Using Fertilizers

Do you need to? If so, what do you need?

Healthy soil has a variety of nutrients in it to help your plants grow strong enough to defend themselves from most (but not all!) pests and diseases. Soilless mixes, like you'd use in a pot or a container, do not contain any soil at all. They are typically a mix of peat moss, perlite, and vermiculite and have almost zero nutrient value. They are essentially organic sponges. Unless you add fertilizer, in the form of compost or other types of fertilizer, your plants will not produce well and will be prone to many pests and disease issues.

What is a fertilizer?

By definition, a fertilizer is "any organic or inorganic material of natural or synthetic origin which is added to a soil to supply elements essential to the growth of plants." (Brady, Soils, 1974)

What exactly does that mean? Organic means something which is or was alive. Animal manures were once living plants. Bonemeal, a by-product of slaughter houses, is composed of ground- up bones of animals. Inorganic means from non-living sources. Rock phosphate, a common source of phosphorus, comes from rocks, a non-living material. The term natural describes the manure, the bonemeal, and the rock phosphate. All are naturally occurring. The term synthetic describes such products as nitrogen fertilizer manufactured by combining natural gas with nitrogen from the air.

Organic vs. Inorganic

There has been much controversy over organic versus inorganic fertilizers. It is important to realize that plants do not recognize the difference between organic and inorganic fertilizers.  As long as these elements are supplied in adequate amounts it makes little difference to the plant if they are organic, inorganic, natural or synthetic Their tiny root hairs can absorb only nutrients that have been broken down into inorganic, water-soluble forms. It makes no difference to your tomato plant if the atom of nitrogen it is absorbing has come from a compost pile or a fertilizer factory. There are, however, advantages and disadvantages to each form of fertilizer, organic and inorganic.

Organic Fertilizer

Advantages - Organic nutrients include such things as cow, sheep, poultry and horse manure. (One should avoid using pig, dog or cat feces because of the problems involved with internal parasitic worms which may be transferred to humans.) Bonemeal, bloodmeal, compost, and green manures will also provide nutrients for your plants.

There is less danger of over-fertilization by adding decomposed organic material to a garden. It provides a slow release of nutrients as micro-organisms in the soil break the organic material down into an inorganic, water soluble soluble form which the plants can use. The addition of organic material improves soil structure or "workability" immensely. It also vastly improves the water-holding capacities of sandy soils, a distinct advantage in arid climates such as ours.

Disadvantages - For the most part, organic fertilizer is not immediately available to the plants. As noted above, this "slow- release" feature can be an advantage. However, if there is an immediate need for nutrients, organic fertilizer cannot supply them in a hurry. Furthermore, information on the amount of nutrients and the exact elements in an organic fertilizer such as manure is not readily available to the home gardener. In contrast, when you apply manufactured inorganic ferilizer you know the kinds and amounts of the elements it contains, and this allows you to be more precise in meeting a plant's nutritional needs.

The possibility of nitrogen depletion is another drawback of organic fertilizers. Because of complex bacterial action, the addition of a large amount of organic material can cause a temporary nitrogen depletion in the soil and therefore in the plants.

Inorganic Commercial Fertilizer

Advantages - The primary advantage of using packaged commercial fertilizer is that nutrients are immediately available to the plants. As well, the exact amounts of a given element can be calculated and given to plants.

Disadvantages - Commercial fertilizer, especially nitrogen, is easily washed below the level of the plant's root system through the leaching of rain or irrigation. An application which is too heavy or too close to the roots of the plants may cause "burning" (actually a process of desiccation by the chemical salts in the fertilizer). As well, overly heavy applications of commercial fertilizers can build up toxic concentrations of salts in the soil, thus creating chemical imbalances. If organic materials are readily available and cheap, the expense of the commercial fertilizer should also be considered.

Whether a gardener chooses to use organic, inorganic or a combination of both types of fertilizers, it's important to follow the guidelines regarding timing of application, placement of the fertilizer, and the proper amount of fertilizer to be used.

The Nutrients in a Fertilizer

Beside the major nutrients (N, P, K, S) there are also "micro- nutrients," or trace elements, which are needed by plants in very, very small amounts. But if these are missing, the plants will not be able to complete their life cycle. Among these micro-nutrients are iron, zinc copper, calcium, manganese, and magnesium. Most of these are present in adequate amounts in Saskatchewan soils. Complete house-plant fertilizers usually contain the major nutrients as well as the micro-nutrients. Read the label prior to purchase to ensure this.


Nitrogen is needed for the green, leafy, vegetative growth of plants. When an element is lacking, the plant will show deficiency symptoms. Deficiency symptoms for nitrogen include an overall pale yellow color of the leaves, and plants which are dwarfed or stunted. Nitrogen is mobile in the plant; that is, it moves from the older growth to the newer growth, where it is most needed. Therefore deficiency signs will appear first in older leaves.

Nitrogen moves easily through the soil in the soil water. For this reason it is said to be very "mobile." It is easily "leached" or washed downward by rain or irrigation water. If it is washed below the root zone of the plants, it will not be available for plant use. Therefore,it is the fertilizer element most often lacking and most often needing replacement.

Because of complex bacterial interaction, nitrogen is usually not "available" for plant use until the soil has warmed up in the spring and the soil temperature has reached 15.5C. This is why plants may appear yellow and stunted in early spring when the soil is still cold, even if nitrogen fertilizer has been applied. As soon as the soil warms up, they will appear green and vigorous.

Too much nitrogen or a nitrogen imbalance can delay flowering, fruiting and seed set. The resultant growth is soft and succulent and may be more vulnerable to fungal and bacterial infection. As well, nitrogen can desiccate or "burn" the roots of plants if placed too close to seeds, seedlings or newly planted plants.


Phosphorus is said to promote root growth, root branching, stem growth, flowering, fruiting, seed formation, and maturation. When phosphorus is lacking, stems and foliage often have a red or purplish tinge. This is particularly noticeable on tomatoes and corn. Deficiency signs are seen in new growth first.

Phosphorus is very stable and non-mobile within the soil, so it is not easily leached by soil water. When used moderately, it may be placed fairly close to seeds and seedlings and will not "burn" or desiccate them.

Potash or Potassium

Potassium enables the plant to more readily withstand "stress" such as drought, cold, heat and disease. (In a lawn such "stress" may be in the form of human and pet traffic.) It also stimulates flower color and promotes tuber formation and a strong root system.

When potassium is lacking, leaves appear dry and scorched on the edges and have irregular yellowing. This is seen on older leaves first.

Potassium is usually readily available in Saskatchewan soils and therefore seldom needed as a fertilizer application. But sandy garden soils may show deficiencies. Corn deficient in potassium may be susceptible to fusarium infections.


Sulfur is essential to plant growth and metabolism. It contributes to the unique taste and flavor of cabbage, broccoli, Brussels sprouts, cauliflower, and other members of the mustard family.

Plants that do not have enough sulfur are stunted, thin- stemmed and spindly. The younger leaves are light green or yellow. Fruit and seed maturity may be delayed when sulfur is lacking.

Various forms of sulfur may be added to basic soils to acidify them (or lower the pH).

The Numbers on the Fertilizer Bag

On a fertilizer bag, you will find three or four numbers with hyphens separating them. The numbers indicate, in order, the percentage of nitrogen (N), phosphorus (P), potassium or potash (K), and sulfur (S) - the letters in parentheses are the chemical symbols for the elements. Here are some common fertilizers and the proportion of the elements they contain.

Percentage of N P K S





















So, a 25-kg bag of 16-20-0-14 would provide (16/100 X 25 =) 4 kg of nitrogen, (20/100 X 25=) 5 kg of phosphorus, no potash, and (14/100 X 25 =) 3.5 kg of of sulfur. A 25 kg. bag of 34- 0-0 would supply 34/100 X 25 =) 8.5 kg of nitrogen but no other elements. A "complete" fertilizer contains nitrogen, phosphorus, and potash. If one of these three elements is missing, the fertilizer is not "complete."

When and When Not to Fertilize

Lawns should be fertilized on approximately May 15 with 2 to 2.5 kg of 27-14-0 or 26-13-0 per 100 m2. Six weeks later apply 2.0 kg of 34-0-0 per 100 m2. Six weeks later apply 2 kg of 34-0-0 per 100 m2.

Trees and shrubs generally do quite well on Saskatchewan soils without the addition of chemical fertilizers. Most problems associated with trees and shrubs are due to lack of proper watering or insect problems and are seldom attributed to lack of nutrition. Excess fertility in the soil promotes excess succulent foliage which is more susceptible to winter injury.

Vegetable and flower beds usually require 11-48-0 or 16-20- 0 applied in spring at the rate of 2 lb per 100 m2.

Fall fertilizing of trees, shrubs and lawns is not recommended in Saskatchewan. In warmer parts of the country, it may be a common practice, but not in the cold prairies. Because fertilizers encourage growth, fall fertilizing may stimulate plants to continue growing when they should be "hardening off" in preparation for winter. Plants that have not hardened off properly will likely suffer from some sort of winter injury.

Never put granular fertilizer or fresh manure in the planting hole. The chemical salts within the fertilizer may desiccate or "burn" plant roots.

How to Apply Commercial Fertilizer

On lawns, to apply granular fertilizer, first divide the total amount needed in half. With a fertilizer spreader adjusted to the lowest possible setting, walk north-south over the lawn area with half the amount, and then east-west with the other half. This will give an even distribution and reduce the possibility of "burning." If you are hand-broadcasting, follow the same procedure.

In vegetable gardens and annual flower beds, fertilizer may be applied in several ways: (1) fertilizer can be broadcast and thoroughly incorporated into the upper 7 to 8 cm of soil in spring prior to planting; (2) or it can be side-banded that is, incorporated along the sides of each row and about 5 cm deep. This involves more work but is more efficient because less fertilizer is used; or (3) fertilizer can be placed around each plant, 5 cm away and 5 cm deep. This last method is even more "labor intensive" than side- banding but it is also more efficient, in that all of the fertilizer is placed where it can be used by the plants. In perennial borders or other permanent plantings, fertilizer should be incorporated around each plant, 5 cm deep and 5 cm beyond the roots.

Seasonal Growth Patterns

In early spring, buds which have been dormant over winter begin to grow in response to warming temperatures and lengthening days. With most trees and shrubs, growth occurs rapidly and continues only until about early or mid-July. At this time growth ceases, and the plants begin to harden off and prepare for the winter. You can check this by examining the tree or shrub. If the leaves at the outer tip are the same size as the leaves lower down on the branch, then you know that growth has ceased. Look carefully and you should also see a bud at the base of each leaf, and a terminal bud at the very tip of the branch.

Some plants, such as raspberries and elderberries, tend not to cease growth in July, but continue growing slowly, sometimes right up to killing frost. When this occurs, the plant has not hardened off properly and, as a result, tip die-back occurs. Any plant that continues growing until late in the fall is susceptible to this form of winter-kill. When fertilizing trees and shrubs, it is best to apply the fertilizer in early spring when the plant is actively growing. By early July, fertilizing should cease.

Lawns are an exception to this; ideally, urban lawns should be fertilized three times a year (May 15, July 1, August 15). Lawns should not be fertilized later than mid-August, for this will encourage growth too late in the fall. Because lawns are low to the ground and usually have a good snow cover, they are more protected from the winter environment and therefore less susceptible to winter-kill that trees and shrubs.

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