Soil salinity is a widespread environmental problem, particularly in arid and semi-arid regions of the world. In Pakistan, about 6.3 million hectares of land are affected by salinity and ground water in most of these saline areas is also brackish and thus unfit for irrigation. Reclamation of large areas of saline lands seems difficult due to many climatic and economic factors such as shortage of fresh water for leaching of salts, poor natural drainage, high costs for construction and maintenance of drainage systems, etc. However, salt-affected lands and brackish water could be utilized for cultivation of salt tolerant plants. Use of halophyte species, that accumulate high amounts of salt, has been reported to improve saline sodic soils. Kallar grass (Leplochloa fusca [L.] Kunth), a highly salinity-tolerant species, is used as a primary colonizer for plant establishment and biomass production on saline sodic lands. The growth of kallar grass not only provides biomass to be used as forage but also ameliorates soil conditions, thus facilitating the growth of other species in succession and improving the general environment. However, systematic studies describing successive changes in soil properties of the saline lands after kallar grass planting are scanty in the literature. Sustainability of growing plants on saline sodic soils with saline water irrigation has not been thoroughly investigated. The question of how long term use of saline irrigation water will, affect, deteriorate or ameliorate, the soil environment already degraded due to salinity still remains unanswered. Studies were, therefore, conducted to monitor the changes, in physical, chemical and mineralogical properties of a saline-sodic soil profile in reclamation fields under kallar grass.
The soil was highly saline (ECe=22 dS m-1), sodic (SAR=184.4), alkaline (pH=10.4), low in organic matter (3.3 gKg-1) with poor soil physical properties. Kallar grass was grown for five years, irrigated with saline under ground water (ECe=1.4 dSm-l, SARadj=19.5 and RSC=9.66).
The cropping of kallar grass improved appreciably physico-chemical properties of soil within a period of three years. The growth of grass increased significantly and linearly (r =0.974**) the plant available water due to increase in water retention at various applied pressure tensions. The increase in plant available water was found to be highly correlated (0.922**) to an increase in organic matter content, porosity (0.989**) and most of the soil physical properties (hydraulic permeability, structural stability. specific surface area) and was inversely correlated with soil salinity, pH sodium adsorption ratio (SAR), soluble cations and anions. The hydraulic permeability increased exponentially (InKfs =-0.421+0.982T) with growth of kallar grass and was significantly correlated with soil porosity, water retention, structural stability and organic matter content.
Soil salinity, sodicity and pH decreased exponentially by growing kallar grass due to leaching of salts from surface (0-20 cm) to lower depths (> 1 00 cm). The reduction in soluble cations (Na+, K+, Ca2+ and Mg2+) and anions (C1-, SO42-, HCO3-) occurred due to leaching from upper to lower depths of soil. A significant decline in soil pH was attributed to release of CO2 by grass roots and solublization of CaCO3. Both soil salinity measured as electrical conductivity (ECe) and soil pH were significantly correlated with most of the chemical characteristics Na+, Ca2+, Mg2+, K+, C1-,HCO3-, and sodium adsorption ratio (SAR). Significant correlations were found between soluble cations (Na+, Ca2+ and K+), soluble anions (C1-, SO42- and HCO3-) and sodium adsorption ratio. In contrast, the results showed negative correlations between soil organic matter content (OM) and all chemical properties. The cropping of kallar grass resulted in an increase of soil organic matter content linearly (r=0.924**). The soil organic matter content exhibited highly significant relationships (r>0.820*) with available water, porosity, structural stabilty index, hydraulic permeability and specific surface area confirming its effectiveness to improve deteriorated soil physical properties.
The uncropped control soil showed a younger mineralogy with early stage of weathering. The clay fraction of control soil contained a mixture of mica, mainly illite (44.8 %) smectite (24.5 %), kaolinite (10 %), chlorite (7.5 %) with minor amounts of hematite/goethite (5.5 %), quartz (4 %) and traces of feldspar. The growth of kallar grass enhanced soil weathering with the increase in smectite, chlorite and hematite/goethite at the expense of illite and kaolinite. The elemental composition of clay fraction in control soil revealed that SiO2 dominated with an average of 53% followed by A12O3 (23%) and Fe2O3 (8%). The A12O3 increased in clay fraction due to its incorporation in the formation of depths of 1:2 expansible clay with the growth of kallar grass. The SiO2 and K2O decreased while, Fe2O3 and MgO increased with advancement of weathering by growing kallar grass compared to uncropped soil.
The illite content exhibited positive correlation with kaolinite, SAR, pH, Electrical conductivity (ECe), and inverse relations with most of the soil physical properties considered in this study. Smectite showed a high correlation with soil organic matter content and soil physical parameters and inverse relations with SAR, ECe and pH under kallar grass cropping. Smectite also showed high correlation with oxides of Mg, Ca, Al and Fe. The transformation of 1:1 type clay to 2:1 type expansible clay due to weathering improved soil physical properties and dispersed fraction of colloidal particles leached down alongwith soluble ions. The increase in smectite content may have enhanced the availability of both micro and macro nutrients due to its higher retention capacity.
The study confirmed that salt-affected soils can be improved effectively with biological approach, i.e. by growing salt tolerant plants. Kallar grass showed a tremendous potential to improve most of the physical, chemical and mineralogical properties without any adverse effects of saline water on soil properties. The growth of kallar grass enhanced leaching and interactions among the physical, chemical and mineralogical soil prope11ies and restored soil fertility The ameliorative effects on soil properties were more pronounced after three years of growing kallar grass. Soil maintained the improved characteristics with further growth of the grass up to five years suggesting that growing salt tolerant plants is a sustainable approach for biological amelioration of saline wastelands.