Amelioration of subsoil acidity by calcium sources in laterite soils of black peper garden

Abstract

Soil acidity is a major problem in humid tropical regions due to high rainfall and temperature. Hydrogen and aluminium are the major ions responsible for soil acidity. Historically, soil scientists and agronomists have addressed the problem of soil acidity and recommend amelioration by conventional liming and ploughing. Black pepper, an important and widely used spice around the globe, is cultivated widely in our state. In Kerala, this crop is grown in laterite soils, which poses many soil related stress of which soil acidity is a major one. The productivity of pepper is very low in these tracts, and lower compared to other places. High exchangeable Al and low Ca content in subsurface horizons act as barriers for the root growth of black pepper towards lower layers. The effect of conventionally surface applied liming materials like CaCO3, Ca(OH)2 will be confined to the top layer alone. While in materials like Phosphogypsum, Ca is soluble and can move to lower depths and offer possibility of ameliorating subsoil layers. Isotopic techniques are useful for a quick and reliable means of studying the movement of ameliorants through the soil and also to examine the distribution of active roots at lower depth of soil column without destroying the plant.

With this background, an investigation was carried out at College Of Horticulture, Vellanikkara about the subsoil acidity amelioration in laterite soil of black pepper garden using three calcium sources- CaCO3, Ca(OH)2 and Phosphogypsum. The whole study was conducted as 5 experiments using the soil collected from the pepper garden, College of Horticulture, Vellanikkara. Analysis of soil sample revealed that the exchangeable aluminium content was 69 ppm at the subsoil layer is in significantly higher concentration than the surface. On the basis of this an incubation experiment using three calcium sources, lime, slaked lime and Phosphogypsum was done and the results revealed that lime is more effective in increasing the pH while Phosphogypsum is effective for reducing the exchangeable Al in soils. In continuation to this soil column study using PVC columns filled with soil layers simulating field condition revealed that liming at 1 LR level was better for good plant growth. The effect of three sources on ameliorating subsoil acidity was evaluated by measuring the root activity of pepper plants grown in the columns by isotopic method. For this 32P was applied at a depth of 50 cm depth and the counts on leaf after a period of 8 days were taken as an indication of presence of active roots at 50 cm depth. The counts obtained from the leaf sample of black pepper revealed that count rates increased with increase in level of application of liming materials. In soil columns treated with phosphogypsum, significantly higher counts were noticed which indicates better root growth at subsurface layer of the PG treated columns.

This result was confirmed by performing a leaching experiment in PVC columns using 45Ca labelled ameliorants. Radio assay and autoradiography done on this experiment also proved that, in Phosphogypsum, Ca is highly mobile compared to CaCO3 and Ca(OH)2. In order to understand the response and tolerance level of Al on pepper plants specifically on roots a solution culture experiment was also done by growing rooted plants in Hoagland solution containing different levels of Al. Solution culture experiment proved that the pepper root tolerates an Al concentration of 5 and 10 ppm and beyond this level plants die off and roots decay. How ever at 5 ppm level of Al profuse root growth was noticed. The anatomical observation of the roots were also done and some modification in the tissue orientation is noticed.

On the basis of this investigation it can be concluded that 1. A sub surface zone with high concentration of exchangeable Al exists in laterite soil of the pepper garden of College of Horticulture. 2. Phosphogysum offers a potential option for ameliorating the subsoil layers and to promote root growth of black pepper to deeper soil layers. 3. Some promoting effect on black pepper root growth is noticed at 5 ppm Al, in solution culture.

On the basis of these observations it is suggested that further investigations are needed on other soil types and also to validate by field trials. The acidic nature of PG at the zone of its application has to be contained by blending this material with CaCO3 or Ca(OH)2. The biochemical responses of the black pepper plant to exposure to Al, needs to be studied in detail by elaborate experiments.