most cases a pronounced benefit has followed their use. More recently a type of low-soluble slag has been introduced to which Mr Garners findings may not necessarily apply, and this is at present being compared with high-soluble openheart lag at a number of centres in a new series of experiments. Mr Gamer concludes that for most typical grassland soils and district basic slag will be a safe choice as a phosphatic manure and that, where it is important to secure an effect in the first season, a type which carries the guarantee of approximately 80 per cent, solubility in citric acid is to be preferred. Advantage in Speed of Action. Superphosphate has in the past been somewhat overshadowed by basic slag as a source of phosphate for grassland Improvement, and where the latter is known to act rapidly and well, it is suggested , tliere is no reason to employ the somewhat more expensive water-soluble phosphate. Nevertheless, in dry situations and on calcareous soils superphosphate is quite in place, and its value is well known to farmers in such regions. Further, it has the advantage in speed of action, and if application has been delayed until early spring it has been observed to affect the following hay crop rather more certainly than basic slags. With

a highly soluble manure like superphosphate there is something to be said for the practice of giving little and often rather than a large initial application to last for a period of years. Evidence is still accumulating with regard to the conditions which favour the use of ground rock phosphate and its performance in comparison with other sources of phosphoric acid. The opinion is that on the whole the effect of ground rock phosphate is slower to appear than that of high-soluble basic slags. This is particularly so on the drier soils and in districts of low rainfall. Many observers report that the action begins to be marked in the year following application rather than in the first season. Moist conditions, a pronounced need for phosphate, a long growing season and a fine degree of grinding all seem to be helpful to its action. Very fine grindings of North African phosphate are now available. A common grade is the same as that of basic slag, i.e., 80 per cent, though the 100-mesh sieve, while 85 per cent, through the standard sieve is readily obtainable. A further grinding furnishes a material at least 90 per cent, of which passes the 100-mesh sieve, while in some cases 90 per cent, is guaranteed to pass a sieve having 120 meshes per linal inch. Value of Bone Phosphate. Bone phosphate has a considerable popularity for pasture improvement among farmers in certain English districts, as, for example, in Cheshire and Shropshire. At present steamed boneflour is a popular dressing for pastures on calcareous soils. In applying any form of phosphatic fertiliser, and in particular the less soluble forms, to grass land, the condition of the surface is important. The manure must find access to the soil before any benefit can be secured. Consequently, growth should be grazed down, coarse patches mowed back and any surface mat opened up by heavy harrowing. This surface preparation is in itself beneficial as it favours the development of the finer herbage, and this is further stimulated by the manure applied.