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Description:

Soil fertility refers to the ability of soil to supply and sustain nutrients for healthy plant growth. Maintaining fertile soil is integral to good lawn and garden management. Fertile soil will promote the development of strong plants. Strong plants are less susceptible to insect and disease damage, more able to tolerate drought stress and aid in the reduction of weed populations.

Healthy earth is comprised of a mixture of water, air, minerals and organic matter and contains an abundance of various soil organisms. Plants obtain nutrition from minerals and organic matter in the soil.

Essential nutrients:

There are 16 identified elements that are essential for plant growth. Hydrogen, carbon and oxygen are obtained mostly from the air and water. The primary nutrients - nitrogen, phosphorous and potassium, are required in the largest amount. Calcium, magnesium and sulfur, termed secondary elements, are required in lesser amounts, relative to the primary nutrients. The micronutrients - iron, manganese, molybdenum, copper, zinc, boron and chlorine, are required in extremely small amounts. Plant growth will be restricted by a deficiency in any one of the primary, secondary or micronutrients.

Minerals:

Roughly half of garden soil is comprised of minerals - small bits of weathered rock, broken down into smaller particles by physical, chemical and biological processes. Soil particles vary in size - sand particles being the largest, silt medium and clay the smallest. It is both the size of these particles and the proportion of sand, silt and clay in a soil that determines soil texture. Soils with high sand content are termed loose and open. Water drains rapidly through sandy soils and nutrients are quickly leached out. In contrast, soils that are high in clay are termed heavy and are capable of holding onto nutrients. Because soil texture influences the rate of water drainage through the soil and ability of soil to retain nutrients, it directly influences how well your plants develop and grow.

Organic matter:

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 improves soil structure because it holds together mineral particles in clusters called aggregates. Aggregate formation improves soil structure (physical arrangement of both organic and mineral aggregates). Humus loosens heavy clay soils, which improves drainage, while acting as a sponge in sandy soils, 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. Furthermore, adding organic matter will, over time, increase the amount of available nutrients by increasing the activity and number of soil organisms that break down organic matter.

Soil organisms:

Soil organisms include bacteria, fungi, protozoa, nematodes and earthworms. They improve soil structure through decomposition of organic matter and are involved in transforming organic material and soil minerals into vitamins, hormones, disease suppressing compounds and nutrients required for plant growth.

Soil pH:

The pH level in the soil is a measure of its relative acidity or alkalinity. This is important when considering soil fertility because pH can affect the availability of soil nutrients. Most essential plant nutrients are soluble at pH levels of 6.5 to 6.8. If soil pH is much higher or lower than this range, the soil nutrients can become chemically bound and unavailable for uptake by plants. Certain plants can tolerate or require soils with extremely low or high pH (i.e. acid or alkaline soils). Plants such as azaleas, rhododendrons and blueberries, for example, require acidic soil for growth.

Acidic soil can also influence plant growth through its ability to reduce the activity of beneficial microorganisms. A decrease in microbial activity slows the break down of organic matter and ties up soil nutrients, especially nitrogen.

Soil testing and analysis:

If the plants in your garden are not thriving, then it is a good idea to evaluate your soil for nutrient content, deficiencies and excesses, as well as the pH level. Many big-box stores, nurseries and garden centers offer a variety of easy to use and inexpensive do-it-yourself kits. One can also send soil samples away for laboratory analysis. Compared to the home tests, lab tests often provide more information and more accuracy. 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 directions carefully.

Improving soil fertility:

Organic Matter

Regardless of the nutrient status of your garden soil, amending with organic matter once, if not twice a season, will improve soil structure, encourage soil organism activity and provide nutrients that, over time, can be slowly released to plants. Sources of organic material include:

• Backyard compost, mushroom compost, composted leaves (leaf mold)
• Composted animal manure. Never use raw manure as it can burn plant roots and may contain weed seeds.
• Aged sawdust/ ground bark - The older the sawdust or ground bark the better because the decomposition process for such materials consumes nitrogen, making this nutrient unavailable to nearby plants. Adding a nitrogen source when using 'brown' organic matter will mitigate against nitrogen loss.
• Peat moss - Sphagnum peat moss is acidic in nature.

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.

Organic fertilizers:

The following compounds, derived from plants, animals or minerals, contain elements essential for plant growth.

• Blood meal - 12-14% nitrogen
• Fishmeal - 8-10% nitrogen, 4-9% phosphorus, 2-3% potassium. Fish meal also provides many minor and micro-nutrients
• Bone meal - 20-22% phosphorous
• Rock phosphate - 20-30% phosphorous, most effective in acid soils where it is more soluble.
• Wood ashes - 4-10% potassium. Continued use of wood ashes can raise the pH of the soil and should not be used if soil pH is more than 6.5.
• Greensand - 6% potassium and trace elements.
• Seaweed - 4-13% potassium. Usually made of red algae, brown algae or kelp

References:

• The Toronto Botanical Garden Weston Family Library is an excellent source for horticultural information.
• Persch, Barbara B. ed. Brooklyn Botanic Garden Record: Soils. Brooklyn, New York, USA: Brooklyn Botanic Garden, Inc, 1990.
• www.zzyx.ucsc.edu/casfs/gardenideas/soilfert.html
• www.gov.on.ca/OMAFRA/english/crops/resources/soillabs.htm (For soil testing labs)

Factsheets are produced by the Toronto Master Gardeners in association with the Toronto Botanical Garden. They provide introductory information about a broad range of horticultural topics and are intended for personal use and study purposes. Should your gardening group or organization wish to use multiple copies we ask that you inform the Toronto Botanical Garden at info@torontobotanicalgarden.ca.

Date revised: January 8, 2006