The Online Gardener's Handbook 2010
Chapter 3: A Word about Soil
Fertilizers

Table of Contents

  1. Introduction
  2. Organic Fertilizers
  3. Chemical Fertilizers
  4. Chemical vs. Organic Fertilizers
  5. Learn More

Introduction

Think of fertilizers as plant food. By that definition, any material added to a soil to enhance the nutrition of plants could be considered a fertilizer. This includes commercially prepared fertilizers, and organic materials such as manure, mushroom compost, fishmeal, or blood and bone meal.

Organic Fertilizers

Organic fertilizers include manure and compost, and their value to gardens and lawns is often under-estimated. Manure and compost contain many micronutrients and organic matter, which helps to build and maintain soil structure. They also contain all the nutrients required for plant growth. The problem is that they may not necessarily be present in the proportions required by all species or soil types.

Cow and Sheep Manure

Well composted cow and sheep manure are readily available in most garden centres. It is applied at a rate of 120-250 kg manure per 1000 m2 of soil. Turn under manure with 9.75 kg of 0-20-0 fertilizer (spread the manure on the soil, broadcast fertilizer overtop and then work both into the soil surface with a spade or fork). Fresh poultry, rabbit, and sheep manure are much higher in nutrients and soluble salts than livestock manure, so reduce the rate of application to one-third, compost it thoroughly first, or apply it in the fall.

Leaves, Lawn Clippings, Peat, Sawdust

This type of organic mulch is broken down by micro-organisms in the soil. This process, unfortunately, uses important nitrogen stores required for plant growth. To prevent this, turn the mulch under with 12.5 kg of 15-15-15 fertilizer per 100 m2 of soil.

Compost

Compost bins can be bought at most hardware stores and garden centres, or you can build your own of wire fencing supported by posts, boards or masonry. Place the bin in a semi-shaded and well-drained spot protected from cold winds. Do not place it under trees where roots will grow up into the compost. Line the bottom of the bin with straw to soak up any liquid that may run out.
Alternate layers of organic matter with soil. For each 15 cm of organic matter, packed down, provide 3 cm of soil. This will add micro-organisms to the compost.

If compost is composed largely of straw, sawdust or fallen leaves, apply a nitrogen fertilizer such as 10-6-4 to speed up the breakdown of the material and therefore improve the nutrient content of the compost. A rate of 360 g fertilizer per 35-40 L of organic matter is recommended. Manure can be used instead of fertilizer and dolomitic limestone (60 g per 40 L compost) added to counteract acidity and speed up decomposition.

Compost made up largely of kitchen scraps and yard waste such as grass clippings or spent flower blooms is nutrient-rich. Additional fertilizer is rarely required, and if used, not more than 40-50 g fertilizer per 35-40 L of organic matter is recommended.

Compost can be built up to 1.0-1.5 metres. It is helpful to have more than one bin to produce compost throughout the year. Keep it moist but not soggy, and turn it two to three times every two weeks to hasten decomposition and reduce odour. Compost started in fall is ready to be used in June.

Chemical Fertilizers

Most garden and lawn soils in new subdivisions are deficient in several essential plant nutrients, especially phosphorus and potassium. This is particularly true when the topsoil has been disturbed and redistributed after house construction. By using the proper chemical fertilizer, these nutrients can be added to the soil at the optimum rate, thereby improving plant growth. The best method of determining the kind and amount of nutrient required is a soil test.

Established garden and lawn soils can also benefit from the application of a low rate of a balanced chemical fertilizer. This will help replace nutrients used by the previous crop or during the previous growing season. Once soil fertility is high, however, you only need to apply enough fertilizer to maintain fertility levels.

Fertilizer Grade and Ratio

All fertilizers sold in Canada are sold by grade, which tells you the nutrient percentage of the product by weight. It is expressed as a set of three numbers: total Nitrogen (N), available phosphate (P2O5) and soluble Potash (K2O). A 10-6-4 fertilizer, therefore, contains 10% N, 6% (P2O5), and 4% (K2O).

Grade is one of the most important factors to consider when buying fertilizer; it is certainly more important than the trade name. A fertilizer under the trade name "Tree Food", with a grade of 30-10-10, can be used anywhere that a 30-10-10 fertilizer grade is required.

Ratio defines the proportion of nutrients in the fertilizer. Thus, 7-7-7 is in a 1:1:1 ratio, while 21-7-7 is in a 3:1:1 ratio and 4-8-12 is in a 1:2:3 ratio.

Substituting Fertilizer Analysis

When the required fertilizer grade is unavailable, it is possible to substitute with another of similar ratio, but you must first calculate the amount of nutrients required. This is done by multiplying the amount of fertilizer applied by the grade for each nutrient, and dividing by 100.

For example: 10 kilograms of 10-6-4 provides:
Nitrogen: 10 kg × 10% /100 = 1.0 kg Nitrogen
Phosphate: 10 kg × 6% /100 = 0.6 kg Phosphate
Potash: 10 kg × 4% /100 = 0.4 kg Potash

Let's say, for example, that it has been recommended that you apply 5 kg per 100 m of 7-7-7 grade fertilizer, but only a 10-10-10 product is available. This will be suitable, as each fertilizer has a ratio of 1:1:1. You must, however, correct for the amount of fertilizer that you use. With this example, the recommended 7-7-7 fertilizer will supply 0.35 kg of nitrogen (5 kg of fertilizer x 7% nitrogen), 0.35 kg of phosphate and 0.35 kg of potash. If substituting a 10-10-10 fertilizer, 3.5 kg per 100 m will provide you with the same amount of nutrients.

You can use these calculations to substitute other grades of fertilizers. Try to substitute a grade of the same, or similar, ratio. Some common substitutions are usually listed on the back of your soil test report form.

Toxicity of Fertilizers

All fertilizers are toxic if too much or too concentrated an amount is applied. As well, some nutrients - especially micronutrients - are toxic even at low levels. A fertilizer's solubility can also play a role in determining its toxicity to plants. For example, ammonium nitrate and calcium nitrate are both used in chemical fertilizers as a nitrogen source. Ammonium nitrate, however, dissolves far more quickly when it comes into contact with moist soil than does calcium nitrate. If you accidentally spill fertilizer on your lawn or garden, therefore, the chance of damage to plants is far greater if ammonium nitrate is used.

Chemical vs. Organic Fertilizers

Chemical fertilizers are usually much cheaper per kilogram of actual nutrient than are manure or compost. As well, they provide you a balanced nutrient content, and allow you to choose the grade and ratio needed for your garden.

Manure is generally more expensive, and a considerable amount needs to be incorporated into the soil to meet the needs of the crop. However, manure has the added benefit of providing a good source of organic matter that maintains soil structure and increases earthworm and microbial activity. And when purchased in pre-bagged amounts at a garden centre, its nutrient content is usually balanced. For those wishing to avoid chemically manufactured forms of nitrogen, therefore, manure is a viable option.

The use of home compost as the sole fertilizer in your garden is not as viable. This is because it is extremely difficult to know its nutrient make-up. Nutrient content also varies according to the ingredients used. As a source of organic matter, however, home compost is excellent.

Learn More

 


For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca
Author: OMAFRA Staff
Creation Date: 21 July 2005
Last Reviewed: 25 June 2010