14 April 2000

Acidity is the prime target of GPS work

LIME is an ideal candidate for precision application, especially on Scotlands generally acidic but variable soils, says CSC agronomist John Hughes.

Compared with uniform liming based on whole field analysis, the bottom line benefits of a GPS guided system can be substantial and easily outweigh the extra cost, he maintains.

Signs are that Scottish and Northern growers are beginning to appreciate that message.

Up to May last year CSC had carried out GPS mapping on 6000ha (15,000 acres). The year before the figure was only 4300ha (10,500 acres).

Grid samples

The variations in pH across a field do not have to be large for the GPS benefits to kick in, maintains Mr Hughes. Using the example of a 10ha (25 acre) field with 100m grid samples ranging from 5.5 to 5.8, growers applying lime at a uniform 4t/ha (1.6t/acre) could expect an annual 15.5% yield loss in spring barley over four years, he calculates.

Depending on variety and price that could be worth £54-74/ha (£22-30/acre) a year. The annual cost of GPS mapping and the premium for variable rate spreading over four years is only £5.50/ha (£2.20/acre).

"Its small by comparison," says Mr Hughes.

"We are finding about one field in five might benefit from variable application of phosphate and potash. But for lime I reckon the figure is nearer 70%. If you are thinking of precision farming think lime first." &#42