During drought, soil dries up and is hard as a rock. You can't plant it, nor will anything grow in it anyway. During floods, it gets goopy and you can't plant or harvest, and it can destroy crops if it happens at the wrong time. If the soil temperature is too cool, you can't plant yet, you have to wait for it to warm up. Within rational limits of what the climate is capable of, nothing bad can happen to plants from having a soil that's really warm, as long as it's adequately moist. Anything that can be grown in the north can be grown at the equator in much warmer soil, unless it needs a long day or a winter dormancy as part of its life cycle. As mentioned by ggelsrinc, soil temperatures staying warm over the winter can encourage development of insect pests. That's the extent of soil temperature's effects on agriculture.
Microbial activity doesn't have much to do with it. Modern chemical farming has greatly altered the soil ecosystem and the plants don't care because they're getting the nutrients they need added. Plants can grow in rocks with no organic matter whatsoever, if the rocks have enough of everything they need. You see this a lot in deserts and mountains, and around the Canadian Shield. The irrigated farmland around the Phoenix metro area and the rest of southern Arizona is incredibly fertile and productive. The microbes in that soil are vastly different than the soil here in Minnesota, but they grow some of the same crops. Tomatoes and strawberries are two that come to mind right away, and they grow just as well in either location, at least as far as the soil is concerned.
As average soil temperature increases, it may indeed contribute more carbon to the atmosphere. I'm sure the soil microbe balance will shift. But that has little or nothing to do with agriculture.