The salt of the earth

Without good drainage, irrigation farming is threatened by soil salinisation. But in areas where it’s needed most, drainage may only be affordable with government assistance. Roelof Bezuidenhout spoke to Dr Jack Armour.
Issue date : 06 June 2008

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Salinisation destroys soils and reduces crop yields, and the state should help irrigation farmers in the semi-arid Lower Vaal and Riet River catchments combat it. So says Dr Jack Armour of Free State University’s Department of Agricultural Economics. His research focuses on salinisation’s impact on the socioeconomy of irrigated farms, hydrology, and biophysical interactions between plants, soils, salts and water. Dr warns that if incorrectly managed or left unattended, soil salinisation can lead to financial, social and environmental degradation and threaten the major contribution irrigation in dry areas makes to national food security. Salinity takes its toll Salinisation, Dr Armour reminds us, ended the ancient Egyptian and Mesopotamian irrigation-based civilisations.

Modern examples of bad salinisation management include the collapsed Aral Sea project in Kazakhstan and the Salton environmental headache in California. alts in irrigation water aren’t the real problem. Salinisation happens wherever evaporation exceeds rainfall, no matter how good the irrigation water is. Pure water evaporates from the soil surface and transpires through the plants, leaving the salts behind. Without surface runoff or leaching, these salts build to levels that eventually become toxic to plants and break down soil structure.

“In arid irrigation areas, the threat to sustainable farming also comes from mineralisation of salts already in the soil,” explains Dr Armour. “Already higher-lying farms irrigated with good-quality water are accelerating salinisation in lower-lying ones.” Drainage, whether by open-furrow or through underground pipes, combats both waterlogging and salinisation, providing a channel for excess water and salts. Innovations include a reverse-pump drainage system which can also irrigate crops.

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Drainage removes excess surface water, manages shallow water tables by retaining and removing water, and manages water quality to achieve economic and social benefits while safeguarding key ecological functions. Many countries therefore invest public funds in large-scale drainage projects.

The estimated returns are substantial, with crop yields expected to increase significantly. However, if the need for drainage is caused by inadequate water management, then policies that improve water management will also require investment.

However, effective drainage is capital-intensive and often too expensive for irrigation farmers – even when crop losses due to salinisation exceed the direct costs of drainage, says Dr Armour. At a regional level, the improvement in agricultural productivity far outweighs the expense, and has a positive ripple effect throughout the economy. As well as salinisation and waterlogging, Dr Armour says farmers in the study area face increasing environmental control and debt burdens, pressure to increase water-use efficiency, diminishing returns and less state support.

The study area encompasses the areas of the Orange-Vaal and Orange-Riet Water Users Associations, including the Vaal River from downstream of the Bloemhof Dam to the confluence of the Orange River; from the Orange-Riet and Orange-Vaal Canal extraction points downstream of the Vanderkloof Dam to the Orange-Vaal confluence; and the Riet River, downstream of the Kalkfontein Dam, including the confluences with the Modder River and the Vaal. Visit or e-mail Dr Jack Armour at [email protected]. |fw

The maths

According to Dr Armour, a 1ha plot that requires 1 000mm of irrigation water per year uses 10 000m3, or 10 million litres. The salt concentration, or salinity, of the irrigation water is 1 000mg/â„“ total dissolved salts (TDS). This means 10t of salts are added to the soil profile yearly. On medium to heavy soils, the average cost of drainage will be R30 000/ha. A 15-year loan at 9% will cost R3 722/year to service.

This is economically feasible in the Lower Riet River block, but not convincing without a subsidised grant in the other two blocks. The figures give farmers a good indication of the costs of poor drainage on their farms. The biggest loss of R6 962/ha/year was experienced in the Lower Riet Irrigation block, followed by Scholtsberg (R2 596/ha/year) and the Orange Vaal Irrigation block (R2 218/ha/year). Implemented for the whole study area, the total real cost (on a 2005 basis) of salinisation over 15 years is about R995 million – a good benchmark for funds for remedial action.

How SA should fight salinisation

Before new schemes are developed, provision must be made for irrigation drainage, using drains, canals and storage reservoirs to capture and manage the saline irrigation flows from the drains.

This will avert a downstream problem for other farmers, urban and industrial water users and the environment. Government needs to invest in restoring our damaged resource base and make funds and services available for drainage. More agricultural engineers are needed to provide drainage design and installation, and enough agricultural extension officers should be trained to regularly measure soil salinity status and trends, and input this data into a centralised database.

The Department of Agriculture should run salinity awareness programmes and inform farmers about assistance and funding if and when it becomes available. Farmers should buy a salinity meter and track salinisation in their own soils, fertiliser companies should get more involved in taking readings and doing analyses, and buyers should enquire about soil salinity before investing in irrigation land. Insurance companies should be made aware of the potential increased crop risk on salinised soils if not managed correctly.

New technology for high-lysine maize

Many decades of breeding and selection for increased maize yield potential has raised starch levels, but reduced the quantity and quality of protein. Maize grain, therefore, became a staple food and feed with reduced nutritional value. However, a natural mutant gene, opaque-2, opened an era of conventional breeding for increased levels of lysine, an essential amino acid in the diet of humans and several animal species.

However, an improved genetic approach can now achieve much more success. Maize kernel protein is comprised of about 40% α-zein, which contains almost no lysine. In feeds, this deficiency is normally overcome by adding commercially produced crystalline lysine or by mixing in soya meal. The high-lysine mutant gene reduces α-zein while increasing other proteins, boosting lysine levels by 60% to 70%. A negative side-effect is a softer grain. Continuous selection for harder grain has counteracted this, but even so, high-lysine maize, called QPM, hasn’t yet captured a significant market share.

A novel approach, using genetic modification that employs an interference system, RNAi, has doubled lysine content to 5,4% of protein, but with a soft grain, little increase in endosperm lysine and no increase in free lysine in tissue. Adding a bacterial gene that suppresses the breakdown of lysine in the metabolic pathway, in combination with another bifunctional gene, raised free lysine in the endosperm 30-fold and 100-fold in embryo tissue, apparently without sacrificing grain quality. This has achieved the highest lysine level yet and paves the way for raising levels of other amino acids like tryptophan, methionine and threonine. – Wynand van der Walt ([email protected]) Source: ISB News, March 2008. |fw