26 December 1997

Precision P and K applications might not pay

PRECISION application of phosphorus and potash fertiliser using yield maps may not pay, despite the inaccuracies of the widely-used index system.

The current approach, using a single soil index for a whole field, can mean 47% of a field is inappropriately fertilised with P and 31% with K, said Martin Froment of ADAS Bridgets. "We are not targeting fertiliser as well as we could."

But making variable applications according to yield maps and standard values for P and K offtake in grain and straw is unreliable, he added. Work at ADAS Bridgets and Boxworth shows there is a wide range in actual P and K offtakes values.

Grain P ranged from 4.3 to 8.5 kg/t, grain K from 3.7-6.7kg/t. Neither was related to soil content. Variability was even greater for straw. Rainfall, nutrient content and different soil types are the main factors causing that variability, said Mr Froment.