Unless you are growing all of your plants hydroponically, soil is an important component of any garden. The soil affects how your plants grow. Having a basic understanding of soil science will allow you to become a better gardener because you will have the tools necessary to create the best environment possible for your plant.
To help you along in your journey to better understanding soil science, I have compiled a list of 11 terms every gardener should know and understand.
1. Bulk Density
Bulk density is the dry weight of soil divided by volume and is often expressed in grams per cubic centimeters (g/cm3). It is important to understand that bulk density accounts for air space within the soil. This distinguishes bulk density from what is referred to as particle density. Particle density (typically around 2.65 g/cm3) is a measure of the weight of soil particles per unit volume.
Bulk density is a measure of soil compaction. This is important because a more compact soil (higher bulk density) will not be as well-aerated as soil that is less compact (lower bulk density). In addition, more compact soils can result in shallow rooting and poor root growth.
In addition, a high bulk density indicates that water may not infiltrate through the soil profile very well resulting in poor drainage. Poor drainage can negatively impact plant and root growth.
If you want to know the bulk density of your soil, I recommend reaching out to your local agricultural extension office. They will be able to connect you with a good soil testing lab in your area. Most soil tests cost between $20 and $50 depending on how many samples you want them to test.
If you get your soil test back and the bulk density is less than 1.5 g/cm3, I recommend trying to lower the bulk density of your garden’s soil. There are a couple of ways to do that. Tilling soil temporarily decreases bulk density. However, rainfall and foot traffic will bring the bulk density back up over time. To decrease bulk density long-term, it is important to increase the amount of organic matter in the soil. You can do this by planting cover crops during the winter or by adding compost to your soil.
2. Soil Organic Matter (SOM)
Soil organic matter (SOM) refers to the part of soil that is comprised of decaying plant and animal tissue. Most agricultural soils contain between 3 and 6 percent organic matter. Sandier soils tend to have a low percentage of organic matter. The amount of organic matter increases with clay content.
Organic matter is a very important component of soil. It provides nutrients for the soil, improves soil structure, and increases water holding capacity. See point number 6 to learn more about water holding capacity.
I have occasionally heard humus and organic matter used interchangeably. However, they are not the same thing. Humus accounts for only a portion of all organic matter within the soil. So what is humus? Humus is the dark material that forms at the end stages of biological decomposition.
Humus modifies the soil in a way that promotes plant development. Humus adds nutrients to the soil, converts nutrients to forms that are available to plants, and improves soil structure in a way that allows for more water retention.
Now that I have established that organic matter is an important part of soil, you may be wondering how you can add some to your garden. The easiest way to add organic matter to your soil is to apply compost. Compost refers to a pile or bin of organic matter, such as grass clippings and vegetable scraps, that are in varying degrees of composition. Finished compost smells and looks like fresh soil.
Typically, gardeners dig in compost while tilling. Work in 1-2 inches of compost for every 3-5 inches of soil. Alternatively, you can apply compost as a mulch. In this case, apply 2-4 inches of compost to the soil surface.
3. Soil Texture
Soil texture is a term that refers to the relative amounts of sand, silt, and clay. It is important to understand that soil texture is different from soil structure which describes how soil is arranged into aggregates. Soil texture is described in terms of the amount of sand, silt, and clay. The table below summarizes the characteristics of these particles.
| Particle | Equivalent Spherical Diameter (AASHTO Standards) | Chemical Composition |
|---|---|---|
| Sand | 0.075 mm – 2.0 mm | Silicon dioxide in the form of quartz |
| Silt | 0.002 mm – 0.075 mm | Quartz and feldspar |
| Clay | <0.002 mm | Aluminum silicate from weathering rock |
Soil texture influences a number of soil properties. For example, sandy soils will have larger pore sizes because the particles are larger and there are no fine particles to block the pores. As a result, water will flow through sandy soils more quickly.
If you want to determine the texture of your soil, you can perform what is known as the squeeze test. Take a handful of moist (but not wet) soil and squeeze it in your hand. Sandy soils will fall apart in your hand. In contrast, soils with a lot of clay can be shaped (like modeling clay) by poking and squeezing the soil. Clay soil will also feel firmer when you squeeze the bit of soil. Silt will leave a residue on your hand.
The squeeze test is definitely not exact. In order to precisely determine the soil texture based on the soil texture triangle (see below), you would have to have a lab perform a particle size distribution. However, an exact texture is not necessary for hobby gardeners. The squeeze test will give you an idea of what you are working with.
4. Loam
As you can see from the soil texture triangle above, a loam consists of an approximately equal proportion of sand, silt, and clay. However, there are a number of different types of “loamy” soils (e.g., sandy loams, sandy clay loams, loamy sand, silty clay loams, and clay loams).
“Loamy” soils are considered the best soils for growing plants. This is because of the different soil particles working together to create an ideal environment. The sand allows for good drainage. Silt and clay increase the water holding capacity of the soil and contain nutrients.
Because loams are comprised of different sized particles, there are pore spaces that not only allow for good drainage but allow roots to penetrate through the soil profile.
Finally, loams make good garden soils because they are naturally fertile. They are home to microorganisms that play a vital role in decomposing organic matter and recycling nutrients.
5. Aeration
Soil aeration describes the exchange of gases between the soil and the atmosphere. Because plant roots absorb oxygen and release carbon dioxide during aerobic respiration, soil aeration is one of the most important aspects of plant productivity (Taylor, 1949).
In general, the internal transfer of oxygen from the plants above ground to the roots below ground does not occur quickly enough to keep up with the roots’ need for oxygen. That means aeration is important in order to replenish oxygen supplies in the soil.
Gardeners aerate their lawns to break up compact soil. You can do this using a digging fork or shovel. Simply break up the top 6 to 12 inches of soil to allow for healthy root growth.
Earthworms and other creatures that live in the soil also improve soil aeration as they break up the soil while digging tunnels.
6. Water Holding Capacity
Of the water entering the soil, some water will be evaporated and some water will drain through the soil profile. Cohesion and adhesion will hold the remaining water in place.
Water holding capacity is the total amount of water soil can hold at field capacity which represents the maximum amount of water available for plant growth. Field capacity is the amount of soil water soil can hold after gravity has drained away excess moisture. At field capacity, all macropores are filled with water.
Soils with higher clay content will have a greater water holding capacity. However, this does not mean that there will be more water available for plants. Clay soils have smaller pores which means the plant has to work harder to suck up the water. Sandy soils have large pores. However, water just drains through the pores because cohesion forces are not large enough to overcome gravitational forces.
Soil structure has a significant impact on water holding capacity. Soil aggregates create pores that allow the plant to store water. Organic matter stabilizes the soil structure and will help increase the water holding capacity.
7. Waterlogged Soil
Waterlogging occurs when soil is saturated with water. This occurs when water enters the soil profile faster than it could drain away.
In cases of prolonged waterlogging, anaerobic conditions could prevail. Anaerobic conditions occur when the amount of oxygen used by organisms exceeds the amount of oxygen diffused through the soil profile. This means that anaerobic environments lack free oxygen.
Without air, plant roots will die. In addition, waterlogging also decrease the soil pH which can be harmful to plants.
Poorly drained soils or compacted soils are susceptible to waterlogging. Because clay soils tend to be poorly aerated and poorly drained, soils with a high clay content are more likely to be waterlogged. If you are dealing with waterlogged soils, you could try the following strategies:
- Build a raised bed
- Increase the amount of organic matter.
- Plant cover crops. These are grasses, grains, and legumes that grow during the fall and winter. These crops increase organic matter and prevent topsoil from eroding away.
8. Rhizosphere
The rhizosphere is the area immediately adjacent to the plant roots. This area contains the substances leaked by plant roots during photosynthesis. The rhizosphere is an area of intense biological activity because the substances leaked by plant roots are food for certain types of organisms that are not present in the rest of the soil.
The rhizosphere plays an important role in plant growth. For example, plants may form mycorrhizal relationships in the rhizosphere. These are symbiotic (mutually beneficial) relationships between the plant roots and fungus known as mycorrhizae. Mychorrhizae colonizes the root system and aids the plant in nutrient and water uptake.
Some gardeners purchase mycorrhizal products to inoculate their flower beds before planting. Simply work the product into the top 4 to 6 inches of soil.
However, you should avoid inoculating existing gardens with mycorrhizae. There are many different species of mycorrhizae, and each one is adapted to a slightly different environment. The existing mycorrhizae might fend off any foreign mycorrhizae. This means adding mycorrhizae to an existing garden might not add any additional benefit to your garden.
If you are growing legumes, mycorrhizae also work in conjunction with nitrogen-fixing bacteria. These bacteria convert atmospheric nitrogen into forms that can be used by the host plant.
9. Porosity
Porosity is the fraction of soil volume that is comprised of air space. Typically, porosity is expressed as a percentage. The space between soil particles is called pore space. You can think of porosity as the number of these pores in the soil.
The porosity of a soil is important because the amount of pore space determines how water that a given soil volume can hold.
Clay soils actually have a higher porosity than sandy soils. This is because clay soils have more surface area than sandy soils. Clay soils have numerous small pores. Water can be held tighter in small pores than in large pores. This means that clay soils hold more water, but the plant has to work hard to take up the water in these pores. In contrast, sandy soils have less overall pore space, but each individual pore is larger.
10. Cation Exchange Capacity
Cation exchange capacity (CEC) is the sum of exchangeable cations that a soil can absorb at a specific pH. It is typically expressed in terms of milliequivalents per 100 grams or centimoles per kilogram.
CEC influences the soil’s ability to hold on to nutrients and bugger against soil acidification. Many cations function as essential nutrients for plants. Because soil particles are negatively charged, these positively charged nutrients but allow these to exchange with other positively charged particles in the surrounding soil.
Sandy soils, which do not have much organic matter, tend to have a lower CEC than clay soils that tend to be high in organic matter. The table below lists the normal range of CEC values for common texture soil groups.
| Soil Group | CEC (meq/100g) |
|---|---|
| Light-colored sands | 3-5 |
| Dark-colored sands | 10-20 |
| Light-colored loams and silt loams | 10-20 |
| Dark-colored loams and silt loams | 15-25 |
| Silty clay loams and silty clays | 30-40 |
| Organic soils | 50-100 |
To determine the CEC of soil, you will have to submit soil samples to a lab.
Unfortunately, CEC is an inherent soil property and is difficult to change. To improve nutrient uptake, make sure the soil pH is within the optimum range for the particular plant you are growing.
11. pH
pH is a measure of acidity or alkalinity. Acidity is a measure of the concentration of hydrogen ions, and alkalinity describes the ability to neutralize hydrogen ions. A pH of 7.0 is considered neutral. pH values lower than 7.0 indicate acidic conditions, and pH values greater than 7.0 indicate basic (alkaline) conditions.
The letters pH stand for power of hydrogen. pH is defined as the negative base 10 logarithm of the concentration of hydrogen ions. This definition is presented in equation form below.
pH = -log10 [H+]
Because pH is logarithmic, a substance with a pH of 6.0 has a hydrogen ion (H+) concentration that is ten times greater than the H+ concentration of water with a pH of 8.0.
Soil pH has a significant impact on plant health. This is because nutrients that plants need to survive become unavailable outside of certain pH ranges. For example, plants can experience iron deficiencies (even if there is enough iron in the soil) if the pH is too high. In general, most home gardens will do well if the soil pH is between 5.6 and 6.5. However, some plants prefer a growing environment with a pH outside of this range. For example, blueberries prefer to grow in more acidic conditions with a pH between 4.0 and 5.0.
In addition, pH levels that are too low or too high can cause certain plant nutrients to become toxic. For example, molybdenum becomes available in toxic amounts at very high pH levels.
To determine the pH of your garden soil, you will have to collect a sample and send it to a lab. If the soil pH is too high in your garden, you can lower the pH by applying organic mulch or sphagnum matter. There are also acidifying fertilizers available such as ammonium sulfate or urea.
If the soil pH is too low (too acidic), raise the soil pH using lime or wood ash.