26 February 1999


TISSUE tests are only as good as the sample drawn, and the thresholds applied. Some growers are testing inappropriately and drawing wrong conclusions from results, warns Arable Research Centres.

"Growers must always ask themselves why are they tissue testing," says eastern region manager Stuart Knight. "I suspect quite a lot is done without any recognised reason for the sampling."

Such sampling risks exposing one-off results. Low values may be recorded but they could well be transient and are unlikely to be yield limiting. Corrective action may waste time and money.

"When tissue testing growers should aim to confirm or diagnose suspected problems, not find them," he stresses. Such problems may be a visually poor area of the crop, a soil type known to be at risk or a consistently low yielding area without any obvious limitations.

Timing of sampling may be crucial and where possible should coincide with periods of crop growth for which accurate thresholds of nutrient status have been established.

"Very often the mistake is to compare tissue test results from a certain growth stage with nutrient thresholds established at another growth stage.

"A key point about tissue analysis is that there is little point testing if there are no meaningful thresholds for that crop at that growth stage," he adds.

Complex choice

Choosing the right time to tissue test is complex. In the spring, sampling early allows foliar feed to be applied in a T1 or pre-T1 tank-mix. But this may fail to identify which deficiencies become apparent in mid stem extension. Delaying testing may mean growth has already been limited, and no further applications are planned until flag-leaf. In some cases treatment may be required in the autumn.

But if a trace element deficiency is clearly defined, then growers should consider its correction as a major application, warranting a separate pass if necessary, advises Mr Knight.

Accuracy limited

Sampling technique also limits accuracy. In HGCA funded trials carried out as part of a precision farming experiment, the ARC has found variation in the results of point samples taken on a grid pattern, which is averaged as in a standard W sampling pattern , could be very misleading. Rather than sampling whole fields patch sampling sick and healthy areas, or areas of differing soil characteristics, may be preferable.

"It is very important to do a healthy area of crop and test that separately to give reference values against which the visually poor areas can be assessed," he advises.

Whatever the crop and suspected deficiency, sampling technique should be done according to the analysing laboratories specifications. Soil and non-crop plant matter contamination must be kept to a minimum.

"Check with the lab on the size and type of sample required to get the most accurate results, and get the sample to the lab as soon as possible. First class post on the same day is best – any delay should be avoided," he stresses.

Results from the same sample have been known to differ depending on which laboratory is used, Mr Knight notes. So growers experiencing particularly odd results might ask for a re-test, or cross check with another testing station, he suggests. &#42


&#8226 Only test for clear reasons.

&#8226 Test healthy and sick areas.

&#8226 Draw sample to lab specification.

&#8226 Check threshold used applies to crop growth stage.

&#8226 No delay dispatching samples.

&#8226 Consider cross checking lab results.