Agroforestry rejuvenates saline soils using saline irrigation

J.C. Dagar ([email protected]), O. S. Tomar and P. S. Minhas

There seems to be little hope for bringing fertile arable lands under forestry, particularly in developing countries where food production for the ever-increasing population is a primary concern. Yet, afforestation is not only a necessity for reducing pressure on natural forests, but also a most desired land use, especially for reclaiming and rehabilitating degraded lands, and particularly saline soils.

According to the Food and Agriculture Organization (FAO) and the United Nations Educational, Scientific and Cultural Organization (UNESCO), the world has 397 million ha of saline soils and 434 million ha of sodic soils. About 12.6 million ha are considered to be under acid-sulphate soils distributed along coastal regions. Of the current 230 million ha of irrigated land, 45 million ha are salt-affected. Of the almost 1 500 million ha of dry land agriculture, 32 million ha are salt-affected in varying degrees by human-induced processes.

Be that as it may, it is possible to increase wasteland productivity in terms of food, fuelwood, forage, medicinal drugs and biodiversity if it is planted with trees. In particular, salt-affected wastelands hold promise for agroforestry. However, without adequate fresh water for irrigation, the dry areas are not arable enough for forest and horticultural crops. But there is plenty of water beneath many of the world's deserts with saline water reserves.

Studies conducted at the Central Soil Salinity Research Institute (CSSRI) in Karnal, India, showed that salt-affected soils and saline waters can be satisfactorily utilized in raising some forest and fruit tree species, forage grasses, conventional and nonconventional crops, oil-yielding crops, aromatic and medicinal plants of high economic value, petro-crops and flower-yielding plants using appropriate techniques.

Salt-affected soils and saline waters

At least five groups of salt-affected soils are influenced by different chemical types of electrolytes, which not only represent different salts, but also have different effects on soil formation. For practical use, salt-affected lands in the Indian subcontinent are identified as saline, alkali/sodic and saline-alkali. CSSRI groups salt-affected soils as either saline or alkaline.

Planting methods and techniques for saline lands

Alkali soil is characterized by a kankar, a hard pan of calcium carbonate forming a layer, which hinders root development. It can be broken up by a tractor-mounted auger pit which digs holes, 15-20 cm in diameter, through the hard kankar layer up to 150-180 cm deep (Fig. 1). These holes are refilled with original soil, 3-5 kg of gypsum (50 percent of the gypsum required for reclamation), 8 kg of farm yard manure, 10 g of zinc sulfate and a small quantity of aldrin to prevent termites. Sodic-tolerant tree saplings, 6-9 months old, are planted in the refilled pit-auger holes, and then irrigated two to three times. This method enables the plant roots to grow at a faster rate toward deeper soil layers where sufficient moisture and nutrients are available.

Many forest and fruit tree species can be raised on highly alkaline soil (pH > 10), but some of them, such as pomegranate (Punica granatum), are unable to tolerate water stagnation. To solve this problem during the monsoon, the raised and sunken-bed technique was found to be quite suitable for agroforestry. The tree species can be successfully grown on raised bunds, and rice-wheat and berseem (Trifolium alexandrium)-kallar grass (Leptochloa fusca) can be grown in rotation on the sunken bed.

Table 1. Relative tolerance of tree species to soil sodicity.

Fig. 1. Auger-hole technique for planting trees in alkali soil.

Planting on saline lands and raising trees with saline irrigation

For the afforestation of saline waterlogged soils, the subsurface planting and furrow irrigation method (SPFIM) was found to be the most appropriate. It improved tree survival and growth. Six-month-old tree saplings were planted in the auger holes in furrows during the rainy season. The V-shaped furrows (20-cm-deep and 60-cm-wide at the top) were made using a tractor-driven furrow maker. Saplings were irrigated with saline water during the early establishment stage. Besides reducing the water application costs, the furrows helped in creating a favorable zone of low salinity through the downward and lateral water fluxes, making salts move away from the root zone, especially when low-saline water was used.

Suitable tree species

Results of long-term field experiments showed that tree species such as Acacia nilotica, Prosopis juliflora and Tamarix articulata were the ideal species for highly alkaline soil (pH >10). Relative performances of the forest and fruit tree species at different pH are shown in Table 1. The biomass of seven-year-old plantations of T. articulata, Acacia nilotica and Prosopis juliflora were 97.3 t/ha, 69.8 t/ha and 51.3 t/ha, respectively. Among the fruit tree species, Zizyphus mauritiana, Syzygium cuminii, Psidium guajava, Emblica officinalis and Carissa carandus grew well in these soils.

For saline waterlogged lands, 40 tree species of arid and semi-arid areas were evaluated. Based on periodical observations of survival, height, girth and biomass, woody species such as Acacia farnesiana, Parkinsonia aculeata and Prosopis juliflora were rated the most tolerant of waterlogged salinity and could be grown satisfactorily on soils with salinity levels of up to 50 dS/m in their root transmission zone. Tree species like Acacia nilotica, Acacia tortilis, Casuarina glauca, Casuarina obesa and Casuarina equisetifolia could grow on sites with ECs varying from 10-25 dS/m.

The experiments concluded that the waterlogged saline conditions affected the survival and growth of trees used for afforestation because salt accumulation near the rooting zone was directly attributed to ground water fluctuations and the saline underground water.

Table 2. Tolerant species for saline soils.

As the most suitable species for saline waterlogged soils, Prosopis juliflora and Casuarina glauca, when planted with subsurface or furrow techniques, had the highest biomass, followed by Acacia nilotica and Acacia tortilis (Table 2). More than 30 species were evaluated in low rainfall areas (annual rainfall about 350 mm) growing in furrows and irrigated with saline water of EC 8-10 dS/m. For three years, the trees were irrigated four to six times. The results showed that various species could be raised on degraded sandy calcareous soil in arid and semi-arid regions.

Fig. 2. Tree species established with saline irrigation.

Ameliorative effect of plantation

The established trees improved the physical, chemical and biological properties of salt-affected soils. The roots of salt-tolerant trees penetrated into the soil and improved permeability, which facilitated salt leaching. Through absorption, the trees were able to exclude salts. However, it was observed that tree plantations could only ameliorate salt-affected soils depending on the soil type. In alkali soils, the ameliorative effects of 20-year-old plantations appeared in the order of Prosopis juliflora > Acacia nilotica > Terminalia arjuna > Albizzia lebbeck > Eucalyptus tereticornis. The results from seven-year-old plantations also indicated that the maximum reduction in ESP and pH was observed under Tamarix articulate, followed by Prosopis juliflora and Acacia nilotica. When raised with saline irrigation, Tamarix articulata had a 0.23% increase in organic carbon in its soil surface layer; Prosopis juliflora, 0.26%; and A. nilotica, 0.10%.

A major benefit that was noticed was that tree plantations lowered the water table in saline waterlogged areas. Based on the seven-year study, it was found that the water table became deeper, an average of 5 cm under the tree canopy, than that in barren lands. Surface-level salts were considerably reduced in soils under tree canopies compared with bare fallow lands. When the trees were raised with saline irrigation, the soil was enriched with organic carbon (>0.4%) in the upper 30 cm and there was no salinity buildup in the profile.

These results showed that the salt-affected soils and poor-quality saline waters could be judiciously utilized for tree plantations. (The authors work at the Central Soil Salinity Research Institute, Karnal, India.)