Abstract:
Investigations into nitrate leaching from sludge and sludge injection into
grassland have been undertaken for many years, however phosphate leaching from
soils and shallow
injection of sludge into grassland are relatively new ideas.
This
lysimeter study sets out to investigate the influence of the type of slurry
and
application rate on the amount of phosphorus and nitrogen lost by leaching. The
field trial assesses the influence of
application methods on grass yield, sward damage
and
ritrogen loss by leaching through analysing the effects on a grass crop using two
different methods of
injection and comparing them with surface application and
granular fertilizer.
Losses of P from
sandy loam soils were found to be minimal therefore
contamination of
groundwater is thought to be unlikely. Similar leacling patterns were
observed from both the cake and
liquid sludges at all four rates_ applied, however the
liquid sludge released more phosphate than the cake sludge, resulting in higher
concentrations recorded.
The
lysimeter study also found leaching losses of ritrate from both sludge types
to be well above the EC maximum
permissible level of 11.3 mg nitrate-N l`l at all four
rates. Residual N from the
previously cropped soil masked the nitrate concentrations
during the initial stages of the study. Losses of N from the cake sludge was lower than
from the
liquid treated plots due to slow release. It was also found that environmental
factors such as
high temperatures and rainfall influence nitrate leaching.
The field trial
analysed various disposal methods in terms of nitrate leached,
crop yield and sward damage. N losses from shallow injection were reduced compared
to the
deep injection plots, as the sludge was placed in closer proximity to the crop
roots.
Olibhe Collins, Silsoe College, 1996.
In overall terms the shallow
injection system yielded the greatest quantity
compared to surface applied or the inorganic fertilizer. Damage caused by the tines
during shallow injection was not evident compared to the deeper injection system
where sward
damage and dieback was considerable.
The results from the field trial indicate that the shallow
injection of sewage
sludge has proved to the most environmentally friendly method for the disposal of
sludge to farm land.
