(a) On the basis of Boron content, classify irrigation water suitability for various soils and crops. (10 marks) (b) Categorise…

(a) Classification of irrigation water on the basis of boron (10 marks)

Boron is essential in trace amounts but is toxic to plants when in excess, and the margin between deficiency and toxicity is narrow. Toxicity appears as yellowing and scorching of leaf tips and margins, starting on older leaves, and leaf drop. Crops differ greatly in tolerance, so Wilcox (1955) classified irrigation water by boron concentration (parts per million) for three groups of crops:

• Class 1, excellent: sensitive crops below 0.33; semi-tolerant crops below 0.67; tolerant crops below 1.0.
• Class 2, good: sensitive 0.33 to 0.67; semi-tolerant 0.67 to 1.33; tolerant 1.0 to 2.0.
• Class 3, permissible: sensitive 0.67 to 1.00; semi-tolerant 1.33 to 2.00; tolerant 2.0 to 3.0.
• Class 4, doubtful: sensitive 1.00 to 1.25; semi-tolerant 2.00 to 2.50; tolerant 3.0 to 3.75.
• Class 5, unsuitable: sensitive above 1.25; semi-tolerant above 2.50; tolerant above 3.75.

Sensitive crops include citrus, stone fruits, apple, pear, grape, walnut and beans. Semi-tolerant crops include wheat, barley, maize, cotton, oats, potato, tomato and sunflower. Tolerant crops include lucerne, sugar beet, cabbage, carrot, asparagus, and date palm. Fine-textured and organic soils adsorb boron and reduce the risk, and leaching with good water removes it.

(b) Industrial effluents and their effects on soil (20 marks)

Categories of industrial effluents, by industry and nature of pollutant, are:

• Organic-rich effluents from sugar mills, distilleries (spent wash), dairies, paper and pulp, food processing and tanneries, with high BOD and COD.
• Inorganic and chemical effluents from fertiliser, chemical, pharmaceutical, textile and dye units, with acids, alkalis, salts and high total dissolved solids.
• Heavy-metal-bearing effluents from electroplating, batteries, tanneries (chromium), mining and metallurgy, with cadmium, lead, chromium, nickel, zinc, copper and mercury.
• Thermal effluent from power plants and cooling water; and radioactive waste from nuclear and mining operations.

By toxicity, effluents are toxic (metals, cyanides, phenols, pesticides) or non-toxic (mostly organic or nutrient-rich); and by physical form they are suspended, colloidal or dissolved.

Effects on soil properties:

• Physical: suspended solids, oils and grease clog pores and reduce infiltration and aeration; sodium in the effluent disperses clay and destroys soil structure and leads to crusting and low permeability; effluent with high organic load may waterlog and make the soil anaerobic.
• Chemical: acidic or alkaline effluent alters soil pH; high salt content raises electrical conductivity (salinisation) and sodium adsorption ratio (sodicity); heavy metals accumulate in the soil and are taken up by crops, entering the food chain; some effluents (spent wash, sugar mill and dairy effluent) add nitrogen, potassium and organic matter, which is a benefit if diluted and applied at a controlled rate, but excess causes nutrient imbalance and nitrate pollution of groundwater.
• Biological: toxic effluents reduce the population and enzyme activity of soil microorganisms and earthworms and inhibit nitrogen fixation and nitrification, while organic effluents may temporarily increase microbial activity.
• Crop effects: poor germination, stunted growth, reduced yield and quality, and contaminated produce.

Effluents therefore need treatment to meet standards before use for irrigation, with monitoring of the soil.

(c) Dryland agriculture in India (20 marks)

Problems. Dryland or rainfed agriculture covers about half of India's net sown area and supplies a large share of the country's coarse cereals, pulses, oilseeds and cotton. Its problems:

• Erratic and uncertain rainfall, with delayed onset, long dry spells and early withdrawal of the monsoon, causing frequent drought and crop failure.
• Poor soils: shallow, coarse or heavy, with low water-holding capacity and organic matter, and susceptibility to water and wind erosion, crusting and low fertility.
• High evaporative demand and runoff losses; the soil moisture is inadequately conserved.
• Low input use: less fertiliser, poor seed and use of low-yielding local varieties, with low yields and high variability.
• Small, fragmented holdings and low capacity to take risk; poor credit, insurance, markets and infrastructure.
• Pest and weed problems under moisture stress, and low adoption of technology.

Ways to increase production and productivity:

• In-situ moisture conservation: contour bunding, compartmental bunds, broad-bed and furrow, ridge-and-furrow and conservation furrows, mulching, and deep summer ploughing.
• Rainwater harvesting and recycling: farm ponds and check dams, with supplemental irrigation at critical stages, using drip or sprinklers.
• Crop planning: drought-tolerant crops and short-duration varieties (pearl millet, sorghum, pigeonpea, groundnut, castor), intercropping and mixed cropping (pearl millet with pigeonpea), timely sowing, seed treatment and hardening, and contingency crop plans for delayed monsoon.
• Nutrient management: integrated use of farmyard manure and fertiliser, in the right time and placement, with legumes in rotation.
• Weed control, thinning of plant population, and anti-transpirants, and mid-season correction in dry spells.
• Watershed management: the watershed approach for soil and water conservation on a community basis, as in watershed development under PMKSY.
• Alternative land use: agroforestry, silvipasture, horticulture such as ber, custard apple and pomegranate, and livestock integration, as risk-spreading.
• Support: crop insurance, weather advisories, credit, remunerative prices for millets and pulses, and research through All India Coordinated Research Project on Dryland Agriculture and CRIDA.