PHOSPHATIC MANURES.

Otago Witness, Volume 23, Issue 2684, 23 August 1905, Page 6

PHOSPHATIC MANURES.

Since phosphates are so extensively used, and are practically the basis of all artificial manuring, it is good that farmers should have a knowledge of them and their principles. In the main phosphatic manures supply phosphoric acid, sulphuric acid, and lime, all most essential elements of a good soil, and likewise are present in the best of soils in small proportions, as the following analysis shows : — Fertile soil — .47 per cent, phosphoric acid, .90 per cent, sulphuric acid, 4.9 per cent. lime. Rich soil — .24 per cent, phosphoric- acid, 0.9 per cent sulphuric acid, 7.30 per cent. lime. Xow, w heat, wheat straw, and milk make heavy draws on this limited supply (leaving alone that taken away also by ths bones of growing stock) to the following extent: 25 bushels wheat, 11.471b phosphoric acid, o.Blb sulphuric acid, .851b lime; 30001b straw — 8.151b phosphoric acid, 5.821b sulphuric acid, 9.341b lime; 750 gallons milk — 251b phosphoric acid. A 25-bushel crop takes 201b of phosphoric acid off the farm -when the straw is sold off, and cattle considerably deplenish the already limited supply. It is, therefore, absolutely essential that the phbaphoric acid at ■least must be returned in some form or other. Long ago it was found that bones would do this part. Bones contain 55 per cent, of phosphate of lime in the form of trioalcic phosphate not soluble in water. The three parts of hme which accompany the phosphoric acid have the effect of making this form of phosphate insoluble. The action of water and the carbonic acid of the air brings about a chemical change. Bicalcic phosphate, which is produced *by the natural decomposition of bones, is slowly soluble, and acts well. It acts quicker in proportion to the degree of pulverisation before application to the soil, but even then is not quick enough for farmers who want immediate effects. Liebig brought about the system of applying sulphuric acid to pj&verised bones, and accomplished in a fewTiours what Nature took a long time to do. Mons-calcic phosphate (commonly called superphosphate) is very soluble. This manure became so much in demand that bones would have gone up to a prohibitive price ; but the late Sir J. B. Lawes discovered that superphosphate could be nianuiactured iruis

mineral deposits of phosphates. It was early discovered that if superphosphates were held over a season they went back several units in their percentage of solubility. They were then known as ■Reduced" superphosphates, and in this foini Y5 ere actually preferred by many pi-acticaL f aimers in Great Britain who tried them. But superphosphates w-ere sold on their percentage of solubility, and British farmers are not willing to pay on the basis of a pe*^enlage that once existed, and. manufacturers were also unwilling to sell ?on the basis of the "reduced" percentage 'and lose the value of the insoluble phosphate which it contained. The "battle of the gauges"' was waged, and now manufactureis find it cheaper to use a few extra, gallons of sulphuric per ton to ensure the percentage being maintained, and wise farmers "reduce" their own on some plan, as mixing one ton of crushed bones with two tons super. ; let them heat together in a heap. In five or six weeks the mixture is in good condition. The solubility of the bones has been increased, and that of the super, decreased or "reduced." One farmer obtained as good results from 27s 6d expended this way as with 40s worth of super, with 10 loads farmyard manure added. The explanation may be given that phosphate as sold in. the form of "super." doss not exist naturally in the soil, and it may, therefore, be called an "unnatural"' product. " Keducing" tempers it, and the farmer sees the action of "an old friend" in its work.

Many practical iarmers hold that in application superphosphate should be "well scattered, as confining it in drills m^ans that the roots will practically likewise be confined in the same groove, which is most undesirable in many. ways. Superphosphates are sometimes too sudden, especially in dry districts. When the moisture is present they iorce a big growth, but it is a weak growth, because forced. When a dry spell sets in the growth is not strong enough to stand its altered circumstances, and is consequently very liable to mildew, rust, or any other fungus disease which may be present in the air. Sulphate of lime, or gypsum, the by-product in the manufacture of super., is an excellent manure in itself, especially for wheat. It lorms about 56 per cent, of the bulk of the superphosphate of commerce. It attracts moisture from the air, and is likewise a direct plant food. Seaweed ash, or seaweed ploughed in from a compost h*^ap, or seaweed applied in any form, forms an -excellent allround manure, and for phosphates especially. It may be said to be the only; manure used in the island of Jersey.