PRECISION FARMING, fine in principle, can become too complicated and expensive for everyday application. That is the thinking behind a new service from the Wilts-based Courtyard Partnership.

Detailed soil mapping and analyses are of little value unless they are financially rewarding, says the firm”s Mark Gillingham. But there is a potential benefit in trying to pinpoint areas restricting output or where savings can be made, he adds.

“At one level it can be very simple – more soil usually equals bigger crops. That”s blindingly obvious, but from that we get the concept of limiting factors.

“With phosphate and potash things become a bit more subtle because the effects of varying applications of them are much less than with nitrogen and certain soils tend to lock them up.”

Close grid soil sampling can identify such soils, but is unnecessarily expensive, he believes.

“So we have developed a hybrid system. Instead of dealing with hundreds of different soil types, it is based on relatively few, depending on their structure, texture, stone content, lime status and depth.”

Each affects the way P and K are stored and made available to crops, he explains. “Looking at maps we began to realise most fields can be broken down into reasonably sized chunks. We have found for most farms the average is about 15 acres.”

The key to the new system is pinpointing boundaries outlining “virtual” fields with different soil types. “We sit down with farmers to look at geological data and yield maps if available. We ask them to tell us where they think their soils vary and we draw up a draft map on that basis.”

The differences are confirmed by zig-zag GPS-logged soil sampling across the perceived boundaries. “We then do a soil analysis with respect to each new virtual” field.”

The results allow growers to adjust fertiliser inputs to achieve the desired P and K indices without resorting to sophisticated spreading equipment. “We wanted a system farmers could apply using machines they already had.”

Adjustments are usually in the order of 50kg/ha (40 units/acre). “We”re not using technology to vary applications by only 5-10kg/ha. With P and K, it”s not that critical.

“For farmers without GPS or electronically controlled variable spreaders we then find the best area of the field and fertilise to that zone as appropriate. We then top up the weak areas every second, third or fourth year depending on the deficiency. It”s a bit like using lime to adjust pH.”

The mapping needs doing only once, says Mr Gillingham. “It”s a one-off. After that, you have your virtual” field boundaries. “They are also management zones, not just for basic fertilising, but for varying seed rates for establishment and nitrogen timings. For example, on colder soils you may want to advance the first N dressing.

” The cost depends on whether the farm already has a mapping programme or access to digital maps, he says. “But setting up the management zones should be about 2.50/ha based on a 400ha farm.” Thereafter, triennial soil sampling and annual fertiliser recommendations cost about 2.80/ha.