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Studies on a drained clay soil at Brimstone Farm in Oxfordshire (Williams et al.,
2006) showed that when cattle slurry applications timings were moved from autumn
to spring to reduce nitrate leaching losses, elevated ammonium-N concentrations
of up to 3.9 mg/L NH 4
-N (c. fivefold greater than the EC Freshwater Fish Directive
limit of 0.78 mg/l NH 4
-N) were measured in the spring drainage water flows. This
was because rainfall after the spring slurry application caused contaminated water
to move rapidly from the ‘wet’ soil surface to pipe drains, via the network of soil
macropores and mole drains in the heavy clay soil.
Figure 2: Ammonia emissions after cattle slurry applications to grassland
(Williams et al., 2005b)
These data indicate that there is a need to recognise that slurry management practices
that aim to reduce nitrate leaching losses by moving from autumn to spring/summer
application timings, may exacerbate ammonia volatilisation losses under warmer
and drier soil conditions, and increase the risks of other nutrient pollution of drainage
waters from clay soils.
Broiler Litter
Assuming that all of the broiler litter was previously applied in the autumn, nitrate
leaching losses after surface broadcast applications on the sandy soil Nottinghamshire
farm were estimated at 9,650 kg N (equivalent to 13 % of total N applied). Changing
the application timing from autumn to spring increased the fertiliser N replacement
value of the broiler litter by £3,860, as a result of reduced nitrate leaching losses.
The capital cost of purchasing a spreader that was capable of applying broiler litter
evenly and at agronomically required rates was £22,000, giving an annual amortised
repayment cost of £2,860 (over 10 years). Overall, the net benefit to the farm was
£1,000/year. However, the practical problems of topdressing broiler litter to growing
crops in spring (i.e. soil trafficability problems, non-compatibility of machine spread
widths with tramline spacings and potential odour/fly nuisance) is likely to make
such an application policy impractical on most farms. On suitable soil types, such
as the sandy soils on the Nottinghamshire model farm, the rapid soil incorporation
of broiler litter before the establishment of spring sown crops would reduce
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