1. Fertilizer NO₃-N was applied to fallow and planted soil. In
a field experiment the rates of application in factorial
combinations were 30 and 60 kg N0₃-N/ha in the autumn (December)
and 60 and 90 kg N0₃-N/ha in the spring (May). Samples of soil
cores down to 60 cm., and plant material (winter wheat) were taken
periodically throughout the December to September growing season.
Greenhouse experiments were carried out using topsoil and
subsoil separately. In the first greenhouse experiment* 1|5 cm.
columns of topsoil and of subsoil were used under plant (rye grass)
and fallow with and without 60 kg N0₃-N/ha. Duration was 7 weeks.
Three watering regimes were applied, i.e. 60, 90, and 120 percent
of the water holding capacity. The excess water from the highest
water regime was collected. The second experiment was carried out
on topsoil only. Soil was ignited to destroy organic matter and
was compared with the unignited soil under fallow and planted (rye
grass) receiving N0₃-N at rates of 23 and 50 ppm.
A laboratory experiment was carried out investigating the
effect of two aggregate sizes (1-3 and 3_6 mm. in diameter) on
leaching losses of added N0₃-N.
2. Soil samples were analysed for NO₃-N and NH₄-N in 4 horizons
from the field experiment (3 from the Greenhouse Experiment I) each
15 cm deep, and from each of the Greenhouse Experiment II and the
laboratory experiment. Leachates were analysed for the same two
mineral fractions of nitrogen. Plant samples were analysed for
3. The field experiment showed that the number of soil cores is
a limiting factor and better results could be obtained only by
taking more replicates.
4. Recovery of fertilizer N at the end of the season ranged from
67-77% in the field experiment (9 months) and from 60-95% in the
greenhouse experiments (nearly 2 months) under conditions of no
leaching. However, when leaching was excessive, recovery of
fertilizer N was as low as 2%.
5- Leaching losses of fertilizer N could be as high as 100%.
Movement of fertilizer N occurred within 3 weeks of application,
translocating fertilizer N from the surface to 45-60 cm deep in
soil. Precipitations ranging from 38 to 1025 mm resulted in
downward translocation of fertilizer N. The size of soil aggregates
showed their important role in holding fertilizer against loss by
leaching. Planted soils lost about 30% by leaching; fallow ones
lost more than 75% within 3 weeks under 482 mm precipitation.
6. The topsoil possessed greater capacity for mineralization and
immobilization than the subsoil. This was proved by experiments
comparing behaviour of fertilizer N in topsoil vs. subsoil and in
ignited vs. unignited topsoil.
7- The proportion of fertilizer N immobilized was greater in the
fallow (about 90%) than in the planted. Indications of remineralization were shown in both field and greenhouse experiments.
8. Denitrification and reduction of NO₃-N was shown in both
topsoil and subsoil and in the ignited topsoil. Fertilizer NO₃-N
moving from topsoil down to the subsoil - under field conditions -
seems to have been reduced to elemental N rapidly in the subsoil.
9. There are indications that application of fertilizer enhances
mineralization immediately (up to 3 weeks) after application; but
within 3-ip months it seems that fertilizer enhances immobilization.
Exudation of fertilizer N by plants may have had an important
effect on its status in soil.