17 January 1997


By the time striping is seen in grass or crop fields significant yield loss is already being suffered. New efforts aim to give growers better advice to help avoid that

SP ratings as a guide to fertiliser quality already have a financial value farmers should take into account. But now the rating is being improved thanks to changes in the testing at Silsoe.

"We want to improve the robustness of the results we are producing so we can be more emphatic about the financial benefits we are claiming," explains Dr Paul Miller, applications specialist at the institute.

Testing changes are based around the use of a simulated fertiliser spreader, complete with hopper, typical agitation and spreading mechanism. It can be used in the laboratory, allowing more measurements to be made independently of weather conditions.

Computerised controls and a device for measuring flow rate into the spreading mechanism also feature.

Smaller samples can be used and the rig could be run for single samples, rather than being set up for batch operation, says Dr Miller.

Total cost is put at about £75,000, shared by Silsoe and the Fertiliser Manufacturers Associ-ation.

Not only will the changes make for more reliable data, it will also allow imported material to be tested. Currently some imports only arrive in the country at times when it is impossible to run the current test due to weather factors.

Work with the current test has already shown that the difference between spreadability between an SP5 and SP3 can amount to a £6/ha (£2.40/acre) difference in crop gross margin.

Although that may not seem much in terms of crop output, it is more significant when related back to each tonne of fertiliser used.

Assuming an application rate of 160kgN/ha (143units N/acre) the saving equates to £36/t in the cost of the fertiliser.

Big variations in fertiliser quality can be found in the marketplace, notes Barry Higgs, director general of the Fertiliser Manufacturers Association. "We test products which we buy ourselves. It shows that there is plenty of SP 1, 2, 3 and 4 products on the market as well as the SP5 which people see as a quality mark."

So far no other European country has adopted an approach similar to the SP rating for fertiliser quality, says Mr Higgs.

However, an attempt was made by the European Fertiliser Manufacturers Association to introduce a guide to spreader calibration based on the characteristics of different fertilisers. But that proved too complex and also was prone to problems with field conditions, he explains.n

Fertiliser quality has a big bearing on the accuracy of spreading. New tests at Silsoe should make the SP rating even more relevant, say researchers.

Crop yield and financial return from nitrogen fertilisers with different "SP" ratings.

161 kgN/hayieldreturn

applied ast/ha£/ha




Response of different crops to inferior fertiliser quality.

cropassumed valuefinancial loss


3rd wheat after a break11011.13

winter barley after a cereal1057.69

winter barley after a break1057.58

spring barley after a cereal1057.53

1st wheat after a break1107.42

2nd wheat after a break1106.74

spring barley after a break1055.33

winter oilseed rape1202.42

NB. The fertiliser showed a C.o.v. of 20%, typical of lower grade products.

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