Rain against N distribution in soil


Nitrogen is gained by soil through fertilizers, organic manures
and fixation by microorganisms, but it does not accumulate in
the soil or is fully used up by the crop plants. The nutrient use
efficiency of applied nitrogen fertilizer is indirectly governed by
losses due to leaching, volatilization and fixation. Recent
studies have indicated that the use efficiency of N fertilizers
is about 60%. Hence, attempts were made to study the
distribution of N at various soil depths against various
quanta of rainfall.

Soil samples were collected from UPASI Experimental Farm
at 0-40 cm depth. The samples were packed in six identical
soil columns in the order of their presence in the garden.
Uniform quantity of water was added to all columns one day
prior to imposing treatments to ensure uniform soil moisture.
A known quantity of urea-MOP (N: K2O ratio 4:3) mixture
was added to all the soil columns. To the first five treatments,
water equivalent to 0, 2.5, 5.0,10 and 20 mm rainfall was
added to the soil columns every day for a period of thirty days,
and to the sixth treatment water equivalent to 50 mm of rainfall
was added consecutively for the first three days of
experimentation. At the end of one month, the soil columns
were unpacked and the soils of different depths
(0- 10, 10-20, 20-30 and 30-40 cm) were separated. The soil
samples and leachates were analyzed for NH4 - N, NO3 - N
and NO2 - N. For the purpose of calculation, the nitrogen
available in leachate was included along with that present
in the bottom layer of soil (30-40 cm).

The distribution of nitrogen at various depths was calculated.
It appears from the result that in the absence of rainfall,
about 67 % of applied nitrogen was located over the first 10 cm
followed by the immediate next 10 cm of the soil profile.
The same trend was observed when water equivalent to
5 mm rainfall was added. The second 10 cm layer of soil
(10- 20 cm) was rich in nutrients while there was a
substantial decrease in the first 10 cm soil profile due
to addition of water equivalent to 10 mm rainfall or more.
The concentration of nitrogen in the third and fourth layer
increased due to increased water addition which may
not be available to tea plants.

The results of this laboratory study indicated that when there
were gentle follow up showers (less than 10 mm every day)
after fertilizer application, a major portion of the applied
nutrient would be distributed over the root zone of soil-plant
system. The results further showed that when 150 mm
rainfall was received within three days after fertilizer
application, 35 to 45 % of nutrients leached below
the root zone.