ACID PHOSPHATE v. GROUNDROCK PHOSPHATE.

Otago Witness, Issue 3398, 30 April 1919, Page 8

ACID PHOSPHATE v. GROUNDROCK PHOSPHATE.

The material, furnishing phosphate, and used for agricultural purposes is known by a number of 'names—raw rock phosphate, phosphatic rock, and floats. These names apply to a finely-ground rock containing from 10 to 14 per cent, of phosphorus. There are the terms acid phosphate and phosphoric acid, which apply to the same rock, but to which has teen ..mixed an equal volume of sulphuric acid. sThe phosphorus in the phosphoric acid is fin an available form for plants. The phosphorus in the Taw rock phosphate is

not in an available form. However, in America it is a common practice to make free use of the raw rock phosphate by applying it when finely ground direct to the land, and no doubt we would do similarly if more phosphate fields were discovered in New Zealand. As it is, the rock at Clarendon is for the most .part treated with sulphuric acid ere being offered to farmers. The method which is adopted in the States will appeal to farmers who declare that superphosphate sours the land. The raw phosphate is applied to the land by mixing it with the farmyard manure or by ploughing it under, with some green-'crop. The decomposition of this organic matter —that is, the manure or the green crop, forms organic acid which liberates the phosphorus in the raw rock phosphate. It is a common practice to mix 501 b to 701 b with each load of farm manure, or some 21b or 31b of this dry material may be sprinkled in the gutters behind each cow after the byres are cleaned. This helps to dry and sweeten the gutter, and is a good way of mixing thoroughly this ground raw rock phosphate with the manure. Of course, it takes time before the phosphorus becomes available to the plants, as the manure or green crop has first to decompose. It would seem to be a safe rule never to "apply raw rock phosphate to land unless the soil is rich in active organic - matter. Small benefit would be apparent the first year in any case. The following paragraphs are taken from a leaflet recently published by the Ohio Experiment Station, and are of interest: "Acid phosphate in field tests conducted for nearly 20 years by the Ohio Experiment Station has proved to be not only a more effective but also a cheaper carrier of phosphorus than raw phosphate rock for crops under conditions which render freight charges a relatively large part of the cost of the fertiliser. These tests have included experiments with these two fertilising materials by themselves, and in connection with other fertilisers, with lime and with barnyard manure. Acid phosphate is made by treating raw phosphate rock, or floats, with an equal weight of sulphuric acid. This orocess reduces the percentage of phosphorus, but increases its solubility. The acid phosphate therefore costs more, and freight on it is higher also. Still, the tests at the experiment station show that, while raw phosphate may be used with profit on land deficient in available phosphorus, acid phosphate has produced increased crop yields more than sufficient to meet the larger cost." Hoard's Dairyman, in further comment, says:—"lt would be unwise to apply raw rock phosphate to land lacking in organic matter —that is, roots, manure, or some green crop. We have used raw rock phosphate on Hoard's Dairyman Farm for a good many years, and have reason to believe that we are getting good results. We make it a point to either apply it with manure, or to plough it under with the fourth crop of green alfalfa. When we apply it with manure, we use about 5001 b per acre, mixing from 501 b to 601 b of it with each load of manure and applying eight to 10 loads of manure per acre. When we plough it under with green alfalfa we use about 10001 b per acre. We do not think there would be any advantage in using more rock phosphate than there is organic matter to take care of it. It should be borne in mind that through the decomposition of organic matter, acids are formed which act upon the raw rock phosphate, and liberate the phosphorus. The difference between raw rock phosphate and acid phosphate is, the former contains phosphorus unavailable to the plant until acted upon bv the acids of the soil, and the latter may be taken up by the plants without anv action from the acids of the soil." In brief, phosphoric acid (acid phosphate) is made by mixing one ton of raw rock phosphate with one ton of sulphuric acid. The sulphuric acid acts upon the raw rock phosphate, and forms what is called a phosphoric acid fertiliser; in other words, makes the phosphorus available to the plants. Farmers are advised to conduct some experiments on the lines of the foregoing. Some of our well-stocked humus soils might be improved out of all recognition if treated as suggested.