26 June 1998

Poor sampling can cause imprecision

MAKE sure you use all the cheap and easily accessible information about your land before embarking on expensive soil sampling, is ADASs David Parishs advice to growers contemplating precision farming.

Field history, local knowledge, aerial photos and soil type information are all valuable guides to input needs, he explains. "There is usually quite a lot of data already available. It is possible that your farm may already have been mapped by the Soil Survey."

Amidst all the publicity surrounding precision farming, growers must ensure that any changes they make to their systems are cost-effective, he stresses.

A detailed ADAS survey of 78 fields found big differences in the phosphorus, potash and magnesium content of the soils. 47% of the samples for phosphorus fell outside the mean P index, he notes.

"There is clearly significant variation in the field. But to capture it you need to sample at 35-40m intervals and take at least nine sub-samples at each point." Most commonly used soil testing services are based on one sample a hectare and cost about £15/ha (£6/acre), he notes. "Should we really be using this sort of data on which to base our fertiliser inputs?"

Even using standard figures, such as the 9.3kg/t crop offtake of potash, can be misleading, he suggests. In practice ADAS has found the figure ranges from 5 to 16kg/t.

One way forward, he suggests, is to concentrate on point samples. Once identified such spots can be resampled year by year to pick up trends in nutrient status over time. &#42

Beware of imprecision caused by inadequate sampling for nutrients, warns David Parish.