Healthy soil is alive and breathing. It is full of bacteria, nematodes, and worms. It consumes oxygen. It is an ecosystem unto itself.
In order to understand any other aspect of landscaping, lawn maintenance, or gardening, we must first understand the ecosystem that keeps those things alive and thriving. Once we are familiar with a healthy, functioning soil profile, we can more fully understand where other soils may be lacking.
Many issues in your yard are rooted in the soil. So, let’s get digging.
Soils, like all good things, are comprised of layers. These layers, or horizons, determine how a soil is classified, which therefore tells us how it will behave.
- R (bedrock) Horizon
- Bedrock is the ultimate foundation of any soil. Drainage, cation-exchange capacity, pH levels, erosion, and mineral availability can all be influenced by the bedrock layer.
- C (parent material) Horizon
- This layer is characterized by how it got there, and how it has settled on top of the bedrock layer. All of the forces of nature get a say in how soil is formed. Ice, for example, can carry particles of very different sizes the same distance. Whereas water will carry small particles much farther than large ones. The deposits of these particles are what makes up a parent material in any given soil. Other factors in creating parent material are gravity and wind.
- B (subsoil) Horizon
- The subsoil is made up of a mixture of the parent material and minerals that have leached through from the upper layers. It is usually mineral-dense, but too far below the surface to support soil life.
- A (topsoil) Horizon
- Topsoil is what most soil-improvement strategies focus on. It is what defines the health of the soil profile, and is the only layer we can reasonably amend and improve. This layer is comprised of smaller parent material particles, organic matter, minerals, and most of the life that dwells within the soil.
- O (organic) Horizon
- This layer may or may not be present. The organic horizon is made up of dead and decaying organic material. Examples would be the layer of leaf litter in a forest or a layer of thatch in your lawn.
These horizons, collectively, make up a soil profile. Examining each horizon, and what it consists of, will give a comprehensive overview of the health of that particular soil sample.
Soil texture is determined by the ratio of three soil separates in relation to each other. These separates (sand, silt, and clay) are a direct result of the C Horizon, or parent material.
Parent material can be deposited by five different forces:
- The official name for parent material deposited by ice is glacial till. This is the result of material left behind by a moving glacier. Ice can carry all particle sizes equal distances. Therefore, glacial till is marked by a parent material layer that has no order, and is a mixture of boulders, pebbles, sand, and clay.
- As water moves, it tends to drop heavier items, like sand, while moving finer particles farther distances. These alluvium deposits are characterized by a sandy layer closer to the water source, while clay deposits are found layered farther away.
- Gravity, like ice, leaves a layer of poorly-sorted parent material. Soil at the bases of mountains or cliffs can be an assortment of boulders, rocks, and sand as a result of material falling down the sides.
- Wind sorts soil similarly to water. Finer, lighter particles can become suspended and travel great distances, while larger sand particles are swept across the ground. Sand dunes are an example of this process, where dunes form around an initial obstacle, like brush, and collect in a pile as it’s blown across the surface. Over time, as sand is progressively blown across the area, many piles of sand build up and form sand dunes.
- Lakes & Oceans
- This parent layer is formed under bodies of water, and is generally made up of very fine particles. Over time, as lakebeds dry up and coastlines evolve, these layers become exposed and begin to break down.
Parent material can also be the result of the bedrock being exposed and subjected to weathering, although this forms a very weak C Horizon and does not have much impact on the layers above it.
The texture of your soil will depend on how the parent material was deposited in your specific location, and can be a mixture of different processes over the centuries. Because parent material is something that accumulates and settles over many years by natural processes, it is nearly impossible to fix or change in your lawn or landscape. Clay textures will remain clay textures, because a change in texture would require a change in parent material.
There are three basic classifications of soil separates:
- These are the finest soil particles. They are carried the farthest by wind and water, and tend to settle at the top of C Horizon layers. Because they are so fine, they can pack together very tightly. If the region gets lots of rainfall, this can make a soil waterlogged very quickly. However, if the region is dry, it can make it impossible for the water to penetrate.
Clay can also hold on to nutrients and minerals. Since water has such a hard time flowing through, it cannot wash the nutrients and minerals into the lower horizons. Therefore, clay soils tend to be very rich in nutrition. However, they also have many problems with rot and fungus, because it is difficult to introduce oxygen into a soil with such poor porosity.
- Silt is the middle-of-the-road particle size. It could be defined as a gritty clay or a fine sand. Silt has a good balance of drainage to nutrient retention, and allows moderate water absorption and retention.
- Sand can have some positive qualities in soil. Its large particle size helps avoid compaction, and creates more porosity. Oxygen is able to fill those pores, which helps ward off rots and fungi.
However, sand is a very poor texture in terms of nutrition. Its low surface area makes it much harder to latch onto minerals and nutrients, and they are easily washed down into the lower horizons. Due to the poor water retention, sandy soils have a difficult time supporting microbial and insect life.
You can easily determine your soil texture by wetting a handful of soil and squeezing it in your hand. If the soil sticks together, it’s clay. If it is crumbly, it’s silt. And if it won’t form a clump at all, it’s sand. Of course, most soils are a mixture of textures, so you may have a silty sand or silty clay. Semi-equal distributions of all three can be classified as loam soils, and these are the most fertile.
Overall, texture will determine how well your soil will retain nutrients, minerals, water, and oxygen. These factors will determine how well your soil holds and utilizes organic matter, and therefore what your soil structure will be.
Soil structure, unlike texture, can fluctuate depending on the season or weather patterns. Structure is basically the relationship between soil texture and organic matter. As organic matter is broken down, binding agents form between soil particles and organic matter. As soil particles bind together, they form aggregates. The formation of these aggregates is what gives the soil its structure.
Depending on the soil texture, organic material, microbial activity, water and air mixture (porosity), and temperature, this structure can take many forms.
The three basic soil structures are:
- Single Grained
- If the soil texture is very loose and sandy, then it will likely lack the ability to form aggregates. This results in a soil that cannot hold any form, and most organic material, water, and nutrients pass right through.
- If the soil texture is loamy, there is a greater chance that the soil particles can form clods, or small structures called aggregates. These aggregates can take many forms; blocky, granular, platy, prismatic, columnar, crumbly, and others. All are able to hold on to a relatively useful amount of nutrition, minerals, water, and organic matter.
- If the soil texture is too high in clay particles, there will be an overabundance of binding agents. Instead of workable, porous aggregates, these soils form large, impenetrable masses. They can be very high in nutrient and mineral content, but lack the ability to take in water in dry climates, or to drain properly in wet climates. When these soils dry out, it can form large cracks along the surface.
Different structures allow different amounts of nutrients, minerals, water, and air to remain in the soil. The ideal soil is a balance between all factors.
A healthy soil structure is easily identified by a layman’s test. It should be easy to till or dig when dry and crumble without being dusty. But also, when wet, you should be able to dig into it without it sticking to your shovel.
Structure, unlike texture, is a soil property that we are able to influence. Structure is the relationship between texture, which we cannot change, and organic matter, which we can change. Therefore, in order to improve soil structure, our focus must be on organic matter.
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First, we must make a distinction between organic material and organic matter.
Organic material is the dead leaves, grass clippings, and banana peels you throw into the compost bin. Organic matter, on the other hand, is the end result of decomposition. Once microbes break down organic material, it becomes a stable substance.
You’ve probably seen this in action. A pile of grass clippings will diminish in size very rapidly as decomposition begins. Water evaporates, fibers are broken down, and a fully-composted grass clipping pile will be about 90% smaller than its original size.
This leftover portion is organic matter.
Organic matter will mineralize, or transform, into usable materials, at a rate of about 5% per year. However, this is greatly influence by the weather and soil activity.
Organic matter is what makes soil able to sustain life:
- As organic material mineralizes, it releases materials that can be absorbed by plants. This includes nitrogen, phosphorous, potassium, and many other nutrients necessary for plants to grow.
- Water Retention
- Organic matter can hold up to 90% of its weight in water. Due to its spongy texture, this water is easily available to plant roots for absorption. Clay also retains high amounts of water, but because clay is inorganic, and very different in how it binds to water, this water is usually unavailable for plant roots to absorb.
- Organic matter determines aggregation. Any soil can become aggregated given enough time and organic matter. But a lack of organic matter will result in single-grained or massive soil structures.
- Erosion Control
- Since organic matter is what holds soil together, it makes sense that without it, soil falls apart. Healthy soils are able to retain their form due to the structure that organic matter provides.
An important byproduct of decomposition is the production of hydrogen ions. As these ions are produced, the soil pH drops or becomes more acidic. This is offset by the addition of new organic material.
Plant material is usually alkaline, which is a higher pH value. The addition of this alkaline material can offset the acidity in the soil, and maintain a fairly neutral pH level.
The pH levels in a soil impact the availability of nutrients and minerals. On a scale of 1-14, pH should hover right under 7 for most plants to function properly. Too far in either direction causes major nutrient imbalances.
If organic material is consistently incorporated into the soil, pH is usually not an issue.
Healthy soil structures have a balance between loamy soil textures and organic matter. This balance will naturally bring about a healthy amount of porosity.
Porosity is simply how porous the soil is. Organic matter needs water and air in order to mineralize, so it’s important that water and air can actually penetrate the soil.
Sandy soils have larger particles and therefore a very high porosity, and cannot hold water. Clay soils have microscopic soil particles which creates a very low porosity, so they cannot hold air.
Healthy porosity is determined by the sponge-like consistency of organic matter, as well as a balance between soil particle sizes. Ideally, the soil should be half solids and half pores. These pores would vary in sizes to allow half to hold water, and a half to retain air; very similar to a damp sponge.
Low porosity can result in too much water in the soil, or soil that has difficulty absorbing water. Too much water can cause root rots, fungi, and prevent organic material from decomposing. Too little will result in obvious water intake problems for surrounding plants, and also make it difficult for soil organisms to survive.
High porosity will result in no water holding capacity. Organic matter will be washed right through the soil without a chance to mineralize, and plant roots will have no reserve water supply.
An abundance of oxygen will prevent rots and fungi, but the lack of water will result in plants unable to support new growth.
A medium porosity will maintain moisture levels in the soil without creating an environment that breeds rots and disease. But, perhaps, more importantly, a balanced porosity will allow the soil to host a variety of insects, bacteria, nematodes, and worms.
Living Organisms In Soil
Healthy soil does not exist without living organisms. They are responsible for almost every process that makes soil able to support plant life.
You know you have succeeded in creating a healthy soil if you dig up a sample and find an abundance of insects and worms. This is the goal of a truly healthy soil- to promote and maintain life under the surface.
The most influential soil organisms are:
- Earthworms are an obvious sign that your soil is healthy. Earthworms feed on organic matter, and excrete highly-nutritious castings. As they move through the soil, they create tunnels that help maintain porosity and prevent compaction.
- These are technically worms, although they are significantly smaller than earthworms. Nematodes are usually beneficial, although some feed on the roots of plants. These tiny worms are able to break down organic matter, transport microbes through the soil, fight off soil-borne diseases, and keep harmful bacteria and fungi in check.
- Bacteria has many important functions within the soil.
- Break down organic matter
- Hold on to nutrients to prevent them leeching through the soil
- Fix nitrogen in the soil
- Break down pesticides and chemicals
- Suppress diseases
- Bacteria has many important functions within the soil.
When you introduce organic matter into your soil, you’re not feeding plants. Rather, you’re feeding these microorganisms. In turn, these worms, nematodes, and bacteria transform organic matter into the nutrients that plants can absorb.
The Big Picture
Healthy soil is the result of a properly-functioning ecosystem. So, the steps to creating a healthy soil are all geared towards creating an inviting home for microbes, which will in turn create a healthy environment for plants.
In the end, soil improvement can be boiled down to, “If you feed them, they will come.” Meaning, feed, and water the microbes, and the soil will self-correct given enough time.