Ontario Grain Farmer February 2021

“When you dig into the woodlot soil, it’s black as coal with a nice crumb structure, and it falls apart in your hand,” he says. “So, there’s clearly lots of room for improvement — we know this soil can have high organic carbon levels, we just have to figure out how to do that and grow crops too.” In terms of bulk density or hardness, the soil was consistently lower under cover crops than under corn – for all tillage systems. The optimal range for soil bulk density is about 0.9 to 1.2 megagrams per cubic metre, and the near-surface soil under cover crops is regularly at the top of that range. In the corn, soil density increases to about 1.4 — a hardness level that starts to restrict root growth in most clay loams, Reynolds says. “Roots need air to function,” he says, noting that the soil’s near-surface plant available air capacity (PAAC) was at about 15 per cent under cover crops, but down to 10 to 12 per cent during the corn phase. He says that crops in fine-textured soils become susceptible to aeration deficits once soil air capacity gets below about 14 per cent. The ideal PAAC for crops is about 20 per cent — which is frequently the case in sandier soils. AVAILABLE WATER Plant available water capacity (PAWC) indicates the ability of a soil to store water that can be used by crops. “What’s interesting is that even though Brookston clay loam can hold a lot of water, typically less than half of that water is available to crops,” he says. PAWC was consistently greater under cover crops (10 to 15 per cent) than corn (below 10); however, ideal PAWC is about 20 per cent, while less than 10 per cent can make the soil droughty. Water availability is a tug of war between the soil and the crop, and the soil ONTARIO GRAIN FARMER 17 FEBRUARY 2021 This article features research funded by Grain Farmers of Ontario. LEARN MORE always gets the upper hand in the end, he says. In terms of permeability, Reynolds says he didn’t see any clear differences between cover crops and corn. This was likely caused by the many cracks, worm holes, and root channels that are typically present in the near-surface of Brookston clay loam soil. Reynolds is surprised that the soil improvements accrued under cover crops largely disappear during the following corn year. “The benefits just don’t hang around,” he says, noting again that it might be a function of the short timeline of the experiment. He feels that perhaps timed sampling of the soil — at different periods during the growing season — might clarify what is going on. “My feeling nonetheless is that cover crops are definitely a good idea from a soil physical health perspective,” he says. l