| dc.description.abstract | The soils of the Station on which the trials were carried out are deep well drained red friable clays belonging to the Nitosol soil
taxonomic group in the F.A.O. classification. The plough layer of the plots was moderately acid (p 5.85) and had an average C.E.C. of 25.6 me/
100 g, average carbon and nitrogen contents of 3.22 and 0.286% and bulk density of 1.09 gm/cc. Water held at field capacity and wilting point averages 35.64% and 27.73% giving 7.91% (dry weight basis) available water in this horizon, though available water stored (storage capacity) i the top 180 cm of the soil averages 136.6 mm.
The investigations on soil inorganic nitrogen
were carried out during the long rains (March -
October 1978) and during the following short rains of November 1978 - February 1979, During the two seasons, rainfall was above average, being 758,2 and 519,8 mm as compared with averages of 584,6 and 343,5 mm,
- During the study period, it was found that inorganic nitrogen (the sum of N03, NH^ and NC>2 nitrogen) in the top 30 cm of the soil investigated:
a) accumulated rather steadily during the dry periods. During this period, it increased from about 29 ppm in mid July to about 64 ppm early in October 1978,
b) increased rather sharply from about 21 ppm on the 11 January 1979 to about 26 ppm on the 26th January 1979, dropped to about 18 ppm during early February 1979 then rose again.
c) decreased as the rains continued from about 64 ppm in October 1978 to about 21 ppm during the first half of January 1979 and from about 26 ppm late in January 1979 to about 18 ppm early in February 1979.
Accumulation of inorganic N in the top 30 cm of the soil during the dry period was ascribed
mainly to continued mineralisation of .organic N. Rather sharp increases of inorganic N noticed during the period 11th January - 26th January 1979 and after 7th February 1979 were attributed to the flush of decomposition of organic matter and subsequent mineralisation of organic N at the onset of rains (Birch effect).
Rapid declines of inorganic N during the periods 16 - 22nd August 1978, 3rd October 1978 - 11th January 1979 and 26th January - 7th February 1979 were due mainly to leaching of inorganic N when there was enough moisture to wet the profile above field capacity.
This general pattern of inorganic nitrogen levels during the two seasons investigated was further found to vary according to the crop, significant differences being found between levels under the eight crops planted in the trials. These eight crops were Irish potatoes, maize, wheat, 1-inseed, soya beans, field beans, sunflower and sweet potatoes. During short cropping periods when all crops were fertilised with nitrogen except sunflower and sweet potatoes, it was concluded that inorganic nitrogen levels were relatively high below Irish potatoes and maize and relatively
low under linseed and wheat. Although the amounts of inorganic N below; the various crops varied more or less according to the sampling date, the crops can be ranked according to the levels of inorganic N under them as follows:
linseed [_ wheat /_ field beans _ sunflower / soyabeans /_ sweet potatoes / maize / Irish potatoes.
Significant differences in organic nitrogen content were found between Irish potatoes and all other crops; maize and linseed; maize and soya beans; sweet potatoes and wheat and sweet potatoes and sunflower. The differences in the soil inorganic N between crops were mainly due to uptake of inorganic N from the soil though it is possible that the influence of the crops on mineralisation rates was a contributory factor. The high inorganic N"'below Irish potatoes was possibly due to low rates of nitrogen uptake, and due to the effects of weeding, earthing up and wide spacing on mineralisation rates. The low inorganic N below linseed and wheat on the other hand was due to high rates of N uptake, and the fact that they were weeded only once and covered the soil well.
Since nitrogen movements, transformations and uptake by the crops are much influenced by the soil moisture levels, subsidiary investigations monitored soil moisture levels below the eight crops studied. Soil moisture- investigations covered the long rains 1977, short rains 1977/78 and the long rains 1978.
Stronger drying conditions were experienced during long rains 1977 than during the remaining two seasons (1977/78 short rains and 1979 long rains) but in none of the three seasons did the soil dry out to wilting point to more than a depth of 70 cm at which depth moisture was continuously
available to crops so that none of the crops wilted permanently. During the 3 seasons the deepest layer the soil was dried below wilting point was about 60 cm depth. Drying the soil below wilting point was due to upward movement of water mainly in the vapour phase and evaporation of water from the soil surface.
In general, total soil moisture in the profile to a depth of 180 cm throughout the growing period seemed to be:
a) lowest under sunflower and
b) highest under Irish potatoes
However,'marked differences between water content of specific sampling horizons were found
to occur between
crops as follows:
Horizon
Least moisture
Most moisture
0 - 10 cm
linseed
maize
field beans
sunflower
10 - 30 cm
linseed
Irish potatoes
wheat
sweet potatoes
maize
30 - 70 cm
linseed
Irish potatoes
sweet potatoes
soya beans
field beans
70 - 120 cm
sunflower
Irish potatoes
sweet potatoes
wheat
linseed
120- 180 cm
sunflower
Irish potatoes
maize
linseed
wheat
The moisture use of the various crops was related to the rooting habit (rooting depth and intensity) whereas the moisture in the whole
profile down to 130 r.m throughout the growing season was related to rooting depth and uptake, duration on the ground and evapotranspiration
The basic agronomic trial, within which the soil nitrogen cud moisture studies reported took place, wee concerned with finding out the extent to which the growth and yield of each of the eight crops planted was influenced by the nature of the preceding crop. Within the trials, each crop was followed first by each of the eight crops, to give treatment combination, and then by a test crop of maize. The yield of the eight crops and
the test crop of maize. The yield of the eight crops and
the test crop during the three seasons covered by the investigations did not appear to be significantly influenced by the preceding crop, but was related mare to the rainfall of each season. | |
