General soil makeup

Soil is the mineral and/or organic matter at the earth’s surface that is capable of supporting plant growth. The parent materials of the soil, as well as the nature of chemical physical processes that have altered it over time will have an impact on the nature of the resulting soil type

The types of plants vary as much around the globe as the types of soil. The differences in soil are a result of: temperature, precipitations, acidity, soil depth, toxicity, water retention capability, insects and diseases, and tolerance of plants to difficult issues in the area like flooding, drought, excessive salt, or high clay content.

Organic soils have more than half of the upper 80 cm as organic material. They are developed from an accumulation of plan residues. They include tundras and peat bogs and make up less than 1 percent of the world’s surface.

Organic matter is living or dead plant and animal residue. It is an active and important part of soil makeup that affects its chemical and biological properties. It brings Nitrogen, phosphorus and sulfur to soil, prevents erosion, and loosens soil for better aeration and water movement.

Mineral soils are all other soils (soils with less than 50% organic material). These soils are developed by rocks disintegrating into small particles and then shifting and mixing with organic material over time.

Soil fertility is defined by the nutrients that plants need that are available in the soil. For most plants there are 16 chemical elements that are essential for plant growth. These are hydrogen, carbon, oxygen, phosphorus, potassium, sulfur, calcium, iron, magnesium, boron, copper, manganese, zinc, molybdenum, chlorine, and nitrogen.

Soil Texture

There are 12 soil texture classes, shown in the Soil Texture Triangle:

See Soil Texture Triangle here

Clay heavy soils hold more water. They are wet and heavy and have poor drainage. Sandy soils are quick to drain and hold less nutrients. Generally, the more clay and organic content in soil, the more water it can retain. The largest amount of plant-available water is found in silt loams and high-silt soils. Soil compaction can alter the ability for soil to hold water as the pore size between granules becomes smaller.

The “squeeze” test: to quickly estimate soil texture on site, squeeze a handful of slightly moist soil. If the soil is dry, add a little water. If you squeeze the soil and it turns into a dense lump, it is heavy in clay content. If it will not maintain any shape and falls apart, it is largely sand content. If it holds its shape for a moment and then falls apart, it has a more balanced ratio of organic material, sand and clay content which is best for general planting.

Soil PH

Soil can be acidic, alkaline or neutral – most plants do well in 6.5 pH, but some plants have very specific pH requirements.

You can get a home pH test or have your soil professionally tested. It is recommended to check the pH of soil every 4-5 years because it changes over time (it’s alive!)

Living organisms in soil

There is a wide variety of animal life under the soil, including burrowing animals (moles, mice, rabbits, etc), earthworms, arthropods (mites, millipedes, ant larvae etc), gastropods (slugs and snails), and nematodes (microscopic worms). There is also plant life under the soil, including the roots of the plants that we see above ground. There are plants, such as pine trees,  with tap roots (a vertical primary root with lateral branches) and plants, such as corn,  with fibrous root systems (many roots of similar length which meet at a common point near ground level. There are also cyanobacteria (also known as algae) living under the soil. Algae grow rapidly in most, fertile soil and produce considerable amounts of organic material. 

Protozoa and slime molds digest bacteria and fungi which influence microbial populations and increases plant nutrient recycling.

Soil bacteria exceed all other microorganisms by number. One gram of soil can contain 100 million bacterial cells. You may have heard that beans fix nitrogen in soil, but it is actually heterotropic bacteria that grow symbiotically on the plant’s root systems. These bacteria reproduce and die rapidly, excreting nitrogen through this life cycle, which is then available to the bean plants as well as other nearby plants.

Soil viruses can be harmful for plants, animals, and humans depending on the type of virus. These soil viruses have limited hosts. Carriers of soil viruses include some nematodes, fungi, and the roots of certain plants. Enzymes in soils attack viruses, so they cannot live in soil for long periods of time without a host (1-4 weeks).

Ideal conditions for beneficial microorganisms in soil

Most microorganisms like moist soil at a pH around 7. They like warm temperatures although some can do well at very low and very high temperatures. They like soil with plenty of organic matter. Harmful microorganisms also like these conditions, so controlling them may take a specific regime to fight the problematic microorganism.

Fungal networks

Fungi live in soil and are organisms without the ability to use the sun for energy. They live on organic matter in the soil.. Fungi help decompose organic matter to hasten available nutrient recycling in soil. Fungi secrete substances that help form water-stable aggregates in soil.

Some fungi can be detrimental to plant growth, but many are beneficial for the health of living soil. For example, mycorrhizae are fungi that form a symbiotic relationship with plant roots. They attach around plant roots and aid in transmitting nutrients and water to the plant by extending the plant roots along their fungal paths. The plant provides the fungi with photosynthetic sugars which allow the fungi to access needed carbon. The mycorrhizae’s protective cover on the plant’s roots can also help a plant seedling to tolerate drought, disease, soil acidity and high temperatures. Inoculation with mycorrhizae in commercial plant soil in greenhouses to help aid plant growth has been practised since the 1970s because it gives plants much higher survival rates, especially under the stress of transplanting.

In fields and forests mycorrhizae help stabilize soil aggregates, sequester carbon, improve plant growth, deliver moisture, augment deeper root penetration and suppress root pathogens. There are different types of mycorrhizae: arbuscular, ectomycorrhiza, orchid, and ericoid. Within these types there are hundreds to thousands of fungal species of mycorrhizae. Most plants will have a symbiotic relationship with multiple mycorrhizae species at a time, and some plants are more picky. There are a number of plants that are “non-mycorrhizal” and will resist fungal colonization. See www.mycorrhizas.info/nmplants.html for a listing of non-mycorrhizal plants.  Mycorrhizae allow carbon and nutrients to be shared from one plant to another via their fungal networks, which can travel much further distances than the plant roots themselves. Plant roots themselves are like cities, and the mycorrhizal networks between them are like super highways connecting the metropolitan areas of all the plant root systems in the area. Mycorrhizal networks also help plants communicate with one another. Plant stress hormones and other signalling molecules are carried by fungal protoplasm to entire communities of plants. For example, the signal of an aphid infestation in a plant can be communicated and trigger defence mechanisms in other plants via mycorrhizal network within 24 hours!

These networks have been proven to be important for ecosystem health and sustainability. Some things we can do to help preserve mycorrhizal networks is to avoid using fungicides and to avoid disturbing soil by practices such as digging and tilling which will break these networks.

 

1. [Donahue, R. L., Miller, R. W., & Schickluna, J. C. (1990a). Soils: An introduction to soils and plant growth. Prentice Hall of India.]
2. [Phillips, M. (2017). Mycorrhizal planet: How symbiotic fungi work with roots to support plant health and build soil fertility. Chelsea Green Publishing.]