By Harry Brook
Often we don’t consider the soil that grows our crops. There’s a lot happening in the soil, below the surface. Under the dry conditions we experienced this year, there was a lot more salinity showing up around sloughs and in low areas. It shows up as the white patches where nothing is growing. Sometimes you may not see the soil surface but if there is a solid stand of kochia or foxtail barley present, you can be guaranteed that soil is saline. Kochia and foxtail barley have very high tolerances to salinity.
Electrical Conductivity (EC) is used to measure soil salinity by measuring the soil’s ability to conduct electricity, in decisiemens per metre. What is important is the crop’s tolerance to withstand the salt. If your soil test shows salinity below 2 in the top two feet or below 4 in the two- to four-foot level, your soil is non-saline. Just looking at the top two feet, weakly saline is 2 – 4 ds/m, moderately saline is 4 – 8 ds/m, strongly saline is 8 – 16 ds/m and very strongly saline is greater than 16 ds/m. If you see a white crust on the surface, the EC is above 8. There are no annual crops that grow above 16 ds/m. There are a few grass species that can handle very high salinity but choices are very limited. Canola, flax and oats are tolerant at the low range of moderate salinity while barley, wheat and rye are tolerant towards the high end of moderate tolerance.
Where does the salt come from? A lot of the subsoil, down at the four foot or deeper level, is very saline in nature, saturated with sodium, calcium and/or magnesium. When it comes close to the surface, as evaporation pulls it up, it can affect the soil structure. High sodium levels leads to soil layers, close to the surface, forming swelling clays. These swelling clay layers prevent root penetration, stranding the roots in the top part of the soil. That makes them less drought-tolerant, unable to obtain enough nutrients, and cause the crop to have a very uneven height. A lot of the soils in Flagstaff County have this type of subsoil. Bare ground speeds up evaporation and increases the amount of salts left on the surface. Once enough salts have permeated the topsoil, it prevents any crop from growing. Why? All plants obtain water through osmosis. If you have too high a salt content outside the root, water doesn’t move into the plant and the plant dies. You can have a seedling perish for lack of water despite being surrounded by water.
When permanent vegetation around a slough is removed and cultivated, bare soil and heat bring more salts to the surface. Over time, those saline patches get bigger, increasing the non-productive area in a field. Even if the crop grows, you’ll find it stunted and not terribly productive close to these saline patches. If you are clearing around these sloughs and water bodies you are setting yourself up for revenue loss for those acres. Is it really worth it?
In a wet year, the rain tends to wash the salts from the soil surface and you may not see those salts lurking beneath the soil surface. However, they are still there, waiting until they can show up the next dry year.
How can you fix the problem? It’s taken time for it to get bad enough to affect yields, so it will take an adjustment of farming practices and time to improve the situation. It’s a two-step approach to reclaiming saline soils. The first step is to prevent any further salt accumulations. If you can identify the area where the moisture is entering the soil, that’s your recharge area. Seeding this area to alfalfa is one way to interrupt the water flow to the discharge area, the place where the salts are accumulating.
Using perennial plants around the edges of the wet, discharge areas reduces further salt build up. Once a good stand of perennial grasses has established, rain can slowly leach the salts out of the root zone.
It’s taken years to develop a salinity problem and it will take a lot of time to reverse the process. Be patient and keep your soils productive for the long haul.
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.