Soil nutrient levels must be replenished
Soil nutrient levels must be replenished
BEWARE of trying to live off soil reserves of phosphorus and potash for too long and do not neglect liming, warns a leading soil scientist.
The effects of reducing P and K fertiliser use to save money may not be immediately obvious, but could seriously jeopardise yield in the longer term, Keith Goulding told last weeks meeting of the International Fertiliser Society.
Over-economising on the two key nutrients will reduce the efficiency of applied nitrogen and may lead to environmental problems as unused N leaches into water supplies, says Prof Goulding.
Figures show a steady decline in the use of P and K on arable crops since the mid-1990s, he notes. "Economic pressures have tempted growers to reduce inputs."
With phosphorus, where excess can encourage polluting algae in ponds and lakes, that is no bad thing, provided soil indices do not fall below the critical levels for specific crops, he says.
There are also signs that potash accumulated in subsoils over many years can provide more of crop needs than previously appreciated. "In some cases, for example where roots are encouraged to go deeper in dry weather, it can be quite significant. Deep potash may supply up to half a crops requirements."
The best guide is to keep soil P and K at the critical levels appropriate for specific soil and cropping systems, as indicated in DEFRA book RB209.
Less liming, which encourages acidity and cuts root growth and yields by making aluminium too readily available, is also false economy.
ADAS figures show calcium losses from UK soils have doubled since the 1960s. "The reasons for putting lime on now are even greater than they were 40 years ago. A pH minimum of 6.5 should be seared into the brains of all arable farmers."
Using local supplies of magnesium limestone instead of more expensive chalk or other calcium sources from further afield can make potash less available, he warns. *