N distribution as influenced by rain

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 (Table 1). 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.

(Table 1 is available on request drsvenkatesan@gmail.com)

Authors : S. Venkatesan, S. Murugesan and K.V. Hemalatha