Imagine for a moment that you’re a plant. Each day your leaves harvest energy from sunlight to make sugars. What do you chose to do with this precious energy? You could grow more leaves to catch more sunlight. Or grow more roots to access deeper water and more nutrients. Or store it up to use later. Or make as many seeds as you can so you can to produce more children. These are all things that plants do with their energy, but another option, which you probably didn’t think of, is to release it into the soil.
Up to 5-21% of the carbon a plant fixes by photosynthesis is secreted into the soil by its roots. It’s the plant’s way of contributing back the the community that it lives in. These secretions are called exudates, and can be composed of amino acids, organic acids, sugars, proteins, and other organic compounds. Exudates make the soil surrounding the roots (known as the rhizosphere) more beneficial for the plant in several ways: they can repel pathogens, modify the physical and chemical properties of the soil, and feed bacteria and fungi that form symbiotic relationships with the plant. Exudates can even be used as signalling compounds so that plants and microbes to communicate.
Two well known examples of these beneficial microbes are nitrogen fixing bacteria, which make nitrogen from the atmosphere available to plants, and mycorrhizal fungi, which can extract nutrients from the soil to provide to plants, protect plants from parasites, and even transfer nutrients from one plant to another. As well as these two examples, roots take part in a complex interaction between many different bacteria, fungi, protozoa, micro-arthropods, and nematodes, which works to cycle nutrients and provide a hospitable environment for the plant. In a healthy soil, all of a plant’s nutritional needs can be met with the help of these microbes. Why then, do we need to use fertilisers to grow our food?
A big factor is the decline of the microbial communities in our soils. Western agricultural practices like repetitive ploughing kills microbes by exposing them to the air and UV radiation from the sun, as well as decreasing soil carbon content, which is important for sustaining soil life. Bactericides and fungicides used against pathogens can also kill beneficial microbes. When inorganic fertilisers are used, populations of many beneficial microbes are further reduced, increasing the dependance on the fertilisers.
Without the strong community of microbes in the rhizosphere, it’s harder to balance the nutritional needs of a crop perfectly, so nutritional deficiencies and decreased resistance to pests and diseases are almost unavoidable. In response to this, many backyard gardeners and some farmers are adopting methods that actively encourage the microbial population to flourish. Use of compost, compost teas, no-dig gardens, crop rotation, companion planting, and high density rotational grazing are all ways that people use to build up the soil microbiology.
Thinking back to the exudates, if plants spend so much of their precious energy to improve the soil, it must be really important for them. If we want to help plants flourish, it might be better to work with them, not against them, to help improve the soil community they live in. Do we underestimate soil?