Report No FR0316

M J Hann*, C J Atkinson*, R J Godwin* and S R Smith *Department of Engineering for Agriculture, Silsoe College

Sept 1992



To provide the scientific basis for developing appropriate measures which protect the environment from nitrate when sewage sludge is applied to arable land.


To study environmental and agronomic factors which influence the recovery and loss of nitrogen when sewage sludge is applied to arable land.


Current and future measures to protect ground and surface waters from nitrate emissions from agricultural land may constrain significantly sewage sludge disposal practices. This will affect particularly those areas where sludge is applied largely in the autumn period to arable land in nitrate vulnerable zones.


The contribution to overall nitrate leaching losses from operational application rates of liquid digested sludge is probably small and lysimeter studies indicated amounts of nitrate in drainage from sludge treated soils were significantly below the permitted EC maximum. As expected autumn or winter application to land drilled with spring wheat resulted in significantly larger losses of nitrogen through nitrate leaching compared with autumn or winter drilling. However, the closer the application date is to sowing date the smaller becomes the apparent nitrogen loss and the amount of nitrogen recovered is directly proportional to crop yield response. Method of application is also important with injection giving rise to smaller nitrogen losses overall compared with surface application. Ammonia volatilisation from surface applied sludge therefore represents a larger potential route for nitrogen loss to the environment than is leaching of nitrate from injected sludge.


This final report presents the results of nitrogen balance studies from a series of field experiments conducted at Silsoe College. In these trials, liquid digested sludge was injected and surface spread to arable plots at intervals over the autumn, winter and spring period. The plots were either sown with winter wheat or left uncropped until sowing with spring wheat. Applications were also made post-emergence.

The trials were conducted on both sandy and clay soils. The crops were monitored for yield and nitrogen content, and nitrogen losses are discussed in relation to nitrogen additions, method and timing of applications, climatic factors, soil type and time of sowing.

The concentration of nitrate in drainage from lysimeters with sludge-treated soil was also measured. All the field experiments were soil sampled and analysed for nitrate content at the beginning and end of each trial to calculate a nitrogen balance, soil samples were also taken for nitrate analysis about four weeks after sludge application by injection to determine the rate of nitrate accumulation in soil and the potential for loss by leaching from winter rainfall in relation to soil thermal-time.

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