Creating a soil texture map as the basis for variable rate fertiliser application currently involves some complex technology. But a Fife grower believes it can be done a lot more simply using little more than a tractor and cultivator. Peter Hill reports

It was something of a eureka moment when John MacNiven compared the soil texture map he had created with one provided by a professional soil surveying service.

“I was quite amazed they were remarkably similar – and it was only my first attempt,” he says.

One of the maps was created using a device that measures soil conductivity and is widely used for soil texture mapping. Mr MacNiven created his using a tractor and subsoiler, taking signals from one of the draft sensing pins in the three-point linkage to map variations in soil mechanical resistance.

“The idea came to me after I’d had a dubious result from the soil conductivity mapping system,” he explains. “It didn’t tally with my own experience of our fields and I felt the zoning technique for identifying areas of similar soil type was not accurate enough for either soil sampling or for creating a fertiliser application map.”

Modifications

A call to electronics firm RDS Technology resulted in modifications to the farm combine’s grain yield meter so that it would record the difference in voltage between the electrical current delivered to and returning from one of the draft sensing pins as it flexed with changing load.

“I then transferred the data to Patchwork mapping software on my computer, used the ‘smoothing’ function to average out the results and produced a contour map using the same colour shades as on the one supplied to me,” Mr MacNiven explains. “The ‘smoothing’ function is the key because it disregards non-representative figures, such as when the subsoiler leg hits a stone. I can use the same data to create an application map – there’s no need for a zoning process.”

Selective lime and fertiliser use has been practised over the past three years after soil testing using Mr MacNiven’s map to locate representative sampling points revealed some imbalances in nutrient status across the 100ha (250 acres) on which he grows cereals at South Lambieletham, near St Andrews, Fife.

“I’d been aware that our yields were declining and that in some areas crops were dying off earlier than they should,” he explains.

“The tests showed the light land deficient in lime, with a pH of 5.9 compared with 6.5 on the heavy land, low potash levels across the farm and phosphate levels that were through the roof on the light to medium soils, and possibly interfering with micro-nutrient availability.”

Compounds

Having previously used compounds across the farm, the approach for the past three years has been to apply lime and straight fertilisers in appropriate quantities where they are needed.

“I’m using 10-15t less fertiliser a year but I think the real benefit is in the increased yield and grain quality we’re now seeing,” says Mr MacNiven.

The delivery of a new tractor with an all-singing, all-dancing ISOBUS-compatible electronics system further excited his belief that many growers already have much of the hardware needed to take charge of their own soil mapping.

“With the addition of a GPS receiver, the tractor can map various things, including fuel consumption,” he notes. “But while the load on the draft sensing pins is displayed, it’s not mapped.

“I’ve asked Massey Ferguson to see if this can be done so that when subsoiling, deep ridging or carrying out any other soil-engaging task, we can be making a map for precision farming.”

Soil mapping

• Mapping soil texture within fields provides basis for prescription fertiliser treatments
  
• Targeted dressings replace hit-and-miss blanket treatments
  
• Potential for tractor draft control sensing to map soil mechanical resistance
  
• Data could also be used to create lime and/or nutrient application maps

If you want to find out more about GPS and precision farming technology in general, come to the Precision Farming Event on 5 March at the East of England Showground.