USE OF PHOSPHATES.

New Zealand Herald, Volume LXVIII, Issue 20767, 9 January 1931, Page 16

USE OF PHOSPHATES.

VARIATIONS IN TYPE.

FACTORS GOVERNING CHOICE. QUESTION OF AVAILABILITY. In New Zealand, as in most other lands, the chief defect of tho soil is a deficiency of phosphoric acid, so that in tho improvement of poor grassland or the maintenance of herbage already in good heart, phosphate is tho 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 timo to time introduced.

Conditions which govern tho choice of phosphalic fertilisers are dealt with by Mr. H. V. Garner, of tho Rothamstead Experimental Station, England, in tho November issue of tho British Journal of Agriculture. He says that tho points of difference between phosphatic manures fall under tho following heads: —(1) Availability or rapidity of action, in which superphosphate and basic slags of high citric solubility usually take tho load. (2) Provision of lime in quantities which, although small when judged by tho ordinary standards of liming, are nono tho less appreciable. Basic slags are tho best known in this respect, and on an equal basis the low-grado slags furnish moro lime than tho high-grade ones. (3) Unit cost, in which tho finely-ground mineral phosphates work out at the most favourable rates. Popularity ol Basic Slag. For general purposes basic slag maintains its place in tho esteem of English grassland farmers. The greater part of the home-produced article is of the same degree of solubility a3 the pre-war Bessemer slag which was used in the early experiments at, Cockle Park and elsewhere, although appreciably lower in phosphato content (9-16 instead of 18 per cent, phosphoric acid). When dressings are adjusted to compensate for the change in grade, tho sam'r results may bo oxpected from English high-soluble slags as from high-grade Bessemer slag, of which a relatively small amount is still imported from tho Continent.

Basic slags made with fluorspar, and showing a lower degree of citric solubility than tho abovo, 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 typo of low-soluble slag has been introduced to which Mr. Garner's findings may not necessarly apply, and this is at present being compared with highsoluble open-hearth slag at a number of centres in a new series of experiments. Mr. Garner concludes that for most typical grassland soils and districts basic slag will bo a safo choice as a phosphatic manure and that, where it is important to secure an effect in the first season, a typo which carries the guarantee of approximately 80 per cent, solubility in citric acid is to bo 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, there 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 tho advantage in speed c*f 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 largo 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 phospkoric 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 tho action begins to lie 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 fino degre 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, through tho 100-mesh sievo, 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 sievo having 120 meshes per linear inch. Application of Bone Phosphate.

Bono phosphate has a, considerable popularity for pasture improvement among farmers in certain English districts, as, for example, in Cheshire and Shropshire. At present steamed bone-flour, which is the most suitable form for rapid utilisation, is rather cheaper than usual and approaches superphosphate and highgrade slags in. unit price. A mixturo of Luperphosphate and steamed bono-flour is a popular dressing for pastures on calcareous soils.

in applying any form of phospliatic fertiliser, and in particular the less solubta forn\s, to grass land, tho condition of the surfaco is important. The manure must find access to tho soil beforo any benefit can be secured. Consequently, growth should be grazed down, coarse patches mowed back and any surfaco mat opened up by heavy harrowing. This surface preparation is in itself beneficial as it favours the development of tho finor herbage, and this is further stimulated by the manure applied.