This blog article is for readers who may have heard of subsoil, but don't really know what it is, or how it may or may not affect their gardening efforts. 

What is subsoil?

There are several layers (also known as “horizons” to soil scientists) that can be found when we dig deeper and deeper down into the soil. We can imagine all the individual layers of a soil stacked one on top of the other like a layer cake, this is called the soil profile. The surface soil is the uppermost layer of the soil profile, and the one we are most familiar with, because this is where most of the action takes place. Soil mixing with tillage, compost and fertilizer application, irrigation, plant root growth, and animal activity (including microbes) are mainly concentrated within the soil's surface layer. Additionally, decomposition of plant and leaf litter occurs most rapidly in the surface soil, this eventually leads to the formation of new soil organic matter. In comparison, the subsurface soil, or subsoil, is composed of one or more soil layers that lie below the influence of surface soil activities. There is not a consistent depth at which every surface soil layer changes into the subsoil layer(s), rather the subsoil occurs at a different depth from place to place depending on numerous factors, including some of the factors mentioned previously. This is the reason why it is difficult to determine where the subsoil layer begins in the soil profile.  

What does subsoil contain?

Subsoil is found deeper in the soil profile and does not directly experience a lot of the processes that occur in surface soil. As a result, the subsoil has less soil organic matter, less insect and animal activity, slower air and water movement, and a slower rate of decomposition compared to the surface soil layer. However, the subsoil underlies the surface soil and is indirectly influenced by the activities that occur at or near the soil surface. Therefore, the subsoil layer(s) has the excess fertilizer nutrients, herbicides, pesticides, and other dissolved salts that were lost from the surface soil due to heavy rainfall or irrigation. As surface soil begins to erode and age, the water moving into the subsoil can also have a lot of clay that accumulates in distinct regions of the subsoil.

What does subsoil look like? 

It is possible to identify subsoil by recording when and where you notice distinct changes in color, texture, and/or structure of the soil. The surface soil is often enriched in organic matter, giving it a dark appearance, typically dark brown or black. The movement of smaller soil particles from the surface soil into the subsoil increases clay content. The subsoil also lacks many of the rooting channels and biological pores that help build and support soil structure due to the limited presence of plant roots and animal activity. Therefore, as you dig deeper in the soil profile, if you see the soil color begin to fade from a dark brown or black to lighter shades of gray or brown, you notice it becomes harder to dig deeper into the soil, and/or you notice the soil is no longer loose and crumbly but becomes denser and lacks plant roots and worm channels, it is very likely you have reached the subsoil layer.    

What is subsoil used for? 

In general, the subsoil is a less suitable medium for plant growth compared to surface soil because of some of the factors mentioned previously. There are properties of subsoil however that make it suitable for other uses such as a source of “fill soil” for “cut-and-fill” construction operations, as a source of clay for building materials, and as an absorption layer for on-site wastewater disposal.

What can home gardeners do to keep their subsoil in great shape year after year?

Healthy soils are effectively balanced among the chemical, physical, and biological processes that allow soil to function properly and achieve a desired outcome, such as being an optimal medium for plant growth. Therefore, to build or support healthy subsoil, it is important to consider each of these processes, keeping in mind that what occurs in the surface soil layer can, at some point, be reflected by what is observed in the subsoil. With this in mind, to encourage soil health in the subsoil it is important to recognize that certain plant nutrients move with rainfall or irrigation water, such as nitrogen, and may be enriched in the subsoil. Effectively balancing plant nutrient fertilization to avoid excesses can limit the amount of plant nutrients transferred from the surface to the subsoil. The subsoil often has a poorly developed structure due to the lack of plant rooting channels and animal activity (including microbes). Growing a diverse group of plants that have distinctly different rooting depths, ranging from shallow- to deep-rooted, can encourage the formation of rooting channels that extend into the subsoil. The burrowing activity of earthworms also creates channels that deep-rooted plants can easily use and follow into the subsoil. Additionally, the presence of actively growing plant roots acts as a food source in the subsoil that stimulates microbes. The physical structure of subsoil can be improved with deep-rooted annual or perennial plants. As soil structure improves and more of these biological pore spaces are created, the rate of water and air movement into the subsoil layer will increase, helping sustain the balance among chemical, physical, and biological processes.           

What is the substratum layer of soil? Does that layer affect gardening at all, and if so how?

Stratum is a term that is commonly used to identify a distinct layer of rock or soil. Substratum then is a term used to indicate the relative underlying position of a distinct rock layer within stratified rock formations or a distinct layer of soil that lies below the surface soil. The substratum layer of soil has some of the same properties that are common to the subsoil. Although, many of the chemical, physical, and biological processes are further diminished in the substratum layer compared to the subsoil. Specifically, growing roots are not likely to be found in the substratum, so the properties of the substratum layer are unlikely to directly influence plant growth. The limited amount of water and air in the substratum layer will however help breakdown larger solid rock fragments, forming a loose and very coarse structure that closely resembles the mineral composition of the underlying layer(s) of the soil profile.      

What is the bedrock layer? Does that layer affect gardening at all, and if so how?

Bedrock is the bottom layer of the soil profile layer cake. The bedrock layer consists of solid rock that has not yet been exposed to the chemical, physical, and biological processes of the surface soil and subsoil. In some places, bedrock is the foundation from which the overlying soil layers developed, while in other places, the bedrock layer may have become “buried” by windblown sand or sediment. Bedrock does not directly influence plant growth, but it can determine the type of clay minerals found in different layers of the soil profile. 

Dr. Anthony Fulford is the Area Nutrient Management and Soil Quality Advisor for Stanislaus, Merced, and San Joaquin Counties.