Soil Modifiers (Ca-Zeolite and Gypsum) as Ecosystem Engineers in Soils of Humid and Semi-arid Tropical Climates

Abstract

The presence of soil modifiers such as Ca-zeolites and gypsum in Indian tropical soils was reported few decades back by the researchers of the Indian Council of Agricultural Research-National Bureau of Soil Survey and Land Use Planning (ICAR-NBSS&LUP), Nagpur, to highlight their unique roles in fine-tuning the exiting soil classification scheme and also the management practices to enhance and sustain the soils’ productivity. But their specific role in ecosystem services as both sustained and temporary ecosystem engineers remained to be evaluated. Both Ca-zeolite and gypsum release Ca2+ ions, which make soils fairly saturated with Ca ions. A hypothesis on the unique role of Ca-zeolites (heulandite) was proposed in the early 1990s on the formation and persistence of several million years old acidic ferruginous Inceptisols, Vertisols, Alfisols and Mollisols in intense leaching environment of humid tropical (HT) climate on the Deccan basalt rock system in Pune district of the north-western part of the Western Ghats in the state of Maharashtra and in Mandla district of the Satpura ranges in the state of Madhya Pradesh. The formation and persistence of these million years old acid soils then led soil researchers to hypothesize that the favourable release of Ca ions from heulandite prevented the progressive pedogenesis towards the formation of Ultisols and also prevented the soils from losing their productivity. This may suggest that Ca- zeolites as prolonged ecosystem engineers would also support a sustained ecosystem services to agriculture, horticulture and forestry enterprises in similar soils under HT climate in other districts of the Western Ghats (WG) and the Konkan Region (KR) representing the Deccan basalt geology. Validation of this hypothesis was made on similar soils of Satara and Kolhapur districts of the WG and the Ratnagiri districts of the KR areas, and it was observed that the soils developed on the Deccan basalt support agriculture, forestry and mango crop as natural vegetation and also as cultivated system in the vast area of the WG and KR regions. Irrespective of their physiographic regions, ferruginous soils of both the WG and KR show highly comparable physical, chemical and mineralogical characters, which indicate that the rate of soil weathering remained almost similar under the dominating effect of HT climate prevailing for the last several million years. The acid ferruginous soils of the Satara and Kolhapur districts of the WG and the Ratnagiri districts of the KR areas are also endowed with Ca-rich zeolites (heulandite) derived from the Deccan basalt similar to those of the Pune district of the WG. Constant slow release of Ca2+ ions from heulandite prevented the total transformation of the smectite (the first weathering product of the basalt) to pure kaolinite even under intense weathering environment of the HT climate. Instead of phasing towards the formation of kaolinitic/gibbsitic Ultisols/Oxisols, soil formation yielded acidic but organic carbon (OC)-rich Inceptisols and Alfisols and also Mollisols with abundant kaolin, which is an interstratified mineral consisting of 0.7 mineral with hydroxy-interlayered smectite (Sm/K). In semi-arid tropical (SAT) environment, Ca-zeolites exhibit their ecosystem services by creating a favourable drainage even amidst sodicity in Vertisols. They help in redefining sodic soils and support good agricultural practices by retaining NH4+ions in their crystal structure and releasing K ions into soil solution and by not fixing solution P. But in some Vertisols, even Ca-zeolites are unable to arrest the adverse effect of regressive pedogenesis because Ca-zeolites behaved as transitory ecosystem engineers, and thus the soils turned out to be sodic in the SAT environment. On the contrary, gypsum as a soil modifier could arrest the development of sodicity in SAT Vertisols by providing enough soluble Ca ions which helped to overcome the ill effects of regressive pedogenesis and also transformed the soils into a vibrant and productive agricultural system. The present sustainability of crop productivity in SAT Vertisols would however be further improved by using good-quality irrigation water as gypsum has been available for the last several thousands of years as a better ecosystem engineer than Ca-zeolites.