Slurry system keeps N costs in check

NITROGEN FERTILISER costs have risen by about 20% in the past two years. This, combined with the increasing pressure to drive down farm costs, has been one of the main factors helping Scott Kirby – Harper Adams’ farm manager – to justify the £75,000 slurry system being installed on the college farm.

The college’s old system had been in place since 1983 and was, as Mr Kirby describes it, “coming to the end of its working life”.

“We now produce 12,500cu m of slurry and yard water per year. The old system wasn’t designed for those kinds of quantities, so we had to upgrade.

“Unlike the during the 1980s, we no longer have the benefit of grant aid. Any new system needed to have minimal capital costs and allow maximum use of organic manures.”

First update to the slurry handling facility was the purchase of a Dutch-built umbilical boom-spreader two years ago. At a cost of £25,000 for the applicator and accompanying tackle, it was necessary for two reasons.

As the farm supports a 220-cow dairy herd unit, slurry usage for paddock regrowth is a key concern. Trailing shoe applicators ensure slurry reaches the soil directly, minimising sward contamination and nitrogen loss through volatilisation.

The dependence on a contractor has also been removed and slurry can now be applied to individual paddocks, as opposed to block work typical of contractor style.

Slurry is also used to top-dress cereals before first node development. This ensures the crop is not permanently damaged by the umbilical hose as it is dragged over the crop.

An area of high importance for Mr Kirby is the effective usage of slurry as a nitrogen source. In the past slurry was stored in an earth-walled lagoon, which presented a significant weakness for the system. Rainfall increased slurry volume and valuable nitrogen was lost to the atmosphere through ammonia volatilisation.

The slurry is now stored in a 5000cu m reinforced plastic bag which is filled and emptied via a 15cm (6in) central pipe.

Although costly, the Alligator bag, fitted with agitators, is a system commonly found in Holland, with about 40% of farming operations using it. At first the Environment Agency was cautious of the set-up, especially as it is the first of its type in the UK.

“The slurry bag is capable of storing all slurry produced from the beginning of August through to mid-February, the period during which plant usage is minimal and leaching losses highest,” says Mr Kirby.

“If you exclude rainfall from the system, the bag will cope with 12 month’s slurry easily.

“At 15/cu m to store slurry, an equivalent tank system with a roof would cost twice that.”

The application system allows Mr Kirby to manage slurry delivery very effectively, a task which has become increasingly important since the area”s designation as a Nitrate Vulnerable Zone.

While pumping slurry to the application tractor, the pump operator is able to take samples and, using a Quantifix nitrogen meter and software from ADAS, can analyse the nitrogen and dry matter content of the slurry.

“Application rates are worked out in the farm office much as you would for granular nitrogen application” says Mr Kirby.

“A crucial piece in the jigsaw is the flow meter built into the umbilical line. This allows us to calculate the speed at which the applicator needs to be travelling for the required delivery rate.”

An added bonus for the umbilical system is that the college can use its existing irrigation ring-main. Operators can connect to any field hydrant, reducing the length of hose required and making application a less time consuming task.

Savings in nitrogen costs and the ability to apply slurry accurately to designated fields ensure that the slurry system will cover its installation cost in time.