Phosphate rock puzzles

Waimarino Bulletin, Volume 5, Issue 13, 25 August 1987, Page 11

Phosphate rock puzzles

A visiting American agronomist is puzzled at the interest being shown in New Zealand in the use of reactive phosphate rock fertiliser and sounded a warning concerning its use here. Dr Jack Stewart, Director of Agronomy with International Minerals and Chemical Corporation, Chicago, was in New Zealand to address a fertiliser seminar. In the United States the use of reactive phosphate rock has declined from a high of over 1 million tons per annum in the 1950's to virtually nil last' year, said Dr Stewart. "There is no way that, as an agronomist, I would recommend that New Zealand farmers use R.P.R." said Dr Stewart. He said that R.P.R., as received in New Zealand,

was not an effective fertiliser and that it needed to be processed to a finelyground state before application. The product would be almost impossible to ship, handle and apply in this state, he said. He was also concerned about the soil conditions required to release the phosphorous content of the rock. "When phosphate rock is placed directly into the soil, it must undergo acidulation to release phosphorous in a plant-available form. Suitable rocks can serve to supply phosphorous when placed in a sufficiently acid soil. Unless these conditions strictly apply, no rock, however reactive it is claimed to be, will ever work," said Dr Stewart. A further major impediment to the use of R.P.R. for direct application, according to a Dr Stewart, has been the inherent

problem associated with shipping, handling and applying a dusty, finelyground material. It is difficult to spread evenly, drifts readily and is disagreeable o handle. Agronomic performance demands that the rock be finely ground and mixed thoroughly with the soil. This was not being done with R.P.R. applicaions in New Zealand. Dr Stewart also commented on the considerable areas of New Zealand where the soil was deficient in sulphur. Use of superphosphate gave the soil a healthy dose of sulphur which was missing if reactive phosphate rock was applied. "I am aware that trials in New Zealand have produced results for R.P.R., but I known that the product is site-specific and that there can be considerable delays before the phosphorous is released into the soil," Dr Stewart concluded.