Pros & Cons to Beneficial Microbe Additives

Plants and microbes have coevolved for millions of years, and over this time have developed many complicated and intricate relationships. While many microbes have developed strategies to attack, kill, and feed upon plant species, many others have reached a profound degree of symbiosis with their plant neighbors. Plant growth promoting microbes (PGPMs) include a wide range of microbial species, both fungal and bacterial. They have developed a multitude of means to improve plant growth and health in exchange for a place to live and a steady supply of food exudates from plant roots. 

While these incredible synergies have developed naturally in every environment where plants and microbes can be found, when we look to modern agricultural and horticultural practices we find certain situations where the delicate balances found in nature can not, or should not, be manually reproduced. 

Benefits of Plant Growth Promoting Microbes

“Living soil” approaches to gardening and crop production look to mimic the natural relationships between plants and microbes within a plant’s root zone, or “rhizosphere”. In situations where sustainable agriculture of non-regulated crops is the main focus, addition of extra PGPMs in the form of inoculated fungi and bacteria can be hugely beneficial. Soil amended with compost may require bacterial action to break down organic matter into forms usable by plants, and fungi like arbuscular mycorrhizae can be used to colonize a plant’s roots to enhance the uptake of water and nutrients while also fighting off plant-pathogenic bacteria and fungi that could lead to crop death and loss. 

Microbial inoculants, or biofertilizers, are supplemental additions of these PGPMs. These microbes may be applied to crops through “root drench” approaches where fertilizer blends containing living microbes are applied directly to the soil plants are grown in, or through foliar applications where suspensions of microbes in liquid are sprayed onto a crop to colonize aerial tissues such as stems and leaves. Applications of PGPMs using these approaches are often hugely beneficial to cultivated plants in circumstances where biofilm production or microbial bioburden aren’t of concern. In systems used in modern or urban environments such as vertical farming, hydroponic, or aeroponic crop production however, simply adding additional microbes to a system may cause more harm than good. 

Cons of Plant Growth Promoting Microbes

Biofouling, contamination, and biofilm production are some of the main issues faced by cultivators growing indoor crops. The addition of PGPMs to these systems can often cause, or significantly exacerbate these issues. Biofouling, or the microbial blockage of water and fertilizer through irrigation systems, can lead to rapid wilting in systems where plants depend on a constant stream of water and food. In aeroponic or drip-irrigated systems this can lead to total crop death in shockingly short periods of time. 

Contamination through excessive microbial buildup can cause certain regulated food and medicinal crops to fail compliance testing, leading to crop destruction or forced repurposing and loss of profits. Lastly, the biofilms produced by these biofouling microbes can work their way backwards through an irrigation system all the way to the origin of a cultivation facility’s water supply, leading to costly and time-consuming repairs that could put a company out of business. 

While the synergies between plants and PGPMs are well known and frequently applied with great outcomes, the demand for newer “non-traditional” crop models may necessitate a new outlook on how these synergies should be utilized and approached.