2 February 1996

Uniform Psoil tests needed

SOIL tests are valuable for assessing phosphorus needs.. But the wide range of apparently unrelated methods used is prompting calls for more uniformity.

Inability to correlate the findings of up to 20 different test systems is a big barrier to the flow of information between researchers, according to TEAGASCs Hubert Tunney.

It could also account for some recommendation anomalies.

With crops taking up only 5-10% of applied phosphorus in the first year, defining an optimum soil "pool" is always going to be tricky, according to Erik Sibbesen of the Danish Institute of Plant and Soil Science. But with so many ways of determining soil P status the picture is "really confusing".

Even where the same method is used, advisers can make widely differing recommendations. Dr Sibbesen points to two groups in Germany using the same extraction test.

One suggested a high yielding crop needed three times as much P as the other.

Setting optimum upper limits for soil P is "very difficult" and will vary from site to site, he says. "We need to identify soils likely to give high P run-off and adopt management to go with them."

Rothamsteds "Johnny" Johnston believes "bonding energy" which holds the element to various soils is the key to progress.

"Were now much more concerned with losses. We need to go back and look more closely at the absorption process and the organic matter to see how quickly phosphorus can be stripped off."

The prospect of using extra tests, say for aluminium and iron which help lock up P, was raised. The idea is that they could help predict losses. "Its a good idea, but who will pay?" said Dr Sibbesen. He believed money would be better spent on predicting areas where so-called transport losses, through erosion and sub-surface flow, are important.