Improving crop production through understanding the soil microbiome

Posted 24th July 2019 by Jane Williams

Sarah Strauss is a soil microbial ecologist at the University of Florida. Her interests lie in understanding the interactions between soil microbes and crops, with the hope that improved understanding will benefit crop production.

We spoke to her about her work ahead of her presentation at the 4^th Partnerships in Biocontrol, Biostimulants & Microbiome Congress: USA.

The interactions between soil microbes and plants are still very much uncharted territory, which makes it an exciting area of study. This is especially the case for the interactions between soil microbes and crops in agricultural systems. For example, we know that there are bacteria and fungi that can influence plant growth or soil nutrient availability, but most of those studies have been done with only a few plant species or under very specific growing conditions. Much of my research is looking at what interactions are occurring between specific crops in field conditions – and how those interactions might differ based on specific conditions or crops in a farmer’s field.

One specific area of research in my lab are the interactions between microbial amendments and specific crops. I’m located at a research center in the middle of a major Florida agricultural production area. This means I work directly with farmers and am focused on applied research that can provide resources for their immediate questions. These farmers are very interested in the efficacy of commercially available microbial amendments, or biostimulants. One of the projects I’ve recently conducted has been to examine some of these amendments and determine not only if they impact plant growth, but also how they interact with our local field soil microbial community and soil types. There’s been a lot of research on biostimulants, but much of it has been done in the lab setting and may not actually apply once you get outside.

My program is also looking at the interaction between plant genotypes, propagation methods, and the rhizosphere for different crops. Rootstocks, and rootstock propagation, is common for tree crops, but it’s also being used for some vegetable production. I’m interested in looking at whether those propagation methods are influencing the rhizosphere microbiome, and therefore plant growth.

Florida is a special case for agriculture – much of South Florida production is on what is essentially beach sand. There can be as little as 0.5% organic matter, and so farmers and researchers are very interested in methods that can improve the soil environment for microbial activity and crop production.

What are the key production practices key in those areas for influencing/benefiting the microbial communities?

My research program focuses on vegetable and citrus, which are considered “specialty crops.” In general, “specialty crop production” refers to anything that’s not a cereal or grain crop, so crops that are not corn, soybeans, rice, wheat, barley, etc.

Those production systems are very different from cereal crops. Since citrus is a perennial crop, that’s obviously managed differently than corn, which you plant every year. This requires different fertilizer application timing, types, rates, as well as different irrigation methods.  All of these management practices could influence the soil microbial community, and result in different microbial composition and interactions that what might be seen in a cereal crop.

Another major difference in the agricultural production systems that my research examines are the methods for vegetable production. In Florida we use plasticulture. This is a system where raised beds are formed on a field, and then covered in plastic. This allows farmers to fumigate and apply nutrients in a contained manner. Holes are made in the plastic on the top of the beds for the plants. This creates a completely different environment for soil microbes that what you’d find in an open field, which means there will be different interactions between soil microbes and the crop.

What role do biostimulants play in those kinds of environments?

Theoretically, they could play a similar role to what we see in grain or cereal crops. Many of the bacteria and fungi shown to have plant-growth promoting properties for corn or Arabidopsis or tomatoes could theoretically also impact trees and other vegetables. If that’s the case, that would make our research a little easier!

However, this is why additional research is necessary. For example, we know that the plant genotype can play a role in the rhizosphere microbiome composition both between crops (tomato and citrus have different rhizosphere compositions), but also within species. How do those differences influence the efficacy of a biostimulant?

The soil environment and management of that soil environment (i.e. fertilizer application, irrigation, tilling, etc.) is also a key component in the soil microbiome. Our sandy, low organic matter soils in Florida are very different from those in the Midwest, for example. This means that the native soil microbiome is likely different, and that microbial amendments that contain organisms that were not isolated in Florida might not colonize or interact in the same manner as they might in a different soil environment.

What challenges need to be met to better understand how these products can benefit the microbiome and encourage better crop production?

I think there are two components that we need to understand for effective biostimulant use in specialty crops. The first is to understand application rate and inoculation conditions necessary for these added microbes to properly colonize. Ultimately, these biostimulants should have a concentration that will be beneficial to the plant. This will probably depend on the soil environment. For many commercially available products, the recommended rate is a miniscule amount when you look at the actual number of bacteria that will be added to a field. However, studies that have examined the benefits of some of these key microbes usually use significantly greater concentrations. For field applications, that number isn’t well defined or examined.

Related to the application rate is the necessity of determining conditions in the soil needed for these biostimulants to increase in number. This is obviously closely related with how the farmer is managing their crop. How do these additions work with the standard irrigation and fertilizer rates? Do those rates need to be adjusted? If so, by how much? Are other additions necessary to allow for these microbes to inoculate? Again, these parameters are likely going to be specific for a soil environment and crop.

The second component is to determine how added microbes interact with the native microbes in a way that will be beneficial to the entire system. Many plant beneficial microbes have very similar taxa already present in the soil. For example, Bacillus species are known plant growth-promoting bacteria, but Bacilli are also extremely common soil bacteria. But we don’t understand how that new added species is going to interact with those potentially similar organisms already in the soil. Alternatively, there may be native soil organisms that are direct competitors or antagonists of the added organism.

Hopefully, as we learn more about the interactions between soil microbes and crops, we will be able to better manage the soil microbiome for improved crop production.

Sarah Strauss, Assistant Professor, University of Florida, will be speaking on ‘Challenges and evaluation of biostimulant use in specialty crop production’.

Find out more about how plant and soil microbiome research is enhancing crop productivity and disease resistance at the 4^th Partnerships in Biocontrol, Biostimulants & Microbiome Congress. USA. Download the agenda.

Tags: biocontrol, biologicals, biostimulants, microbiome, rhizosphere microbiome, soil microbiome