Green manuring practice in sodic soil has an unique importance since it adds acids in the reclamation process, besides improving the fertility status of the soil. Usually the fertility status of sodic soil is very poor because of its high pH and exchangeable sodium percentage. The soil organic matter content, a measure of available nitrogen, is very low i.e., 0.1-0.5% in sodic soil because sodium carbonate and sodium bicarbonate salts in solution dissolve the humus.
Further the available nitrogen is much lower in the subsoil layers of the sodic soils. Reclamation of alkali soils basically involves replacing Na on the exchange complex with more favourable cations. The solubility of lime, which is always present in alkali soils in significant amounts, is very low, because the potassium content of alkali soil is high. There is an intimate relationship between soil pH, partial pressure of CO2 and calcium ion activity in calcareous alkali soils. Increase in CO2 production in the soil enables to increase the soluble Ca status of soils. This in turn, replaces exchangeable Na, resulting in the improvement of alkali soils.
Soil incorporation of easily decomposable plant material results in increased and rapid production of CO2. For this reason, green manuring has been suggested as an important management practice for the reclamation of alkali soils. Sesbania aculeata and Delonix elata are very effective green manures and green leaf manures respectively used for reclamation of sodic soils. Daincha (Sesbania aculeata) is highly resistant to both drought and water stagnation and salinity and alkalinity. It can be grown in soils with pH 4.5 to 9.5.
It produces green matter of 20 t/ha in 90 days. Daincha contains 3.2% N and 34% Ca on dry weight basis which helps to replace Na from sodic soils. The acid juice (pH 4.0) and high seed protein content (58%) seems to be the cause of its resistance to sodicity stress. During the reclamation of sodic soils gypsum @ 50% of gypsum requirement (GR) has to be spread uniformly over the field.
The surface soil is to be ploughed to mix the gypsum in the sodic soil. Irrigate the field with 10-15 cm depth of water and maintain the same water depth for 3-4 days. At this stage, the sodium content in clay particles are replaced by the calcium ions from the gypsum, allowing the sodium to wash out of the field as Leachate.
The field has to be kept with stagnant water for 3–4 times after each drainage process. Apply the vadanarayanan (Delonix elata) leaves and daincha @ 5 t/ha without allowing the soil to dry. After four to five days of incorporation of green leaves, the field crop like rice with preferably a tolerant variety CO 43, TRY 1 etc.
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