Soil is such a complicated topic. Healthy soil even more so. The factors that make up the living breathing entity we call soil are outlined here. This is the first of a four-part series that includes The Basics of Soil, The Role of Beneficial Nematodes, The Role of Beneficial Protozoa, and The Importance of Organic Matter.
Defined by Function or Complexity?
Soil health is often described by how it functions, rather than what it is. Soil is a very complex system and we are learning more and more about it every day. Organic matter, microorganisms, and plants themselves all play a part. Their roles will be discussed in this series.
Dr. Mark Kibblewhite, from Cranfield University in England defines soil as "a multicomponent and multifunctional system, with definable operating limits and a characteristic spatial configuration." Which broken down means soil has many properties and functions. It’s also healthier when biological, chemical, and physical structures are in alignment. How a soil looks, and acts, is dependent on the parent rock, climate, topography, and soil biology. Soil is a vital living ecosystem full of biological activity.
Conventional agricultural practices have tilled, fertilized, or used soil in some way that has changed the alignment of the natural chemical, biological, and physical characteristics to obtain higher crop yields at the expense of soil health and its long-term fertility. Even in a backyard garden, soil is disturbed by pulling up vegetable plants and raking and hoeing the garden beds to make them smooth to plant seeds. In order to realign these soil characteristics, we have to know more about them.
We do know some of the basic biological, physical, and chemical characteristics of healthy soil.
Biological Characteristics of Healthy Soil
Soil is only as healthy as its biological inhabitants. There are billions of microscopic organisms in healthy soil. Their purpose is to live and multiply. Smaller organisms are prey for larger ones.
Microorganisms also form symbiotic relationships with plants to supply nutrients and water in exchange for sugars (or root exudates) from the plant. Most soil bacteria and fungi live in the rhizosphere (plant root zone). Arbuscular mycorrhizal fungi (AMF) spores germinate, infect the plant root, and form structures within the root. AMF grow hyphae far beyond the plant root zone to acquire water and nutrients for the plant in exchange for root exudates. All soils contain both beneficial and pathogenic microorganisms. Healthy soil is in balance so no particular microorganisms become overwhelming. This is particularly important for healthy plant growth.
But there are more than biological properties of soil. Microorganisms are necessary to break down the chemical and organic constituents of healthy soil.
Chemical Characteristics of Healthy Soil
Soil comes from rock, not necessarily the rock far below the soil in that particular place. Through wind or water erosion the topsoil of a particular field may be radically different from the bedrock. Glaciers deposited a great deal of rock that has become soil far from its parent rock. Soil can also be deposited through human actions.
But soil is more than just rock. Rocks are made up of minerals, and there are thousands of different combinations of minerals, which are made up of chemical elements. There are only 118 chemical elements according to the Periodic Table. There are a number of elements that are assumed to be in the soil; oxygen (O), hydrogen (H), and one absolutely necessary for life on earth - carbon.
The chemical characteristics of soil are the remnants of the rock it came from. Soil has a large number of chemicals, some of which are necessary for plant growth and some are not. At least, soil scientists haven’t discovered a plant use - yet.
Using plant growth function as a yard stick, soil scientists have termed soil chemicals “nutrients.” And in the realm of soil science there are two types of nutrients; macro and micro nutrients. There is debate among scientists about how some of these nutrients should be classified but all agree they’re necessary for healthy plants.
Nitrogen (N), phosphorus (P), and potassium (K) are considered the three big macronutrients. These play an important role in the success of the chemical fertilizer industry. They’re necessary in larger amounts than other nutrients for plant growth but healthy soil contains so much more.
Many soil scientists also classify sulphur (S), calcium (Ca), and magnesium (Mg) as macronutrients. All 6 of these elements are necessary for plant photosynthesis, chlorophyll production, and many other plant growth processes. Without them, your crop will be stunted and have a low yield.
Micronutrients are necessary in small quantities for plant growth, but deficiencies of these elements are common in certain soil structures and terrains. Molybdenum (Mo) helps plants metabolize N. Copper (Cu) is a dual-purpose element – it is necessary for enzyme formation for chlorophyll as well as a fungicide (killing plant pathogens but also destroying beneficial biological indicators of the health of their soil). Boron (B), manganese (Mn), iron (Fe), zinc (Z), nickel (Ni), and chlorine (Cl) are all necessary in very small amounts for chlorophyll production and plant growth.
Essential is a very selective word – it depends on the crop and other soil elements if its needed. Sodium (Na), cobalt (Co), silicon (Si), and selenium (Se) fall into this category of “are they necessary or not?” Soil is the basis for human existence, it’s where we grow almost all our food, and I have only accounted for 18 of the 118 elements on the periodic chart. Where are the 100 other elements? Have we just not discovered a “use” for them in soil for plant growth so don’t count them?
Having the elements from weathered rock and microorganisms doesn’t mean we have soil. We need the soil particles to create structure in the soil. Any given soil will have its own soil microbes and nutrient availability.
The Structure of Healthy Soil
Rocks weather by chemical or physical means. Raindrops on a rock will weather it both physically with the force of the rain and chemically because most rainwater is slightly acidic. Soil type and particle size is determined by the type of rock and the type of weathering. When sandstone weathers it becomes sand and has the largest particle size. When granite or basalt rocks are exposed to surface conditions, they create the basis for clay soils. A single clay particle can only be seen with a microscope. There is a third type of soil particle and that is silt. It is formed by physical breakdown of rocks by climatic and environmental conditions, like rain, winds, animals digging to name a few. Silt is rarely found as a specific soil. Because it is formed by environmental factors it is usually found in combination with sand or clay. Silt will create more pore structure in clay and help sandy soil to form aggregates.
The structure of healthy soil is partially dependent on soil particle size but mainly on the biological and chemical processes that take place within that structure. In the process of living and reproducing, microorganisms modify soil structure. Bacteria produce gooey substances to clump small soil particles, creating microaggregates, to protect them from desiccation, and predators. Fungi hold these microaggregates with their hyphae to create macroaggregates.
In the process of living, biological inhabitants of the soil break down organic matter, their food source. Organic matter contains carbon, essential to all life on earth, and many nutrients in non-plant-available forms. Soil biology converts those nutrients into plant available forms while improving soil structure. The biology in the soil performs multiple functions.
Particle size affects the amount of pore space between the particles. That space is filled with air, water, microorganisms, and organic matter. Sand, with large particles, has large pore spaces and a lot of the biological and chemical matter leaches away with rain.
Clay particles, on the other hand, are so small that the pores fill up quickly with air and water. After a rain, clay particles will hold onto the water. Pooling of water may appear on the surface. This leaves no room for air and creates an imbalance for plant growth and the health of the soil microorganisms. If the clay soil has a small amount of silt, which has larger particles, then pooling may not occur. But if the soil is waterlogged for too long the type of microorganisms will change from aerobic to anaerobic.
Practices That Destroy Soil Health
Soil health is a complicated affair. We don’t know all the factors that go into soil health but we do know a number of practices that can degrade soil health. The mineral component of soil is only accessible to plants if the biological component is alive and well. Microorganisms convert organic minerals into inorganic nutrients and therefore available for plant uptake. Tillage practices destroy microorganisms and pulverize rock. Nature also grinds rocks into smaller pieces but not on the soil surface and not as vigorously. It takes a day to till a field with today’s farm equipment, it takes thousands of years for nature to do the same thing.
In the process nature has allowed the biological and chemical processes to be dominant. In this way a rich dark topsoil, or humus is created. The minerals in the sublayers are brought to the surface by plant roots to be available for shallower rooted plants.
Nature doesn’t disturb the surface of the soil and maintains a plant cover. Erosion doesn’t happen in a natural ecosystem. Healthy soil is site and vegetation specific but always has a strong microbial community, lots of nutrients, and plant roots covering the soil.
Stay tuned for the second blog in this “Soil” series. It’s all about Good Nematodes and how to attract and retain them for healthier soil.
This is part 1 of a 4 part series, you can read the rest of the series here: