Cover crops can positively affect a wide range of soil health characteristics. According to some American farmers, the ability of cover crops to moderate soil temperature is also important but less visually noticeable.
Why it matters: Extreme soil temperatures limit biological activity. Keeping the ground covered helps moderate those swings, benefitting crops.
According to Adam Dahmer, a grain farmer and multi-decade cover crop user from Marion, Illinois, cover crop use has helped his crops absorb more nitrogen and develop healthier root systems. This was communicated in an online presentation given during the University of Wisconsin’s National Cover Crop Summit broadcast earlier in March.
Regarding root development, Dahmer says small, near-surface roots start to die whenever his soil reaches 27 C. In his part of the United States, ground temperatures can reach almost 40 C. These hazards are negated by an active cover crop regime including, but not exclusive to cereal rye interseeded into corn via aerial application (by plane).
“It’s the backbone to almost every farm’s cover crop program,” says Dahmer.
Keeping the soil cool through broadcast interseeding is also a critical strategy for Tom Cotter, a bean, corn, and cattle farmer from Austin, Minnesota, and Chris Teachout, a grain farmer from Shenandoah, Iowa.
The necessity of moderating heat, says Teachout, is reflected in the high temperatures he has measured in uncovered surface soil control plots on his farm — 64 C. He says even partial cover reduces the heat.
Cotter and Teachout, both of whom participated as speakers in the online cover crop summit, prefer to interseed cover crops like cereal rye into corn at or before the V6 growth stage, and ideally, closer to V3. This is done to keep soil microbes as protected as possible, which in itself is part of a wider effort to reduce nutrient and microbial loss in runoff.
“Corn grows fast so be ready. Just make sure to get the cover established before canopy develops,” says Cotter.
If a corn canopy does develop before he can broadcast a cover crop, the growth opportunity for his cover crop is too limited and he may decide not to apply one.
Soil temperature concerns apply across Ontario as well. As described in a 2013 Ontario Ministry of Agriculture, Food and Rural Affairs article called “Soil is meant to be covered,” soil temperatures between 20 and 30 C are ideal for most biological processes, including bacteria that drive the nitrogen cycle. Achieving ideal temperatures is tough, though, because of how rapidly surface layers can dry out.
Adam Hayes, field crops soil management specialist for the OMAFRA, says overheated soil negatively impacts both macro and micro levels of soil life in general. Earthworms, for example, move further down, slowing the breakdown of organic material and nutrient cycling, and root growth can also be stymied.
Some benefits to heat retention too
On the other end of the temperature range, Dahmer says he sees benefits from heat retention. Specifically, incorporating oats into post-harvest multi-species covers helps mixtures perform better as a whole. He believes this is because oats, which emerge and grow quickly, give slower growing cover crops more time to develop by keeping warmth in the soil during rapidly cooling autumn conditions.
Hayes also says living green cover in autumn tends to keep soil warmer for a longer period, extending the amount of time in which soil organisms can work. Sometimes this difference can be seen after a snowfall, where snow will melt off green cover but not bare soil.
Because living cover also affects how quickly soil dries in spring, and by consequence, how quickly it warms to desirable planting temperatures, Hayes reiterates there are still challenges when it comes to planning. Timing termination to optimize both cover and soil moisture levels for planting can be tricky.
Develop an honest plan
Regardless of mixture or agronomic intention, Dahmer believes the most important factor is the relationship between the type of cover crop and the time it’s planted.
“Be honest with yourself on when you can get covers planted. Diversity is good, but you don’t get bonus points for buying seeds that will not work at the time you’re planting,” he says.
“I like to say I focus on lots of less,” he says. That means less tillage, less water runoff and soil erosion, less fertilizer use, less manpower, lower equipment costs, fewer temperature spikes, and less bad press pertaining to his farm’s environmental footprint.