27 February 1998

Examine yield variation before N change

VARIABLE nitrogen application is on its way. But farmers shouldnt rush out and change fertiliser strategies based on yield maps alone.

That is the view of Chris Dawson, an independent precision farming consultant and a founder member of the Shuttleworth Precision Farming Alliance.

Before any judgement to vary N input is made, growers need to look at the underlying cause for variation in yields, assess the likely efficiency of nutrient uptake and determine plant nitrogen status, he advises. "The principal objective of varying fertiliser application is to get a better return for each unit of nitrogen."

Yield variations within a cereal field may be as much as 4-5t/ha (1.6-2t/acre), but it cant be assumed that lower yielding areas will automatically benefit from more fertiliser, he says.

"Simply relating N application to yield presupposes equal efficiency in nutrient recovery, which isnt necessarily the case.

"Variation in soil type, structure and moisture dictates the ability for a plant to utilise nitrogen. A low yielding area of crop on soil with a poor structure is unlikely to respond to extra nitrogen, because it is already receiving more N a unit of output than a high yielding area."

Numerous farmers involved in the Shuttleworth Precision Farming Alliance have linked yield reductions to areas of damaged soil structure. Many now feel the most useful lesson has been to minimise damage to soil structure in order to maintain efficient nutrient uptake, explains Mr Dawson.

"A sugar beet field at Shuttleworth consistently suffered yield reduction in 1997 in areas corresponding to the previous years cereal tramlines. This was despite ploughing and other preparatory cultivations."

Assess crop potential

But any opportunity to vary final fertiliser application is likely to be associated with assessing crop potential and N status, for which an automatic system of recording is needed, suggests Mr Dawson.

"Technology indicates that within the next five years satellites should have the ability to capture details of nitrogen status and crop growth," he explains.

"Once these data can be generated and captured easily, and a robust interpretation of the information developed, farmers and agronomists will have the detail they require to make decisions about fertiliser adjustment."

But Mr Dawson believes such techniques will demand a rethink on farm operations in general. "The greater the precision required, the more critical accurate application becomes. Spreaders are already available to variably apply fertiliser, but work carried out at Silsoe suggests that product quality alone can result in considerable variation of application." &#42

PRECISION POINTERS

&#8226 Consider underlying cause for low yields.

&#8226 Assess efficiency of nitrogen uptake.

&#8226 Minimise damage to soil structure.

&#8226 Use quality fertiliser for accurate placement.