27 February 1998


DONT be tempted to eliminate P and K inputs in a bid to counter falling combinable crop prices.

Although potash inputs may give no yield response, there is a considerable yield penalty if soil levels are allowed to fall, says John Hollies, director general of the Potash Development Assoc-iation.

"That is especially true if the crop comes under stress from either the weather or disease." Nitrogen responses could also suffer if potash levels fall, he says.

Levels need to be maintained by replacing what a crop removes from the soil, plus a buffer to cover the period of maximum uptake at flowering, he says.

For phosphates, Dr Jim Lewis of Hydro Agri says that although it takes 20 years to reduce levels from satisfactory to low (MAFF index 2 down to index 1), correcting the deficiency would take a similar length of time.

Most farms maintain phosphate levels by applying a traditional 65kg/ha of autumn P for cereal crops, which is just short of the removal rate, Dr Lewis says. A large number of trials show that, provided the soil index is above 1, the timing of P application has no effect on yield. &#42

Phosphate in context

Avoiding excess phosphate applications is important to avoid water pollution. But it is worth putting the 20ppm drinking water limit in context, says Hydros Dr Jim Lewis. It compares with P levels of 170ppm in tea, 880ppm in coffee, 820ppm in wine, 380ppm in beer, and 1570ppm in most fruit juices.