Seeding alfalfa in saline soils
Salinity is a limiting factor in crop production in many regions of the Northern Great Plains. Plant performance can vary dramatically on locations affected by increased salt and sodium concentrations. Most of the salinity problems are caused from the accumulation of soluble salts in the root zone. High salts will reduce plant growth and vigor by altering water uptake and causing ion-specific toxicities or imbalances. Poor management will be the cause of more serious problems for crop production under high saline conditions. The excess of salts injured plants by disrupting the uptake of water into the roots.
Alfalfa for example, tends to be highly adaptive to different soil conditions, but it grows best in level, well-drained and deep soils such as loam or sandy loam. Deep soils help establishment and development of alfalfa’s extensive tap root system, which is capable of utilizing water and nutrients from a large volume of soil. The soil acidity level should be neutral or slightly alkaline (pH of 7.0 to 7.5). Alfalfa is not suited to acidic soils (pH < 6.5) and low-lying land where water stands. However, alfalfa can be grown in soils that are slightly saline (sodium chloride), sodic (sodium carbonate) or alkaline (pH > 7). Interestingly, alfalfa has a moderate salt tolerance when compared to other forage species, so it can be grown in slightly saline soils. Overall, alfalfa has varying levels of salt tolerance, being most sensitive to salt in soil in the seeding stage, but generally becoming more resistance to salt as the alfalfa plant matures.
What are saline and sodic soils?
Saline soils will contain large amounts of water soluble salts that inhibit seed germination and plant growth. Soils with high sodium concentration are not a problem if the soil has a very high concentration of calcium and magnesium. On the other hand, soils high in sodium will be a problem if the calcium and magnesium concentrations are low. Therefore, soils that have a high sodium concentration will have a high pH level as well.
Most of perennial forages, especially cool season grasses, have highest tolerance to saline soils. Research has shown that the best time to seed forage or even a cover crop in salt-affected soils is in late fall (mid- October to December), as a dormant season. Plant species with larger seed, such as tall wheatgrass, have greater seedling vigor and are easier to establish on salty areas. Other example to consider is barley, which is a salt tolerant small grain that can work well as a cover crop. Oats, sudangrass, and sorghum are moderately salt sensitive. Overall, plant species with larger seed, such as tall wheatgrass, have greater seedling vigor and are easier to establish on salty areas when compared to alfalfa.
Management of alfalfa stands growing in saline and saline-sodic soils
Generally the two approaches that will help to alleviate the problem of crops growing in saline and saline-sodic soils are: (1) to plant better adaptive varieties or switching over to a different variety which is better adapted; and (2) drainage with intermittent water supply of good quality (low dissolved salts). If soil data shows an exchangeable sodium percentage (ESP) higher than 15%, this will be an indicator of severe Na problems. The only remedy for this type of situation is to remove the sodium from the exchange sites and move the sodium out of the root zone. This can be done by applying gypsum (CaSO4) and leaching with good quality water or letting rainwater take the sodium deeper into the soil if the water table in the soil is deep enough. Other management considerations involve: (1) managing the soil surface to minimize soil water evaporation and concentration of salts; and (2) the use of cool season grasses which are more salt tolerant than other species.