Soil and Why it Matters


Want to know the secret to a bountiful garden? It is more than just dirt, but closely affiliated. It is soil! In the plant world, dirt is often viewed as a lifeless, loose surface material that plants grow out of, but soil has life and fosters growth. Soil is composed of four parts: minerals, organic matter, air, and water. It not only helps anchor roots, but contains nutrients readily available for plants. Soil varies greatly in physical and chemical properties, especially according to geographic location.

Good soil structure is important because it allows air and water to move throughout its profile and gives beneficial organisms a place to live, eat, breathe, excrete waste, and reproduce. Some soils are naturally better structured than others, and it is possible to change physical properties with good management to produce healthy soil.

Soil Structure and Texture (Minerals)

Soil is very complex, so it is important to ensure that management practices do not lead to the decline of soil health. For example, heavy and excessive foot traffic causes compaction and reduces pore space, leaving less room for water and air flow and little room for roots to establish. Mineral solids are what you might picture when you initially think of soil because this is the part you can actually see. Soil particles are the broken down materials that result from rock (igneous, sedimentary, and metamorphic) while soil texture is made up of different sized particles–sand, silt, and clay–along with organic matter.

Sand is the largest soil particle. Sandy soil has very little structure and is free draining. This kind of soil is often infertile and prone to drought. Clay is the smallest particle and has a high ability to hold water and often has poor drainage. Most soils contain a percentage of sand, silt, and clay. When these three particles bond together, they form aggregates. The more structured the aggregates, the more space there is for water, oxygen, and microorganisms. An equal proportion of sand, silt, and clay is considered ideal for growing plants. However, perfect soil is not always a reality, especially not in the Treasure Valley as it tends to have a higher percentage of clay.  

Environmental factors also affect structure and drainage. Root and earthworm activity improve soil structure through the creation of large pores. Excessive cultivation, the removal of crop residues, and increased traffic break down soil aggregates, reduce pore size, and destroy soil health. Be mindful of where you walk and where your garden beds are located.

Water and Air

Soil structure and texture influence water infiltration and it’s availability to plants. As most people know, plants uptake water through their root systems. Therefore, the amount of water in the soil directly affects a plant’s intake. Pore spaces in the soil are where you can find water and air, and an ideal soil has 50% solid particles and 50% porespace.

Porosity is the space between soil particles. Fine soil has smaller pores, but they are more numerous than coarse soil and can hold more water. With smaller pores there is less room for air when water is present, and this can lead to anaerobic (no oxygen) conditions if soil becomes too bogged down. Plant death can occur if oxygen from the atmosphere cannot enter. On the other hand, bigger pores lead to fast draining conditions, hence why sandy soils are more prone to drought. Too much gas exchange can occur, and plants will dry out and die. Therefore, soils with medium pore size are ideal for getting the right balance of air and water.

Organic Matter

Organic matter comes from animals, plants, bacteria, fungi, and other microorganisms that live in the soil. Organic matter makes up the smallest percentage of soil, but do not overlook this component. This percentage comes from the decomposition of animal and plant products such as feces and decaying plant parts. Plants add organic matter in the form of roots and debris. Microbial activity such as bacteria, protozoa, and nematodes decompose these animal and plant products to release nutrients for plant uptake. Soil organic matter stabilizes aggregates by binding soil particles together, giving way to more pore space.

Many people add compost, manure, or grow crops to till into the soil to increase their organic matter content. Once organic matter is broken down by the soil’s inhabitants, it becomes humus. Humus makes soil a rich, dark color.  Because the Treasure Valley has clay soil, adding organic matter is one of the best ways to improve soil quality.

How to Improve Soil

Now that the basics of soil have been covered, let’s talk about how to improve soil health. Soil is a critical resource and the better we as gardener’s steward it, the better our environment and garden health will turn out. Keep in mind the natural characteristics of your soil, such as its texture, natural drainage, and slope.  

As previously stated, the Treasure Valley’s soil texture is primarily clay. Clay soils are more prone to poor drainage and hold onto water more readily. Our soil is more alkaline on the pH scale, which is directly correlated to its chemical makeup. In alkaline soils, some nutrients are more readily available than others, so adding nutrients in the form of fertilizers (natural or synthetic) is often necessary.

Slope is immediately related to your own geographic location. If clay is a problem in your landscape and water is applied more rapidly than the soil can infiltrate, water will run and strip your topsoil, only to accumulate in low spots or runoff into other areas. Keeping these factors in mind when considering how to improve soil health is important because some practices are more beneficial than others.

  1. Reduce Tillage & Traffic:
    Reducing tillage minimizes disruptions to soil aggregates by not breaking them up. Minimized tillage maintains natural aggregates and prevents loose or broken down soil particles from blowing away. When soil is continuously turned, it results in the decomposition of organic matter which further leads to loose, exposed soil particles. Keep in mind that not all tillage is bad and some is even necessary. However, when tillage becomes a constant practice, soil integrity is lost.
    Excessive foot and wheel traffic also compact the soil. Traffic reduces pore space which impedes root growth, leads to poor water infiltration and more runoff, and increases soil hardness.
  2. Increase Organic Matter Inputs:
    Organic matter improves water-holding capacity and nutrient retention in sandy soils, and loosens up minerals that become sticky when wet and hard when dry in clay soils. Organic matter provides a supply of nutrients for plants and increases microbial activity. Two common amendments are compost and manure. When plants die and when leaves fall into the garden, incorporating these into the soil also increases organic matter when they decompose.
    Cover crops are another great way to incorporate organic matter. These plants are specifically grown in the garden to improve soil health, attract pollinators, smother weeds, and slow erosion. Recommended cover crops for clay include clover, winter wheat, buckwheat, alfalfa, and fava beans. These plants bring nutrients up from the subsoil into the topsoil while breaking up compact clay. Plant these crops in your garden beds in the fall and allow them to grow all winter. In the spring till (an instance where tilling is encouraged) these crops into your soil before they go to seed for increased organic matter.
  3. Reduce Pesticides & Provide Habitat for Beneficial Organisms
    Diversified plants in the landscape invite different beneficial organisms that naturally fight and prey on pests. Use of broad spectrum pesticides can harm and kill beneficial insects that dwell in the soil. Integrating a pest management plan with pesticides as a last resort decreases the use of chemicals. Incorporating more plants with greater diversity not only provides habitat, but stabilizes soil and combats erosion.
  4. Mulch
    Mulch shades plant roots and acts as an insulating layer to topsoil. It also provides nutrients over time, breaks up clay, allows increased water and air movement, and improves sandy soil’s water holding capacity. Mulch keeps weeds down so plants are not competing for soil space.
  5. Rotate Crops
    In a vegetable garden setting, crop rotation is one of the most beneficial practices you can incorporate for better soil management. Different crops do different things for the soil. Rotating crops combats soil borne pests and disease life cycles, which improves crop and soil health. Plus, this practice also reduces nutrient excesses and deficiencies in different parts of the garden.

Fostering soil health leads to the overall benefit of your garden. Soil is the literal groundwork for successful gardening and healthy plants. There is so much to learn about soil and its benefit to this earth and our ecosystem. Do not be afraid to get your hands dirty and start improving the health of your soil for healthier plants and a healthier life.

By Riley Rehberg

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