Rain after seeding can often cause top soil to crust, stopping the emergence of canola seedlings.
Canola seedlings can’t penetrate the crust, and often curl back and die. No research has been conducted to show the best ways to break up crusting and free the crop. If a few plants have emerged, it may be best to leave them be.
Fields with plant counts as low as 1-2 per square foot evenly throughout the field are probably better to be left alone than reseeded at this calendar date. Very low plant counts can produce reasonable yields as long as those plants are closely protected throughout the rest of the season and weed competition is kept in check. Fields reseeded at this date have lower yield potential that field seeded in mid May, and the face a much higher risk of fall frost losses.
What conditions can lead to heavy crusting:
Soil type: Soils with a higher percentage of clay will crust more, as clay tends to bind together much more than sand or loam soils. Clay soil granules are small enough to almost glue together with moisture and remain “cemented” when dry. Loam and sandy soils have larger granules and therefore do not bind together as tightly. Higher organic matter (OM) will amount to less crusting because OM, even though it acts as a binding agent for soil particles, only holds particles loosely. Soils with high sodium, low calcium also have a higher tendency to crust.
Moisture situation: Fields that are wet then quickly dry due to wind and heat and are likely to crust. Generally, soils in a moist state will not crust. In-fact, the best cure for soil crusting is rain.
Tillage practices: Clay soils that are wet and then dry quickly can crust with any tillage practice. In general, fields with long-term no-till will have a lower risk of crusting because OM tends to be higher in a no-till situation. Every tillage practice performed breaks down the stubble and OM and therefore will increase the chances of soil crusting.
Packing during seeding: Packing is used to firm the soil around the seed. Increased packing pressure will compact the soil particles more and therefore can increase the chance of soil crusting. Reduce packing pressure in wet conditions.
Opener/packer combination: An opener that creates more soil disturbance can decrease the chances of soil crusting due to simple fracturing of the soil base. However, too much disturbance can also increase soil crusting by breaking down the soil base, which then has a higher chance compacting when packed.
Solutions to crusting
There are no reliable solutions other than to wait for rain. Some growers have reseeded the worst sections of fields. A light harrowing might help if nothing has come through. Do a couple passes then assess whether canola seedlings are being ripped out of the ground. Harrowing too close to emergence can be harmful to a shallow seeded crop such as canola and may not be worth the risk.
Using a roller may be worse than harrowing when soils are wet below the crust. Instead of cracking up the soil surface, a roller could turn the whole topsoil zone to concrete. Again, there is very little research on how to manage crusting. It’s trial and error.