After the driest year on record, some western alfalfa growers may find themselves battling soil salinity. It is important to consider how dry conditions contribute to soil salinity and how it can be managed.
Salt impairment may be identified by white or black crusts on the soil surface, wet spots on the soil surface, marginal leaf burn, or the presence of salt-tolerant weeds. Salt impairs plant growth in many ways: by exerting osmotic stress that results in decreased turgor pressure in plant cells, degrading soil physical conditions that impair water penetration and the plant"s ability to access water, and specific ion toxicities that vary by plant species. Limited water supplies due to drought and deficit irrigation methods can exacerbate soil salinity, thus magnifying these stresses on plants.
* A brief set of definitions related to salt and salt measurements is in the note at the end of this article.
Alfalfa site selection
Management practices that could help to alleviate salinity effects on alfalfa include site selection, monitoring soil and water salinity, variety selection, and soil salinity management through leaching. The following table provides ideal, marginal and undesirable site characteristics for alfalfa production.
There's an app
A smart phone application called Soil Web can assist in identifying alfalfa sites because it provides this soil information for one"s current location. Search "soil web" in your application store, or visit http://casoilresource.lawr.ucdavis.edu/drupal/node/902 for more information on smart phone and computer interfaces that provide soil information. These interfaces provide easy access to the USDA-NRCS soil surveys and were developed by members of the California Soil Resource Lab at UC Davis. While accessing soil survey information is an important first step in determining appropriate sites, periodically monitoring soil and irrigation water salinity is also important, especially since agricultural practices can change soil characteristics.
Seedling alfalfa is weak, and it is important to try to meet ideal soil conditions. Use the best quality water available on seedling alfalfa if more than one source of irrigation water is available. As the root system and crown develop, alfalfa may be able to tolerate more marginal conditions. A grower with more than one source of irrigation water could try irrigating a mature stand with a mix of good and poor quality water. Current research by Dan Putnam (forage specialist, UC Davis) and others at Fresno State is illustrating that certain alfalfa varieties may have tolerance for salty irrigation water. Tolerance appears to relate to the varieties" affinity for accumulating K+ in the shoots rather than Na+.
Additionally, research that I am conducting in the Delta is showing the importance of leaching even when relatively good quality water is used for irrigation. Some soils in the Delta are increasing in salinity because soil conditions and/or shallow ground water impair leaching. Understanding soil properties and the current salinity profile can help in identifying irrigation practices that could improve leaching. For example, in some fields, the soil at the top of the border check has lower salinity than that in the middle and bottom of the check. If irrigation can be modified to increase the irrigation opportunity time on the middle and bottom of the field, this may decrease salinity. This would be an appropriate strategy to try on sandy soils, but it could be risky on soils where water does not penetrate the surface well and Phytophthora infection could occur.
Another leaching strategy actually involves irrigating in the winter. In years when rainfall is sparse, irrigating before a storm will fill the soil profile and allow rain water to leach, rather than just wet the profile. Leveraging winter rainfall with irrigation could help to lower baseline soil salinity in the spring.
To access a presentation at the Alfalfa and Forage Field Day held at the Kearney Agricultural Center, http://alfalfa.ucdavis.edu/FieldDay/2014/KAC.aspx.
* Some soils are salty because parent materials weather to positively-charged cations (Ca2+, Mg2+, K+, and Na+) that join with negatively-charged anions to form soluble salts (NaCl, CaCl2, MgCl2, CaSO4, and KCl). On croplands, salts may be carried in irrigation water to create or exacerbate salty soil conditions.
The electrical conductivity (EC), sodium adsorption ratio (SAR), and exchangeable sodium percentage (ESP) characterize the degree to which soils are affected by salt. Electrical conductivity is a measure of a solution"s ability to conduct an electric current. When the solution comes from a soil saturated paste, the abbreviation used is ECe, and when the solution is water, the abbreviation is ECw. Electrical conductivity is generally expressed in units of decisiemens per meter (dS/m). The SAR describes the concentration of Na+ compared to the concentrations of calcium (Ca2+) and magnesium (Mg2+) on the soil exchange complex, and the ESP is the degree to which the soil exchange complex is saturated with sodium. Both SAR and ESP characterize the sodium status of an alkaline soil, but SAR is becoming more widely used. Note that commercial laboratory results may provide Total Dissolved Solids (TDS) instead of EC. This may be expressed as parts per million (ppm) or milligrams per liter (mg/L), which are equivalent. To convert ppm (or mg/L) to dS/m, divide by 640. To convert dS/m to ppm (or mg/L), multiply by 640.