Dealing with Hot Temperatures

Dealing with Hot Temperatures

Farmers are finishing up planting, but some crops have been slow to emerge. Cooler nighttime temperatures may be delaying emergence. In most cases, moisture has been adequate but highly variable. However, summer has arrived and very soon hotter and drier weather conditions may prevail. The last two years plus this year look like a hot June. Last year, moisture was very short in May. This year, farmers have some moisture at the surface, however, subsurface moisture is very low.

When it gets hot and possibly dry this early in the growing season, what can a farmer do? Crops' roots are still shallow and the crop is not well developed yet. Early planted crops appear to be in better condition to survive higher temperatures and possibly drier soil conditions because they have more developed roots. First, try to terminate cover crops and weeds quickly to save moisture. Second, reduce or avoid tillage because each tillage pass dries out the soil .5-1.0 acre-inch of water. Dry soil tends to blow and when it rains, the soil gets hard and turns to concrete, causing crusting and reduced water infiltration. 

For no-till and cover crop farms, surface residue is beneficial for reducing soil temperatures. A thermometer on bare soil could be 135°F when the air temperature is 90°F and one inch down the temperature can still be 100°F. Residue helps keep the soil cooler and reduces water evaporation. When the soil surface temperature gets that hot, many microbes that recycle nutrients to the plant die off. Rhizobia bacteria, which make nitrogen for soybeans, die off quickly when soils get too hot (above 140°F). Most plants are hurt as much by a lack of available nutrients as they are by a lack of water, although the two are very closely tied together. Plants become stunted as growth is reduced due to a lack of nutrients. 

Several nutrients are critical when high temperatures occur. Without adequate potassium (K), plants are less drought resistant. Low K also promotes higher incidences of disease and insect pests. Adequate zinc (Zn) is also critical to surviving hot temperatures. Zinc levels are commonly low on many soils and in plant tissue. Zinc is also easily tied up by glyphosate (Roundup). An element seldom talked about is organic silicate (silicon). Silicon strengthens the plant vascular system (xylem, phloem) somewhat similar to veins and arteries in humans. Plants with high organic silicates are stronger and healthier (bigger, larger vascular system) and can resist many environmental stresses including hot and dry weather. Organic silicate also helps plants reduce frost injury in early spring or fall. 

If a farmer is doing no-till with cover crops, should they leave the residue standing or roll it down? Getting the crop residue on the soil surface is beneficial, by reducing soil temperature, and conserving moisture; plus as the residue breaks down, it releases nutrients to nearby plant roots. On rye, most of the beneficial allelopathic effects (natural herbicide) come from decaying stems and leaves, reducing weed pressure. Getting crops to canopy quickly is also beneficial to reducing soil temperatures and conserving moisture. 

On another topic, new research shows that managing individual leaf size may have a huge effect on crop yield. On wheat, the flag leaf is the most important leaf because it supplies most of the sugars and energy for wheat production. The same may be true for other crops as well, especially corn. On fruit trees, new leaves do not contribute sugar to the fruit equally. It’s the leaf or leaves closest to the fruit. On most plants, 35% to 80% (wheat for example) of the sugars comes from leaves closest to the fruit or grain. Soybeans have many pods, but it’s still the leaves closest to the pods that make yield. 

Farmers may need to pay more attention to individual leaf size, especially on corn and wheat, to maximize photosynthetic activity. This explains why some of the highest-yielding crops have a great deal of vegetative biomass, especially very large individual leaves, which results in an abundant sugar source for the grain or fruit. The practical application is that farmers need to make sure zinc (Zn and other nutrients) levels are in generous supply for those fruit or grain associated leaves to produce supersized individual leaves before or during reproductive development. (Source John Kempf, 2024). 

Wheat is turning yellow quickly and will probably be harvested early this year. Hot temperatures shut down wheat growth and it matures faster. With hot temperatures expected, give your pets and livestock plenty of cool water and some shade. Also, keep yourself, children, and the elderly well hydrated and try to stay cool.