How Does Foliar Feeding Affect Soil Microbe Biomass?
Featured in the Summer 2022 edition of The Growers Solution were two articles about foliar feeding. The first contained guidelines for foliar feeding that we publish every summer as a reminder for our customers, but also in this article was a brief explanation of a lab experiment our Research Team conducted to see the effect foliar feeding would have on root growth. Additionally, the second foliar feeding article in that edition covered the experiment but looked at the microBIOMETER results from foliar feeding the plant. This article will dive deeper into this experiment as well as look at a second experiment done similarly.
We have known for many years that foliar feeding is an effective and efficient way of providing nutrients to a crop, but as we grow in our understanding of plant growth and soil health as a holistic system, it made sense to try to determine what effect, if any, foliar feeding would have on the soil. Plants live in a symbiotic relationship with soil microbes living in what is called the rhizosphere. This is a small (roughly 2 millimeter) area around the root zone where microbes thrive. There is an ever-expanding understanding of the exact nature of the relationship between plant and microbe in this zone, but suffice it to say that generally what is good for the plant is good for the microbes and vice versa.
I have already explained the methodology of Matt's experiment set-up in the Summer 2022 Solution, but in short, he sprayed 2 gallons/acre of 5 different products twice on 4 corn plants that were planted in pure soil. The sprays were 4 days apart. The soil was watered once and then allowed to dry. The 5 products used were GMS, GMS without trace elements, molasses, 28% nitrogen, and a 9-18-9 liquid fertilizer. Once the plants had dried, he plucked them, weighed the roots and the top growth, and measured the soil microbial biomass using the microBIOMETER test kit. You can find the root weights in Photo A, and the results of both measurements in Table 1 (on page 1).
Matt then repeated this experiment with a couple of differences: He added a control plant that was not treated with anything, and he only sprayed the other plants one time instead of two. The roots are pictured in Photo B and the results of the measurements in Table 2 (on page 1).
DISCUSSION
Since GMS is primarily phosphorus and is hot-mixed to keep the elements bonded in the solution, it will travel from the plant surface to the roots quite quickly when foliar applied. Our questions were: Does this benefit the soil microbes? If so, why and how? There are two pathways it could provide benefit to the microbes. The first is that it travels itself into the rhizosphere and plant roots and is used directly by the microbes there. The second is that it invigorates the plant to produce more sugars for the microbes to feed on. We believe that both are active here, as only both can account for the results, we have seen with all of the products. You can clearly see in both experiments that GMS, with or without the trace elements, has been beneficial to the soil microbes. There is another pattern to point out in both experiments with regards to the soil microbes: In both cases, both the 9-18-9 and the 28% had a detrimental effect on microbe biomass, which was 215 in the soil before any corn was planted in it. Likewise, molasses had a positive effect on the microbes in both experiments.
In order for the 9-18-9 and 28% to have a detrimental effect on the microbe biomass, they would either have to reduce sugar release from the plant to the microbes or have been excreted themselves into the soil and harmed them that way. I believe that it is primarily the latter explanation, as only the 9-18-9 was harmful to the microbes and produced the worst (or very nearly) top and root weights. The 28%, on the other hand, produced a lot of top growth in the 2-spray experiment and more competitive root growth in both. I believe that while the 28% (which is a nitrate) is itself being released into the soil and harming the microbes, it is also benefiting the plant in terms of tissue growth. Therefore, if it were solely a result of more sugar release, the plant treated by the 28% should have had better biomass numbers. Contrastingly, the 9-18-9 was poorly utilized by the plant in both tissue growth and as a benefit to the microbes.
Molasses contains little to no plant nutrition aside from sugar (carbon), which is why it is used as microbe food pretty regularly. Similarly to the 28%, I don't think either would have produced a healthy vibrant corn plant in the long run since they were single element treatments, though clearly the molasses had a much better effect on the microbes. They are really used for two different reasons: 28% for top growth, and molasses as a boost for soil microbes.
There are many services that microbes can provide to a plant in the rhizosphere. I'm sure you have all heard of the "living manure" means, which is that the microbes will absorb otherwise unavailable minerals in the soil into their tissue, convert them, die, and then the plant will absorb the minerals from their bodies. Another more recent pathway found is that roots will actually absorb microbes into their tissue, suck some of their innards out that contain useful elements, and then eject them back into the soil to replenish themselves and do it all over again. Of course, in both these systems the plant is also providing sugar to the microbes. In the latter system, the microbe is existing inside the plant tissue for a time, where it would come into contact with any product coursing into the root system. This is in addition to the plant excreting aspects of excess product into the root zone. I believe this is why we are seeing excellent results with the microBIOMETER test with GMS. The microbes are looking for more than sugar, and in GMS there is a balanced micronutrient package that travels with the phosphorus into the root system and zone, where it can be fed upon by the microbes.
This is shown in both experiments. Generally, whatever differences we see in the single-spray experiment become more extreme in the double spray. This makes sense when you consider that GMS usually realizes its best results when it is used in small doses frequently, while in the case of negative effects they become stronger when the detrimental treatment was repeated.
It is important to remember that the soil microbe community is as varied and non-static as any living community. It is also not isolated, but interconnected with the plants, water cycle, nutrient cycles, seasons and climate, anthropogenic factors, and more. If we treat crops with an eye to the effect that treatment will have on the microbe community, then the symbiotic relationship they share can be strengthened and many benefits can be realized from it.
This is an excerpt from the Spring Growers Solution (2023) written by Zachary Smith, Product and Training Specialist.
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