The Online Gardener's Handbook
Chapter 3: A Word about Soil
Table of Contents
- Organic Fertilizers
- Chemical Fertilizers
- Chemical vs. Organic Fertilizers
- Learn More
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 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 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.
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
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.