Studies on the chemo dynamics of phosphorus in the laterite soils of Kerala and utilization of fixed phosphorus for crop production

Abstract

A study entitled “Studies on the chemo dynamics of phosphorus in the laterite soils of Kerala and utilization of fixed phosphorus for crop production” was carried out at the Department of Soil Science and Agricultural Chemistry, College of Agriculture, Vellayani, during 2018-2021, with the objectives to characterize in detail the dynamics of inorganic phosphorus and phosphorus fractions in the laterite soils of Kerala, identify the dominant phytoaccumulators of phosphorus, screening of suitable extractants for available P, and evaluate various organic and inorganic solubilizers/mobilizers on the solubility and availability of fixed phosphorus. The study was carried out in four parts. The first part consisted of the collection and characterization of georeferenced soil samples and the identification of dominant phytoaccumulators of phosphorus (P). For that, by following a preliminary survey, a total of 100 georeferenced representative soil samples (0-15 cm depth), 25 from each of the four agro-ecological units (AEUs), viz., southern laterites (AEU 8) of Thiruvananthapuram district, south central laterites (AEU 9) of Kollam district, north central laterites (AEU 10) of Thrissur district, and northern laterites (AEU 11) of Kozhikode district of Kerala were collected for assessing its fertility status. From scrutiny of the data, it was observed that among the four AEUs, AEU 8 reported the highest values for electrical conductivity (EC), organic carbon (OC), available P, sulphur (S), iron (Fe), manganese (Mn), exchangeable hydrogen (H+), anion exchange capacity (AEC), microbial biomass carbon (MBC), microbial biomass phosphorus (MBP), and population of P solubilizers, AEU 9 for available boron (B), exchangeable aluminium (Al3+), hydrogen (H+), acidity, and MB C/P ratio, and AEU 11 for pH, available nitrogen (N), potassium (K), calcium (Ca), magnesium (Mg), zinc (Zn), and copper (Cu). According to the results of frequency distribution, it was observed that, among the four AEUs (8, 9, 10, and 11), sandy clay loam was the prevalent soil textural class. Data on the mean values of pH revealed that the pH ranged between 5.42 and 6.21 among the four AEUs, and the majority of the collected soils were found to be strongly acidic (37%) and moderately acidic (40%) in nature. The EC values of the four AEUs were found to be within the critical limit. The mean values of OC varied from 0.63 From the present study, it can be concluded that the soils of southern laterites (AEU 8) exhibited the highest values for available P. Amaranthus viridis from AEUs 8, 9, and 10, and Synedrella nodiflora from AEU 11, were identified as dominant phytoaccumulators of P. Among the various fractions of P, with respect to the Po fractions such as the labile Po fraction, HCl extractable Po fraction, fulvic acid Po fraction, moderately labile Po fraction, humic acid Po fraction, residual Po fraction, and non-labile Po fraction, the soils collected from AEU 8 showed the highest values compared to other soils. It was also observed that the soils treated with vermicompost @ 15 t ha-1 showed the highest values for all the Po fractions when compared to soils without any manure, including total Po. Similarly, for Pi fractions, the soils collected from AEU 8 treated with vermicompost @ 15 t ha-1 showed the highest values for saloid-Pi, Al-Pi, and Fe-Pi, while the soils collected from AEU 8 without manure showed the highest values for residual/occluded Pi. Furthermore, soils treated with vermicompost @ 15 t ha-1 displayed the highest values for Ca-Pi in AEU 10 and reductant-soluble Pi in AEU 11. Notably, the soils of AEU 8 exhibited the highest capacity for P fixation among all studied soils. From the study, it was also observed that ion exchange resins were found to be the best extractants for available P determination, and biochar was the best amendment for alleviating P fixation, thus augmenting soil fertility