Effect of submergence on alleviation of soil acidity and availability of nutrients in a rice-rice ecosystem
Wet land soils have got much importance in agriculture because they offer sustenance for rice, the staple food grain for majority of world population. Soil acidity is among the important environmental factors which can influence plant growth and can seriously limit crop production. Kerala, the southern most State of Indian peninsula with a geographical area of 38,864 km2 experiences a humid tropical climate with a mean annual rainfall of 3000 mm. 98% of the soils are acidic in nature and so the production potential of rice is highly reduced. In wet lands the cheapest method to alleviate acidity is by water management. Hence, the present study is been conducted in the rain fed low land rice soils of Kerala to unveil the effect of submergence on the kinetics of pH and available nutrients. Soil samples were collected from the 12 major rice growing tracts (3 each from each wet land tract) and invitro study was conducted by maintaining 5 cm water above soil for 12 weeks using CRD split plot design. The initial as well as fortnightly pH, available N, P, K, Na ,Fe,Mn,Zn,Cu ,Si and Al were determined by standard procedures.
All the soils studied were acidic except the black soils of Chittoor which are neutral to alkaline in reaction. Based on the severity of acidity, the wet land rice soils of Kerala can be arranged as Kari > Pokkali> Karapadom > Vellayani > Kayal > Kole > Wyanad > Pattambi > Kaipaid > Karamana > Kattampally > Chittoor. The wet lands under study showed an increase in pH due to submergence to a fairly stable value. All the wetlands reached above pH 5.5 within two weeks of submergence except Pokkali, Kari, Kayal and Kole lands. Since most soils contained more Fe (iii) oxide hydrates than any other oxidant, the increase in pH is largely due to the reduction of Fe. Kari and Pokkali soils are identified as Potential acid sulphate soils. For acid sulphate soils, keeping the soil under submergence, liming @ 1000 kgha-1 and washing away of acidity is been recommended. For all other soils two weeks of submergence prior to transplanting rice seedlings can create the optimum pH for rice without addition of lime. All the soils showed an increase in available N, P, K, Na , Fe , Mn and Si due to flooding. The peak increase was noticed during the first two weeks and then a slow build up was noticed. Available aluminium decreased to very low values in all soils except kayal, kari and Pokkali soils due to flooding. Al 3+ activity is directly related to pH, as pH rises due to flooding, aluminium is precipitated as hydroxides or sulphates.
Flooding a soil sets in motion chemical and electro chemical process that affect the supply of nutrients and their uptake by rice. After 2 to 4 weeks period of rapid changes, the processes tend to stabilize. The stable milieu favors rice because the nutrient supply is adequate and the level of toxins is low. A yield increase of about one tone ha -1 is been obtained by merely delaying transplanting for 2 weeks after flooding. After addition of green manures and organic manures during land preparation, two weeks submergence is a must in the rice fields of Kerala to increase pH, availability of nutrients and to minimize toxic elements like Al.