By Craig Madsen
When we think about agricultural production what factors do we consider to be limiting? We might consider the standard NPK trio of nitrogen, phosphorus, and potassium, or plain old water. What if we look at the soil as a living organism instead of a growth medium, how would that change our answer? A person can live without food for 1 to 3 months, without water for about 3 days, and without air for 3 minutes. Air is critical for our survival and for all living organisms, which includes soil biology from microorganisms to earthworms. Our soils need to be able to breathe.
Average soil is composed of 45% minerals, 50% pore space (25% water, 25% air), and 5% organic matter. The pore space is critical for water storage, gas exchange, and living quarters for soil biology to thrive. The better the soil aggregation the more pore space and the greater the ability of the soil to breathe and hold water. The soil is built by a community of organisms. Soil biology is what forms soil aggregates. Bacteria form the small aggregates, the building bricks. In most situations balance is important. A bacterially dominated soil is going to have limited structure. This type of soil is more likely to crust over when it rains causing the water to runoff versus infiltrating into the soil. When the rain hits a bacterial-dominated soil, it dissolves the small soil aggregates and fills the small pore spaces, resulting in water running off versus being absorbed.
A balanced soil needs fungi. Fungi bind the building blocks together by their hyphae and a sticky protein called glomalin and form larger stable aggregates. The hyphae are so fine that you cannot see them with the naked eye. These aggregates held together by the fungi are able to hold their structure when it rains resisting the erosive forces of water and allowing water to infiltrate into the still-open pore spaces. A diversity of living organisms in the soil is critical to building the infrastructure to cycle nutrients and store water. Worms, nematodes, arthropods, protists, and amoebas all play a role in building a healthy functional plant community. This is just as true on cropland as it is on rangeland.
A simple test for soil aggregation is the slake test. Take a fist-size soil sample from your field and a second one from a less disturbed site such as a pasture or along a fence line. Suspend the soil samples in water using hardware cloth and a quart jar to see how long it takes for the soil samples to dissolve in water. If the soil starts to dissolve in the water right away, you have a bacterially dominated soil with little to no structure or aggregation. If the soil stays together and the water stays clear, you have a soil that has stronger aggregation and a higher fungal component.
Another simple test is to look at your plant roots. Dig up one plant and look at its root system. Is it covered with soil or is it mostly bare? (See picture). If it is covered with soil, then the plant has developed a relationship with mycorrhizal fungi. This relationship is critical in building the soil infrastructure and in the cycling of nutrients as well as protecting the plant from harmful fungi and nematodes. Mycorrhizae fungi increase the root surface area for water and nutrient uptake by 40 times. Without this relationship, there is water in the soil the plant roots cannot access.
Plants exude root exudates into the soil to feed the soil biology. What does the plant get in return? The plant gets nutrients, water, pest, and disease protection, and gets tied into a communication system that warns the plant of threats. The bacteria, fungi, amoebas, protozoa, nematodes, worms, etc. are essential for the cycling of nutrients and the formation of humus. Humus is the black or brown color of biologically active soil. It has a great ability to absorb nutrients and water.
When the system is functioning well the soil is able to capture all the rainfall and it is able to access nutrients that the plant cannot access by itself. We’ve talked about water, now let’s talk about air. We breathe out carbon dioxide and so does the soil biology. What is a key component of photosynthesis? Carbon dioxide. A biologically active soil releases CO2 as it breaths out and the higher concentration at the soil level is readily available for the plants to absorb through the stomata located underneath their leaves. Higher levels of CO2 increase plant yields which is why CO2 concentration is increased in greenhouse production. Increasing CO2 levels in greenhouses to 700 to 1200 ppm has increased yields by 40% to 100%.
A well-aerated soil breathes in the surrounding atmosphere, which is 21% oxygen and 78% nitrogen. Over every acre of land to a height of 6 feet, there are about 6 tons of nitrogen in the air. How do we capture some of that nitrogen so we can reduce or eliminate synthetic nitrogen applications? That will be the topic of the next article.
Craig Madsen is a co-owner of Healing Hooves which was started in 2002. Healing Hooves uses a herd of goats as a tool to manage vegetation and help clients create their landscape goals. The goats are managed to target undesirable plants and to reduce fire risk for clients.
Before starting Healing Hooves, Craig was a Range Management Specialist with the USDA Natural Resource Conservation Service for 14 years. Craig is the current president of Roots of Resilience.