Assessment of soil health and status of heavy metals in the certified organic farms of Kerala

Abstract

The study entitled “Assessment of soil health and status of heavy metals in the certified organic farms of Kerala” was conducted from 2017 to 2019. The objective of the study was to assess and compare the soil health parameters and heavy metal status of the inputs and produce of certified organic farms and conventional farms of Kerala. Samples of soil, inputs and plant produce were collected from different locations of Kerala representing Northern, Hill, Central and Southern zones. The organic farms selected for study were under NPOP certification for more than 10 years. The rate of application of manures in organic and conventional farms were 30 t ha-1 and 2 t ha-1 respectively. The first part of investigation was conducted to comparatively evaluate the effect of organic and conventional farming on soil health, for which composite soil samples from 2 depths (0-15 cm and 15-30 cm) were collected from 10 numbers each of certified organic farms and conventional farms. The collected soil samples were analysed for physical, chemical and biological properties from which soil quality index was calculated for each sample and an in situ enumeration of earthworms and arthropods was conducted by excavating 1 m3 soil in all the farms from where soil samples were collected. Among the physical and chemical properties analysed, available N, K, Ca, Zn and Cu at 0-15 cm depth were significantly higher in soils from organic farms than conventional farms. Bulk density was found to be lower in soils of organic farms which is a good soil quality attribute. pH was significantly lower in soils of organic farms at 0-15 cm depth. Total as well as available content of heavy metals were (Ni, Cd and Pb) were not significantly different among the organic and conventional soil samples. Unlike physical and chemical properties, all biological properties were significantly higher in soils of organic farms than conventional farms. Bacterial, fungal, actinomycetes count, earthworms, soil arthropods and dehydrogenase activity were found to be significantly higher in soils of organic farms than in conventional farms at both depths and respiratory activity was found to be significantly higher in organic farms at 0-15 cm depth. Soil quality index was calculated from various physical, chemical and biological soil properties and the highest index (61.1 per cent) was obtained in the surface soils of hill zone under both management. However the highest soil quality index was observed only in soils of organic farms in Northern and Southern zones. The second part of investigation was input analysis in which inputs used in organic and conventional farms, available at the time of sampling were analysed for Zn, Cu, Cd, Ni and Pb. The results revealed that except for the content of Cu in manures and fertilizers, other heavy metals were not observed to be significantly different in manures and fertilizers. The third part of investigation was plant study, in which edible parts of 10 crops were collected from both organic and conventional farms and analysed for Zn, Cu, Cd, Ni and Pb. Except for Zn, other heavy metals were found to be very much within the limit of food safety standards. Zn was found to be high in amaranthus from both organic (96 mg kg-1) and conventional (105.6 mg kg-1) farms. Content of Zn in yard long bean, salad cucumber and curry leaf from organic farms (60-64 mg kg-1) and was found to be just exceeding the safe limit (60 mg kg-1). Zn in plants was found to be significantly affected by management practices also. A significant positive correlation have been observed between the content of Zn in plants and available Zn in soil in both conventional and organically managed soils. From the study it may be concluded that soil quality was found to be generally high in hill zone soils compared to other zones irrespective of the management practice and in all other regions, organic farming was found to be increasing the soil quality. Heavy metals in inputs and plant produce analysed were very much within safety standards. Zn content of amaranthus in organic and conventional farms exceeded the food safety limit. A significantly higher biological properties of soils in organic farms clearly manifests the long term sustainability of organic farming in Kerala’s climatic condition.