Soil fertility refers to the ability of soil to supply and sustain nutrients for healthy plant growth. Maintaining the fertility and health of garden soil is important for good lawn and garden management. Soil fertility promotes the development of strong plants that enjoy better overall performance, are less susceptible to insect and disease damage and tolerate the negative effects of extreme heat, cold and drought. Lastly, fertile soils that promote the vigorous growth of desirable plants often produce less than ideal conditions for growth of many common weeds.
Plants obtain most of the nutrients they need through the soil. Healthy soils contain varying quantities of minerals and organic material (from plants and animals), as well as quantities of air and water which occupy the pore spaces between individual soil particles. Healthy soils are also living soils that support a complex ecology of beneficial bacteria, fungi and animal life (such as nematodes, worms and centipedes.) It is the presence and combination of ALL of these elements that determines the fertility of any soil and the ultimate number, variety and nature of the plants soil can sustain.
Soil fertility varies from one location to another. Why some plants fail while others flourish on a site speaks to the fact that plants grow best in the soil that best supports their nutritional needs. Soil fertility changes over time as nutrients are used and organic matter breaks down. Soils are naturally replenished by means of decomposing leaves, branch litter and animal waste accumulating on the soil surface; these components are broken down and reintroduced into the soil by weather, fungi, bacteria and animal activity. Animals help mix in and redistribute new organic matter through burrowing, introducing vital air and water into the soil by means of the small channels they create.
Physical properties of soil
About half of any soil is made up of minerals found in the form of very small bits of rock weathered by physical, chemical and biological processes. These particles vary in size from the relatively large (sand), medium (silt), down to the very small (clay.) It is both particle size and the proportion of each type of particle that determines a soil’s texture, the name by which a soil is described (e.g., ‘clay-loam’). Soils with high sand content are termed loose and open, as air and water enter and drain rapidly through them. By contrast, soils high in clay are termed heavy and air and water move through them slowly. Each type of soil particle has its own ability to chemically “hold onto” nutrients. In sand, nutrients leach away easily, resulting in soils that have low natural fertility and that tend to be acidic. Clay soils are slower to drain and the nutrients are held more effectively. For this reason, soil texture directly influences how well your plants develop and grow. You can’t really change the texture of your soil, but adding organic matter will change the effect it has on your plants.
Instructions for an easy home soil test to determine the relative amounts of clay, silt and sand in your garden can be found easily on the web.
Pore space and soil structure
Plants need both air and water at the root zone for active growth, which is why pore space in the soil is so critical. In an ideal soil, pores spaces of differing sizes and shapes can be up to 50% of the soil volume and hold fluctuating amounts of water and air, depending on the amount of rain. The large pores allow water to drain away and air to re-enter the soil. The small pores hold moisture in the soil where it is available to plant roots, allowing for important nutrient exchanges. The way soil particles stick together determines the soil structure and the balance between the large and small pores. Overworking the soil, digging when the soil is too wet or even walking on your garden beds will break down the soil structure and lead to compaction of the soil.
“Every garden relies on the organic matter in the soil and the living organisms those materials support…” (Reid 2014 at 166)
Organic matter is any material of biological origin such as decaying plant parts, soil organisms or animal wastes. Fully decomposed organic matter is called humus. Humus holds together mineral particles in clusters called aggregates and this aggregate formation improves soil structure. Humus loosens heavy clay soils, improving drainage and allowing air to enter the soil, creating spaces where a vigorous root system can grow. In sandy soils, humus acts as a sponge, improving water retention. Humus, consisting of microscopic particles with negatively charged surfaces, is also capable of attracting and maintaining a pool of plant nutrients (e.g., calcium, potassium and magnesium) that can become available to plant roots.
As important as humus is for improving soil structure and holding nutrients and water in place, it actually provides very little food for soil organisms, as the nutrients have already been absorbed in the many cycles of decay required to produce it. It is the fresh organic matter in varying stages of decay – plant leaves, stems, roots and flowers – added to the soil surface that fuels the soil food web. This fresh organic matter increases the activity and number of soil organisms; as it is broken down, more nutrients are made available to plant roots.
Soil organisms are the smallest part of the soil’s organic matter by volume but they have a huge impact. The bacteria, fungi, protozoa, nematodes and earthworms all have different roles in the soil food web and function as shredders, decomposers, digesters, nitrogen fixers and predators. They improve soil structure by aerating and creating tunnels in the soil and by recycling and decomposing organic matter and making the resulting nutrients essential for good plant growth available to plants.
Mycorrhizal fungi work with plants’ root systems, effectively extending the roots to reach and extract more water and nutrients. They are particularly valuable for harnessing phosphorus, an important component for plant growth, and making it available to woody plants.
Chemical properties of soil
There are 18 elements essential for plants to grow successfully and complete their life cycles. The 9 macronutrients consist of hydrogen (H), oxygen (O), carbon (C), nitrogen (N), phosphorous (P), potassium (K), calcium (Ca), magnesium (Mg), and sulphur (S.) Macronutrients are heavily used by plants and can run low in the soil (although hydrogen, oxygen and carbon are gaseous elements and are obtained by plants in other ways). Micronutrients consist of cobalt (Co), copper (Cu), iron (Fe), manganese (Mn), boron (B), molybdenum (Mb), chlorine (Cl), nickel (Ni) and zinc (Zn). Micronutrients are used in trace amounts by plants and are slow to deplete.
Natural ecosystems tend to self-regulate soil fertility levels without intervention. Plants can thrive where adequate nutrients are available or may grow and develop at a slower than normal rate when nutrients are less available or entirely absent. The contrived nature of home gardens often requires small adjustments be made to soil fertility, mostly through the renewal of organic matter and occasionally in the form of supplementing low or missing elements.
The term pH describes the level of acidity or alkalinity of a soil. Measured using a pH scale, pH levels range between 0 (Strongly Acidic) to 14 (Strongly Alkaline), with a pH of 7 described as a balanced or ‘Neutral’ state between both conditions. The pH of a soil has a direct effect on its fertility. Most essential plant nutrients are soluble at pH levels of 6.5 to 6.8. At levels much higher or lower than this, elements often bind up, making them unavailable. Most plants tolerate a wide range of pH levels but there are exceptions: the ericaceous or “acid-loving” plants such as azaleas, rhododendrons, heaths, heathers, blueberries and cranberries do best in an acid soil where the soil pH is well below 6.8.
Soil pH may also have an effect on the fertility of the soil. Acidic soil can reduce the activity of beneficial microorganisms, slowing down the decomposition of organic matter and tying up soil nutrients, especially nitrogen.
The pH level of your soil is determined by factors such as climate and the nature of the rock from which your soil formed and is therefore not easily altered. If you wish to grow plants which are not suited to your soil type, (e.g., blueberries in an alkaline soil) it is easier to grow them in containers or raised beds rather than trying to permanently alter the pH of your garden soil.
Soil testing and analysis
If you are planning a new garden or the old one has begun to decline, it may be time to evaluate the condition of the soil. Over time the best soil becomes depleted as plants use up the nourishment around them and years of diligent additions of manure and synthetic fertilizers can lead to excessive levels of less mobile nutrients. Conditions of both excess and deficit can exist at the same time in soil. It is advisable to periodically test your soil using either a home test kit, or a professional soil testing service.
Home soil testing kits are available from big-box retailers, nurseries and garden centres and are easy to use and relatively inexpensive. They are also limited in the amount of information they provide. Professional soil tests yield more detailed and accurate results using samples you extract and send to the lab and they come with professional recommendations (although you may need the help of a professional to fully understand them.) Lab tests measure nitrogen, phosphorous, potassium and pH, but can also read values for secondary and micronutrients. They can also test for organic matter and biological activity.
Regardless of the type of soil test you choose, using the correct technique to obtain your soil sample is critical. Always follow any directions given carefully to ensure the results are trustworthy.
See below for links to Ontario soil testing labs.
Improving soil fertility
Amending your soil with organic matter laid on the soil surface as mulch will improve soil structure, encourage soil organism activity and provide nutrients that can be slowly released to plants.
The best sources of organic material can come from your own garden and include:
- Backyard compost
- Leaf litter, chopped leaves or composted leaves (leaf mold)
- Healthy plant residue and debris from your garden, left on the soil to decompose
- Arborist wood chips – use as a mulch to promote the growth of beneficial mycorrhizal fungi in gardens with trees and shrubs
- Mushroom compost
- Composted animal manure. Never use raw manure as it can burn plant roots and may also contain weed seeds.
- Green Manure: When the garden area is very large or organic matter is not readily available, a cover crop can be planted in spring. Alfalfa, clovers, rye and buckwheat are all examples of plants that can be used as green manure. In the fall, the whole plant is turned over into the soil, adding nutrients and organic matter that will increase soil fertility for next season’s plantings. Green manure can also be planted in the fall, usually in September, and is turned under in the early spring, three weeks prior to planting/seeding. This gives the cover crop time to decompose and release nitrogen. Green manure is best used in planted areas that are turned over annually, in particular vegetable and annual plantings.
The following compounds, derived from plants, animals or minerals, contain elements essential for plant growth. They can correct specific deficiencies but are not a substitute for the organic matter that improves the soil’s structure. Problems can arise when multiple products are applied at the maximum rate shown on the label. Use them only if a soil test indicates a deficiency of a particular element in your garden.
- Blood meal – 12-14% nitrogen
- Fish fertilizer – 8-10% nitrogen, 4-9% phosphorus, 2-3% potassium. Fish fertilizer also provides many minor and micro-nutrients
- Bone meal – 20-22% phosphorous – use with caution as high levels of phosphorous are damaging to the beneficial mycorrhizal fungi in gardens with trees and shrubs
- Greensand – 6% potassium and trace elements.
- Seaweed fertilizer – 4-13% potassium. Usually made of red algae, brown algae or kelp
Healthy soils, healthy plants
- Adding a fresh supply of organic materials feeds the soil microbes and improves soil structure; use what has been grown in your own garden and apply to the soil surface
- Use moderation when applying compost or fertilizer to the garden. It is better to apply organic material consistently over several years than a large amount all at once. Excess nutrients in the soil will leach into the groundwater and eventually into the water supply.
- Mulch with organic materials to feed microbes at the soil surface, buffer the soil from wind and water and to hold moisture in the soil.
- Water deeply and less frequently to stimulate healthy root growth deeper in the soil.
- To reduce soil compaction, avoid excessive digging and don’t walk on your garden beds.
- Try to keep your garden free of pesticides as they can harm beneficial soil organisms
The Toronto Botanical Garden Weston Family Library is an excellent source for horticultural information.
Reid, Keith. Improving your soil: a practical guide to soil management for the serious home gardener. Firefly Books, Richmond Hill, ON, 2014.
Chalker-Scott, Linda. The informed gardener: https://puyallup.wsu.edu/lcs
Pavlis, Robert. What is humus? https://www.gardenmyths.com/what-is-humus/
Pavlis, Robert. Humus does not exist. https://www.gardenmyths.com/humus-does-not-exist-says-new-study/
Ontario Ministry of Agriculture, Food & Rural Affairs. Accredited soil testing laboratories in Ontario.www.omafra.gov.on.ca/english/crops/resource//soillabs.htm
Date prepared: March, 2022
Prepared by the Toronto Master Gardeners, these Gardening Guides provide introductory information on a variety of gardening topics. Toronto Master Gardeners are part of a large, international volunteer community committed to providing the public with horticultural information, education and inspiration. Our goal is to help Toronto residents use safe, effective, proven and sustainable horticultural practices to create gardens, landscapes and communities that are both vibrant and healthy.
Statement on Invasive Plants: When choosing plants, avoid invasive plants, which can spread quickly and dominate gardens. Invasive plants are sold by nurseries, big box stores or even at community plant sales. Invasives may already be present in your garden. They can invade gardens by spreading from under a neighbour’s fence or may be transported by wildlife. For beautiful, sustainable options to invasive plants, see the Ontario Invasive Plant Council’s “Grow Me Instead – Beautiful Non-Invasive Plants for your Garden” at https://www.ontarioinvasiveplants.ca/resources/grow-me-instead/ before purchasing or accepting “gifts” of plants.
Statement on Home Remedies: The Toronto Master Gardeners do not recommend home remedies, as these have not been proven effective through scientific investigation, and may even damage other living organisms in the soil or plants in your garden. There are other garden friendly options you can use.
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