Agriculture Reference
In-Depth Information
Conventional wheat
Direct-drilled wheat
Direct-drilled wheat with white clover
800
600
400
200
0
Conventional wheat
Direct-drilled wheat
Direct-drilled wheat with white clover
300
200
100
0
Autumn 1995
Spring 1996
Autumn 1997
FIGURE 6.2
Earthworm populations and biomass (means ° SE) in three wheat cropping systems at Long
Ashton, U.K. (From Schmidt et al. 2003. With permission.)
concluded that the earthworm populations benefited less from the reduced soil disturbance than
from the enhanced quantity, nutritional quality, and continuity of food supply in the wheat-clover
crop.
M
ANURES
AND
F
ERTILIZERS
Earthworm population responses to mineral fertilizers can be variable. Generally, the effects of
moderate levels of application are positive, reflecting increased litter quality and quantity (Gerard
and Hay 1979; Edwards and Lofty 1982b; Lofs-Holmin 1983; Bostrm 1988; Muldowney et al.
2003), but populations may be depressed by heavy applications of nitrogen (Nowak 1976). Adverse
effects of some nitrogenous fertilizers, such as sulfate of ammonia, appear to be caused by soil
acidification. Ma et al. (1990) reported a severe depression in earthworm populations in some
grassland field plots treated with various types of nitrogenous fertilizers over a period of 20 years.
The degree of depression reflected pH reduction, with ammonium sulfate and, to a lesser extent,
sulfur-coated urea having the most marked effects.
Organic manures benefit earthworms by providing additional food, by their mulching effects,
and by stimulating plant growth and litter return. Farmyard manure is a particularly beneficial form
of organic amendment (Edwards and Lofty 1982b; Lofs-Holmin 1983; Whalen et al. 1998). How-
ever, when populations are already very high under favorable conditions, such as those found in
cereal-clover bicrops, neither the input of additional organic matter (as cattle slurry) nor mineral
fertilizers may increase earthworm populations further (Schmidt et al. 2003). Heavy applications
of animal wastes as semiliquid slurry containing high levels of ammonia and organic salts can be
