20 March 1998

Tissue S test needs a cautious touch

TREAT this years tissue testing results with caution if you are being urged to apply sulphur in response to a low reading.

That is the message from the experts as the need for a more accurate early test is confirmed.

Up to a quarter of the UK arable area is neither at definitely high or low risk of S deficiency, says Kemira Fertilisers Roger Chesher.

Current tissue tests can indicate the sulphur status, but only when used later in the season, after the typical application timing, says Mr Chesher. "It does, however, give a good idea of what to do the following season."

Many trials highlight the inadequacy of conventional treatment thresholds, says Arable Research Centre director Mike Carver.

"The number of trials where there has been a positive response in financial terms is very disappointing. There is clearly a problem in accurately detecting deficiency." ARC is exploring other ways of overcoming the problem.

But tissue testing is just one branch in the risk assessment tree, stresses Paul Withers of ADAS Bridgets. Location, soil type and rainfall must all be considered first, he stresses.

"If you are away from industrial activity and on leachable soil, especially over chalk, then the alarm bells should be ringing. Only then should you tissue test."

Unlike soil tests, which can be misleading, leaf tissue tests taken when plants are fairly well grown give a reasonable idea of deficiency, says Mr Withers.

"In oilseed rape everyone agrees the test should be on the youngest fully expanded leaves at flowering. If total sulphur content is below 0.4% you are deficient.

"In cereals there is some debate over the best timing. Most evidence is that the later you test the more reliable it is." That is because the nearer the crop gets to flowering the more likely it is to have taken up all its needs, he explains.

Tests on leaves two and three, immediately below the flag, give the most reliable results, he says. "If the N:S ratio is greater than 17:1, and the total sulphur content is less than 0.25%, it is a good indication of deficiency."

Recently second node stage (GS32) testing, using the same thresholds, has been examined in trials. "It is looking fairly promising." &#42

SULPHUR DEFICIENCY

&#8226 Risk assessment important.

&#8226 Soil tests unreliable.

&#8226 Tissue test useful pointer.

&#8226 Early season guide needed.

&#8226 New methods being explored.