26 February 1999

SKILL NEEDED TO MAKE MOST OF TISSUE TESTS

Poorly rooted crops are

likely to need nutrient

supplements this spring.

But how best can you judge

their needs? Louise Impey

reports Dalgetys timely

advice (below) while

Andrew Swallow highlights

some possible pitfalls (right)

TISSUE testing has an important role to play on high value crops and where soils are known to be limiting, says Dalgetys technical manager Bob Bulmer.

But he stresses that skill is needed with the interpretation of the results as crop requirements change during the growing season and other factors can affect the availability of certain nutrients.

"Its still a poorly researched area," he admits. "But used correctly it takes the guesswork out of crop nutrition and prevents yield loss. With some nutrient deficiencies, such as manganese, yields have already suffered by the time visual symptoms are showing.

"Taking a soil sample as well can be a good idea, as it will show up things like pH problems."

Dr Bulmer warns that tissue testing should not be done in drought conditions, after liming or when there is high rainfall. "Root disease, compaction and take-all can all affect trace element levels too. Only if you have ruled out all other possible effects and the crop still isnt growing well should it be used."

Best time to sample

The best time to sample is when the crop is just going into rapid growth, he advises. A large handful of foliage from the top third of the plant is required for a test. In Dalgetys case this done by an independent laboratory at a cost of £25 a sample.

The results, which come back within a week, are presented as a bar chart. Five different categories – severe deficiency, moderate deficiency, satisfactory, high and very high – are used to show nutrient levels.

"It detects all the major and minor nutrients, as well as giving the nitrogen:sulphur ratio," says Dr Bulmer. "Growers are advised to react when any nutrient is in the severe deficiency category."

He adds that some nutrients interact, so solving the main problem usually corrects other, less serious problems. His advice on the best course of action varies according to the deficiency.

"Problems with nitrogen, phosphate, potash, magnesium or sulphur can be put right with fertiliser. Manganese deficiency can be partly remedied by using a seed treatment, while copper and boron can be corrected with either a fertiliser or a spray.

"Manganese and zinc deficiencies are not due to deficient soils, but because the crop cant get them. So often a foliar approach is required to get things back on course."

Dr Bulmer stresses that timing is important with foliar applications. "Generally, its best to treat early. Boron and manganese must go on early in the season. Copper is another nutrient which needs to be applied by March. Sulphur is more flexible and a response will still be seen from later applications."

Results from the last three years of Dalgetys testing service show that very few wheat crops are severely deficient in the major nutrients, N, P and K. "But growers put a lot of effort into these and have access to good advice. What they must appreciate is that if any other essential element is in short supply, yields will suffer."

In contrast, nearly 30% of the wheat samples submitted for testing had a severe magnesium deficiency.

"Magnesium levels go wrong for different reasons," notes Dr Bulmer. "On heavy soils, poor rooting can be the cause, and its more likely to occur in combinable crop rotations. On the lighter soils, dry conditions can restrict its uptake."

Copper is also a problem in wheat, with 57% of the samples recording severe deficiency. Zinc was in very short supply in 15% of plants. "These results reflect a lesser understanding of the role of these nutrients in the plant."

In oilseed rape, 10% of the samples tested had a severe deficiency of potash. "The oilseed rape crop has a high demand for K, so we might have to re-think fertiliser policy. Its a light land problem, as it leaches over time."

Magnesium was severely deficient in almost half of the rape samples. "This shows up on heavy land. Theres not enough magnesium fertiliser being used. And boron levels need looking at on light land. It showed up as an issue in a quarter of the rape samples."

Cost benefit good

Dr Bulmer concludes by pointing out that the cost benefit of applying trace elements is still very good. "Manganese is very cheap at £2.50/ha although repeat applications may be required. Boron, zinc and magnesium are more, at £5/ha, but thats not excessive.

"When you consider that yield loss will be around 20% without taking action, it is still money well spent. And with manganese deficiency, you can get total crop loss."

He suggests that tissue testing is not necessary as a routine procedure on combinable crops. "Knowing your soil can be just as useful. But on potatoes and sugar beet its worth the time and money. A preventative approach is always more effective."