20 October 1995

…soil trigger differences

PHOSPHATE losses through drainage seem to be quite small until soil contains a trigger amount at which leaching takes off.

That is the conclusion of studies at Rothamsted. The big unknown, according to "Johnny" Johnston is whether that trigger or so-called "change" point is the same for all soil types.

Long-term trials on the silty clay loam at Rothamsted show there is a "lot of buffering", with little leakage until the soil P reaches about twice that needed to support good crop growth.

"What we do not know is whether the change point is the same for all soils," said Mr Johnston. "The key question is whether we can get an index of vulnerability." His suspicion is that on lighter soils there may be less leeway. Only long-term experiments could prove it, and funds for such work were hard to get, he noted.