Much of Canada and the United States are experiencing a huge summer heat wave right now. Most growers have a good idea of what this weather means for their crops, but many will not know what it means for the soil microbes living in their plants’ rhizospheres. Heat and moisture are two of the biggest factors affecting soil microflora, so should soil conscious growers be worried? In this week’s edition of Growing Possibilities, we will be investigating the effects of a summer heat wave like this one on the helpful bacteria in your soil.
In a year that lacks precipitation and moisture like this one, the effects of heat and dryness on a crop can both be exacerbated. Moisture in the soil acts as an insulator against temperature changes, so if your soil is dry then it will heat up faster as the weather turns hot.
The good news is regular summer heat is unlikely to have a damaging effect on your soil microflora. Soil microbes like a warm environment, and their growth and activity will be encouraged by a little heat. A study on soil bacteria activity found that most helpful soil bacteria will increase their activity and respiration rates until just over 30°C. When soil temperatures started to approach 40°C, their activity began to decline and at 45°C had begun to almost come to a standstill (1). This study shows that in a heat wave like the one we are currently experiencing, the soil where these microbes reside is unlikely to reach temperatures high enough to have a serious, negative effect. On the contrary, it appears helpful bacteria will actually thrive in the low 30 degree temps we are expecting.
With the dryness that is accompanying this heat wave, getting enough moisture is high on the list of priorities for both plants and microbes alike. Fortunately for your plants, Plant Growth Promoting Bacteria (PGPR) can offer some help in this department.
Some PGPR release a sticky substance consisting of extracellular polysaccharides (EPS) when under stressful conditions such as dryness. EPS help bacteria hold onto moisture and nutrients from their surroundings and bacteria emitting this substance around the roots of their host plant can help the plant when under the same moisture stress. EPS prevent the desiccation of bacteria as well as surrounding plant roots, and increases soil aggregation while maintaining a high water potential close to the plant (2).
Another way PGPR may help your plants find moisture is by encouraging the development of plant root systems and root hairs. A well-developed root system is essential to combatting water deficiency in soil. PGPR that produce phytohormones like indole-acetic acid (IAA) encourage early root hair and root system development in plants, which enhances their ability to find and uptake water present in the soil.
Although hot summer temperatures are usually OK for bacteria that is already in the ground, this is not the case for inoculants and biologicals that are still in their packaging. If an inoculant or biological product is still unopened, this does not mean it is invincible! Inoculants and biologicals left outside, where direct sunlight and UV rays can get to them, could potentially be damaged or even rendered unusable by these conditions. The living, breathing bacteria in these products require certain storage conditions, not only to remain viable and ready to colonise your plant roots, but alive in general. If these bacteria are exposed to UV rays or extended periods of high temperatures, they will begin to die off or suffer due to this stress factor, so always make sure you are following the manufacturer’s storage instructions from the time you purchase the product until the product is in the ground. For XiteBio products, we recommend storing products out of the elements, away from direct sunlight, and in a place where temperatures will not exceed 28°C (82°F).
References:
1) Pietikäinen, J., Pettersson, M., Bååth, E. Comparison of temperature effects on soil respiration and bacterial and fungal growth rates, FEMS Microbiology Ecology, Volume 52 (1), 2005, Pages 49–58, https://doi.org/10.1016/j.femsec.2004.10.002
2) Vurukonda, S., Vardharajula, S., Shrivastava, M. SkZ, A. Enhancement of drought stress tolerance in crops by plant growth promoting rhizobacteria. Microbiological Research, Volume 184, 2016, Pages 13-24,ISSN 0944-5013