In 74 experiments carried out in England from 1990 to 1994, cereal
test crops were grown on sandy
soils prone to nitrate leaching, following cereals, sugarbeet and potatoes.
Measurements were made
of the effects of the previous crops on soil mineral N, and on N recovery
by the cereal test crop.
Soil N supply in autumn (SNSa) was greater following potatoes
(c. 100 kg/ha N) than following
sugarbeet or cereals (c. 60 kg/ha N). However when potato crops
to which organic manures had been
applied were excluded, mean SNSa after potatoes was only
c. 60 kg/ha. Soil N supply in spring (SNSs)
following sugarbeet and potatoes was similar and at c. 56 kg/ha,
c. 10 kg/ha greater than following
cereals. Seasonal differences in SNSs were related to excess winter
rainfall and soil water-holding
capacity. Modelled leaching losses gave good agreement with overwinter changes
in SNS (r=0·87),
although SNSs was usually greater than predicted by the model.
This discrepancy was considered to
be due to overwinter mineralization, which was estimated from the intercept
of the regression lines
to be c. 40 kg/ha following cereals and potatoes, and
c. 50 kg/ha following sugarbeet.
Apparent net mineralization (AM) of N during the test crop growing season was
c. 37, 53 and
63 kg/ha following cereals, sugarbeet and potatoes respectively. However,
AM was c. 24 kg/ha N
greater if the preceding potato crop had been given organic manures, but
there was no difference in
AM following manured and unmanured sugarbeet crops.
Nitrogen offtake by the cereal test crop without fertilizer N
(NoffN0) was c. 15·20 kg/ha greater
following potatoes and sugarbeet than following cereals. The greater N
offtake following potatoes is
considered to be due to mineralization of organic manures applied to the
potato crop, while the effect
following sugarbeet appears to be due to mineralization of sugarbeet residues.
At harvest, SMN was
c. 38 kg/ha and similar for all three previous crops. It is
concluded that mineralization of sugarbeet
residues has taken place more quickly on these sandy soils than previously
reported on soils of greater
clay content. Only potatoes grown with organic manures leave greater
N residues than cereals.