What Comes First, Healthy Plants or Healthy Soil?

A common truism says that healthy plants come from healthy soil. But, just as nature doesn’t work in straight lines nor color in black and white, there is a more nuanced understanding of this dynamic between plants and soil. 

Plants are Farmers 

Plants are well known for their wondrous ability to capture carbon dioxide and release oxygen through photosynthesis. What is arguably the more remarkable aspect of photosynthesis is plants’ capacity to harness the immense energy of the sun and convert it into roots, shoots, fruits, and perhaps most importantly, the liquid carbon exudates that plants release into the soil. 

Plants allocate as much as 40% of the total fixed carbon from photosynthesis to the production of root exudates. These sugar-rich exudates serve as the primary food source for beneficial soil microorganisms. Throughout a plant's lifecycle, the exact chemical makeup of these exudates changes in response to what a plant needs. For example, if a plant needs more zinc to fulfill a physiological need, it will produce exudates that attract specific fungi or bacteria that specialize in solubilizing zinc. Likewise, plants will employ the same strategy to entice specific disease suppressive organisms when threatened by pathogens. In essence, plants are farming soil microorganisms.

Ecological Succession

Ecological succession is a concept that describes how ecosystems become more complex over time. It is why the bare soil in a recently tilled farm field will only stay bare for so long. Opportunistic plants, also known as pioneer plants, quickly move in to cover bare soil, filling an essential niche in the early successional ecosystem. Without further human intervention or disturbance, the first plants will complete their function by producing abundant seeds to spread further and cover bare soil. The rapid propagation and subsequent spreading of weeds over bare ground prevent the loss of vital topsoil, provide organic matter in the form of leaves and shoots, and the first injection of precious liquid carbon produced through photosynthesis and distributed throughout the plant’s root zone. Pioneer plants are commonly known as weeds and are considered one of the biggest menaces in farm fields and gardens. However, a closer look at the process of ecological succession shows that weeds are responsible for kickstarting this intricate soil building and paving the way for more complex plant communities.   

The next plant type to take hold is the perennial grasses and forbs. Thanks to the fast-growing annual weeds that came before them, these perennials can invest more photosynthetic energy into building extensive root systems. The expansive network of new roots in the soil provides the next level of erosion mitigation. The increased root surface area augments the soil's microbial activity and diversity as bacteria and fungi amass to feed on the plant's root exudates. Mycorrhizal fungi enter the picture at this point, forming a symbiotic relationship with plant roots. Plants initiate and maintain this relationship by exchanging sugars for soluble nutrients, water, and disease and pest protection supplied by mycorrhizal fungi. This mutually beneficial relationship effectively expands a plant's nutrient-absorbing surface area in the roots zone by 1,000 times. 

With an increasing mass of living roots pumping liquid carbon into the rhizosphere, the soil microbiome begins to flourish and thus prepares the system for more complexity. 

A tipping point occurs when fungi start to outweigh bacteria in the soil, resulting in a more neutral to acidic pH and changing the predominant form of nitrogen from nitrate to ammonium. This progression creates a soil environment that starts to favor ligneous plants, namely shrubs and trees, until we reach a complex forest system, commonly known as a climax forest and described in more nuance as dynamic equilibrium.

The Bio-Mineral Flywheel

The movement of ecological succession happens little by little over time, with plant communities influencing soil development just as much as the soil affects the next wave of plants in the system. All around us, this generative, plant-driven process is taking place. 

Cover crops, green manures, polyculture planting, and agroforestry systems all follow the principle of the bio-mineral flywheel. These practices maximize the harvest of energy from the sun, provide food sources for soil microorganisms who store carbon in the soil, build water holding capacity, cycle nutrients, and ultimately create the conditions for more complexity and more life. By following this principle and implementing best practices, we can accelerate or arrest the process of ecological succession for optimizing soil conditions for a given crop.  

While there is no denying that healthy plants come from healthy soils, that statement is incomplete without including the fact that plants are responsible for creating healthy soils in the first place. 

Image Credit: SoilSymbiotics – www.soilsymbiotics.com