May 08, 2013


by Ankaret Dean, Lanark Master Gardeners


For centuries people have used fertilizers to increase their crops and encourage plant growth. Until the 20th century these consisted of organic material and inorganic mined mineral nutrients. It was not until the industrial revolution that chemically synthetic inorganic fertilizers were discovered. It has been estimated that almost half the world’s population are currently fed as result of synthetic inorganic fertilizers.

Nowadays, gardeners can purchase an easy soluble fertilizer from the store, add it to the watering can or hose and they only need to keep a record of how often they apply the fertilizer. It is as easy as that. Alternatively it can be spread over the ground using a fertilizer spreader.

Fertilizers typically provide six nutrients and seven macro-nutrients, however for most gardeners the most essential knowledge is to understand the numbering system on the fertilizers container. These are always written in large numbers, and are in a row of three, such as 10-10-10. The first number represents the ratio of nitrogen, the second number, represents phosphate and the third potassium. Each of these chemicals has a different effect on the plant.

Nitrogen, the first number, is to make the green part of the plant grow and develop. Phosphates encourage root growth, and potassium encourages seed and fruit production.

Therefore if you buy a fertilizer with a high middle number it will stimulate root growth, which is good for planting out seedlings or transplanting shrubs or trees, A high last number will be good for the production of fruit and flowers, and a high first number will encourage plant growth.

To buy a fertilizer with even numbers such as 10-10-10 means it is an all round fertilizer and can be used on many perennials or shrubs, fruit trees etc. I always fertilized my winter bulbs as they die back with a good dose of 10-10-10. This means that the nourishment goes back into the bulb to wait for next spring.

Organic fertilizers, such as manure and compost are more long lasting (except for slow release pellets for containers) and benefit the soil with organic matter. Manure tea can be made by filling a pail with manure, and covering it with water. After a few days the liquid can be used on plants and the manure covered again with water.

Using commercial fertilizers has to be done with care. Never fertilize plants in a dry season as it can kill the plants, and most liquid fertilizers should not touch the roots or the leaves.

When to fertilize? Spring or early summer is usually the best time, before the active growing season. Also it is good for planting out seedlings or transplanting young or new plants. Fall feeding is suggested for some shrubs but not for roses. New growth at that time of the year could be winter killed.

What kind of fertilizer? Many gardeners either spread fertilizer over the garden patch in the spring or spread manure or compost. Perennials and shrubs, vines or fruit trees can be side dressed. Trees can be fertilized using spikes hammered into the ground at the drip line. Manure tea pretty well helps everything.

Avoid over-fertilizing, though; too much nitrogen produces lots of leaves but not many flowers or fruits. 

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