By Harry Brook
Everyone has seen those white crusts around sloughs and in low areas. They are costly, non-productive parts of the field and they seem to spread each year. All that will grow in them is kochia and foxtail barley.
Salinity in the soil is due to shallow groundwater movement combined with the fact a large area of the province has subsoil with heavy accumulations of salts. These salts can be sodium, magnesium or calcium. Sulfates also show up with salinity. If the salt is primarily sodium, expect soil structure problems. These usually result in gumbo soils.
Whenever you see salts on the surface of the soil, this is known as a local discharge area. Moisture has penetrated down into the subsoil where there is lots of salts and salts are picked up. The moisture hits an impermeable layer, then moves laterally until it is close to the surface. Then evaporation draws the salt-laden water to the surface where the water evaporates, leaving salts on the surface. It is often seen as a white “bathtub” ring around sloughs.
How is salinity measured? Soil salinity is measured in the amount of electricity a soil will conduct. The greater the salt concentration, the more electricity will pass through the soil. On soil tests this is recorded as EC (electrical conductivity). Readings below a “2.0” are considered non-saline soils. Peas cannot tolerate salinity. Barley, wheat and oats will grow in moderately saline soils (6.0 – 8.0) but canola is less tolerant (~5.0 max).
Crops won’t grow in these sites due to osmosis. Plant roots extract water by having a salt content in the root greater than the salt content in the soil water. When soil water is saline enough, crop plants can’t obtain water and die. Sort of like you drinking salt water to quench your thirst. It isn’t healthy. The plants that can grow on these saline areas are adapted to grow with high salinity in the water.
What’s the solution to this growing problem? There are several things you can do. One is to prevent or reduce the evaporation on the surface around these discharge areas. That means putting in perennial crops around the areas to keep the subsurface water deeper in the ground. Alfalfa is a good plant to grow as it has deep tap roots and is a heavy user of water but is only moderately tolerant of salinity. Some grasses, like Altai wildrye and tall wheatgrass have high tolerances to salinity, but you can have an issue with palatability. Perennial forages can, over time, drop the water table and rainfall will slowly wash the salts down into the soil, reducing surface salinity. However, that only lasts until the perennial crop is ripped up and annual cropping commences again.
Another option is using an EM 38 or remote sensing to map where the salinity is coming from and intercepting this moisture, preventing salt accumulation. It can get quite costly with subsoil drainage.
Any way you look at it, soil salinity is a creeping scourge to field productivity. Make changes to your operation to keep fields productive and healthy. Reduce the salt.
Harry Brook is Flagstaff County’s Agricultural Fieldman. He can be reached via email at: hbrook@flagstaff.ab.ca or by phone at: 780-384-4138.