Interrelations of humans with the fertility components of wetland soils

Abstract

Surface soil samples from the major wetland soil types of Kerala were studied to establish the relationship between soil organic matter and its fractions with the total and available nutrients and other physicochemical characters. Destruction of organic matter has resulted in a substantial drop in the status of almost all nutrients and an appreciable change in soil properties. The SOM separated from the soils were fractionated and the correlation between soil characters and the SOM and its fractions were determined. Regression equations relating soil properties which were significantly correlated with SOM and its fractions were also derived in each soil type. The study has revealed that CEC,WHC,SOM, lime requirement, buffering capacity as well as total and available nutrients in all the four soil types showed considerable degree of variability between samples in each soil type. The content of SOM was highest in Kayal soils (8.03%) and lowest in lateritic alluvial (2.24%) soils with Kari and Karappadam soils with intermediate values of 7.58 and 6.81% respectively. Fractionation of organic matter has shown that humification of SOM was maximum in Karappadam soils, followed by Kayal, Kari and lateritic alluvial soils. The degree of degradation of humified organic matter was high in all the soil types except Kari soils, as evidenced from the HA/FA ratios. The SOM in Kayal soils may probably exist as a stable clay humus complex which has resulted in its lower contribution to CEC, lime requirement and nutrient status. All the four soil types were acidic and the contribution of SOM to free acidity was maximum in Karappadam soils (18.4%) followed by Kayal (17.2%) and lateritic alluvial soils (4.2%). Inspite of the high content of SOM, its contribution to free acidity in Kari soils was only 7.1%, probably due to the low deprotonation of SOM at low pH values. pH in all soil types is negatively correlated to SOM and its fractions. The Kari, Karappadam and Kayal soils are highly buffered when compared to lateritic alluvial soils and the slope of the buffer curves for these soils showed a drastic drop due to the destruction of organic matter. The contribution of SOM to lime requirement ranged from 13.9 to 25.5% which is evident from the drastic drop in lime requirement values consequent to the destruction of organic matter. Maximum contribution of SOM to WHC was shown by lateritic alluvial soils (49.8%). In Kari, Karappadam and kayal soils, the contribution was 24.7, 29.4 and 25.3% respectively.WHC is positively related to SOM and its fractions in all the soils. CEC was highest in Kari soils and lowest lateritic alluvial soils. The contribution of SOM to CEC was maximum in Kari soils (27.2%) and least in Kayal soils (9.7%). CEC of all soils except Kayal soil showed significant positive and linear relationship with SOM as well as with its components. The lack of such a relationship in Kayal soil is attributed to the existence of stable clay-humus complexes. Karappadam soils showed the highest content of total nitrogen and lateritic alluvial soils the lowest. Available nitrogen was highest in Kari soils and lowest in Karappadam soils. In all the four soil types, SOM contributed to more than 90% of total and available nitrogen and it is positively correlated to total and available nitrogen. Total phosphorous was highest in Kayal soils and lowest in Kari soils. Inspite of the lowest content of SOM in Kari soils, it made the maximum contribution to total phosphorous (35.2%). In the Kayal soils the contribution of SOM to total phosphorus was minimum and representing only 5.4%. The available phosphorous in all the soil types showed maximum degree of variability (85.1 to 160.8%) between samples and the contribution of SOM to available phosphorus was also very low. It showed a negative correlation with SOM and its fractions in Kari, Karappadam and Kayal soils probably due to the strong linkage of phosphorus to humus via iron and aluminium bridges. In lateritic alluvial soils, the relationship between SOM and available phosphorus was positive. The total and exchangeable potassium, calcium and magnesium are positively correlated to SOM and its fractions. However these are influenced only to a very low degree by the SOM content. The pattern of relationship of each soil property with SOM is the same as that with its fractions and hence, in studies on soil organic matter, the derivation of relationship between soil properties and soil organic matter alone is sufficient to bring out the interaction between soil organic matter and components of soil fertility.