Do You Have Soil or Dirt: The Science Behind the Biological Approach

The most productive soils on the planet are not found in agricultural lands.  Instead, locally, look to our conifer forests.  They require no nutritional supplements, or herbicides and pesticides.  The Forest Service does not apply nitrogen at 100 units/acre, or gypsum, or sulfur.  Yet even so, if measuring “yield” from these lands, it is clear that they far outproduce agricultural production in terms of biomass.  How is this possible?  How can such significant plant production occur when no nutritional supplements or pest control measures are implemented?  This answer is simple:  The soils of these lands have not been disturbed and have a fully functioning community of soil microbes that constantly cycle nutrients and make them available for the community of above ground plants.  By contrast, our agricultural soils are constantly under pressure from mechanical and chemical disturbance.  These practices are detrimental to the soil biological community and interrupt the process of nutrient cycling.

So what exactly is “nutrient cycling”?  Although it is difficult to believe, there is no soil on the planet that does not contain the necessary nutrients for healthy plant growth.  While not necessarily present in plant available forms, there is more than enough nitrogen, phosphorus, potassium, etc…., locked in the crystal matrix of the sands, silts, and clays that make up the mineral component of soil to supply plants with all the nutrition they need.  But if it is not in plant available form, how does this feed the plants?  Making these nutrients available is the role of soil biology.

In an undisturbed soil, minerals are extracted from the sands, silts and clays by organic acids and enzymes produced by bacteria and fungi.  These nutrients are then consumed by the bacteria and fungi, and locked away inside their bodies.  Enter the higher-level predators in the soil biological community.  These predators are the protozoa, nematodes and micro-arthropods that feed on the bacteria and fungi and incorporate what nutrients they need from the bodies of their prey.  Any excess nutrient load is expelled, in plant available forms.  This is the “poop loop” that most growers will recognize as occurring above ground when we use animal manures as fertilizers. The same thing is happening below ground.

But the process is even more amazing than that.  Throughout millions, perhaps billions of years, plants and soil microorganisms have developed a symbiotic relationship where both sides benefit.  In high school biology we learned that plants, through photosynthesis, extract CO2 from the air and produce a variety of sugars, carbohydrates and a smaller amount of proteins.  What we didn’t learn in high school is that while some photosynthetic products are used directly by the plant, some are transported to the plant’s roots where they are released into the soil as “exudates”.  Why would plants do this? What we now know is that these exudates are a major source of food for bacteria and fungi, and create large communities of microorganisms immediately next to the plant roots.  The communities are large enough to form a physical barrier for the roots and can suppress soil-based pathogens and pests.  The plant also has the ability to release exudates that are foods for specific organisms.  Imagine that a plant is deficient in manganese.  By producing an exudate that targets microorganisms that have the enzymes or organic acids to extract manganese from the soil matrix, the plant encourages the growth of these organisms over others.  These targeted organisms extract manganese from the soil, incorporate it into their bodies and finally it is released when they are consumed (and excess excreted) by predatory microorganisms.  Extra manganese is now available for the plant.  In a healthy soil, this process is continuous and provides all the nutrients a plant needs to reach its full potential.