Plants may be wilted, even though soil moisture seems adequate. Leaves may look dull, and the leaf tips or margins may appear to be grayish. Plants suffering from salt stress can be stunted and ark blue-green in color, with tip burn and firing on the leaf margins. If the salt toxicity is severe, the tissue along the leaf margins may become yellow and die.
About saline soils
All soils contain salts, but salinity becomes a problem only when certain salts concentrate in the crop's rooting zone. Sodium chloride or table salt is sometimes the problem but other salts can be. Salt can destroy soil structure causing swelling of clays and dispersion of fine particles that then clog the pores in the soil through which oxygen and water move. It also encourages crust formation.
The key to control of salt in agricultural soils is to hold or leach the salt below the root zone. Keeping a net downward movement of water through the soil does this. Even slightly saline water can be used for leaching purposes. Problems occur when the direction of water flow reverses to an upward movement as occurs with rising water tables. Salt can also migrate upwards to the surface by capillarity, climbing slowly through the fine pores. This is a particular problem if the water table is already high and is salty, as the salt does not have far to creep to destroy the whole rooting zone. White salt crystals can be seen at the surface when the soil dries.
When plants grow in salty soils their growth is reduced; leaves and tillers do not expand. As salt accumulates in tissues it kills the leaves and eventually whole plants. The higher the salinity level the more rapidly effects become obvious and the lower the yield. The accompanying photograph shows poor growth and early death of plants in a salty patch in a wheat crop. Salt crystals are on the soil surface.
There are considerable variations in tolerance to salt. Bread wheat is more tolerant than durum wheat and than species such as rice and maize. Some varieties of bread wheat are more tolerant than others and plants filling their grain are more tolerant than seedlings (Rawson et al 1988).
Sodic soils deserve a mention here. They are not saline as such but do contain relatively high levels of sodium. This causes them to be physically unstable, crumbling and cracking when dry and collapsing when wet. They set hard and are relatively impermeable to water so that run off is a problem carrying with it suspended clay, organic matter and nutrients. High sodium irrigation water should not be used if these soils are highly impermeable and care should be taken not to over water otherwise the soil will rapidly worsen (Russell 1961). One way to improve these alkaline soils is to add gypsum and use deep-rooted crops such as legumes and canola in the rotation to move the gypsum down the soil profile and improve drainage. Sulphur is also used to acidify the soil.
Is your soil salty?
- Look for whitish salt crystals on the dry soil particularly on tops of ridges. Touch them with a dampened finger and taste it to confirm salt.
- Salt concentrations within a field are rarely uniform; therefore, one of the first symptoms indicating a salt problem is variability in crop growth within the field (see pictures below).
- Are there bare soil patches that remain wet or boggy for days after irrigation? Are there patches of crop with reduced growth and yellowing leaves?
- Check for groups of plants that appear limp in spite of having adequate water, and with leaves that are dull, lacking the shine of healthy leaves.
- Is tillering reduced and is there an unusually high proportion of older leaves that are dead? Check below for number of tillers needed for a normal crop.
- Do you have a rising water table less than 2 metres from the surface? Is the water salty to taste? Dig to the water table for a sample and keep a handful of soil from every 30 cm as you dig down. Label the samples with their depth and do the taste test on each sample.
Causes of soil salinity
- Your soil is inherently saline.
- Your irrigation water is saline and has been applied in too little quantity to flush the soil.
- Drainage is inadequate so that a net downward movement of water through the soil is not achieved.
- Too much irrigation water is used and this accumulates as a water table over a shallow impermeable subsoil.
- There is a high and rising water table lifting salt from depth.
- High-transpiring, deep-rooted plants have been cleared from the vicinity allowing water tables to rise, bringing salt with them.
What you can do about salty soil?
- Test water: If you are worried about overly saline irrigation water, send a sample away for measurement of electrical conductivity (EC) to indicate its level of salinity.
- Taste water: If you suspect salinity because of areas of poor growth in your crop, put some soil in a container and add some clean water to more than cover the soil. Shake it up. When the water above the soil clears, taste it. If it doesn't taste at all salty or very slightly salty, the problem is not salt. If it extremely brackish, you certainly have a problem. See whether any of the following solutions can be used. (Taste test from Rana Munns.)
- Leach: Flush the soil system with heavy infrequent irrigations rather than light frequent applications. If the soil is already very saline, fresh water should not be used for leaching. Water without salt could destroy soil structure with formation of crusts over a soupy soil that forms cracks on drying.
- Drainage: Improve drainage by deep cultivation and incorporation of organic matter to ensure a net downward flow of the irrigation water to leach salts.
- Water table: Check whether your water table is saline by the taste test. Do your samples indicate it is getting less or more salty towards the surface? If it is not more salty, work on lowering the water table. If it is more salty, concentrate initially on flushing the salt downwards.
- Gypsum: If soil tests show that the soil sodium concentration is high, add calcium, usually as gypsum, to replace the exchangeable sodium in the soil.
- Leveling: Level the field so that particular areas do not remain wet for prolonged periods.
- Mulch: Use mulches to reduce evaporation from the soil surface.
- Variety: Do the above but also change to a more tolerant genotype or crop species.