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Plants, like all other living creatures have nutrient requirements necessary for growth. Their roots remove nutrients from the soil to fuel processes within their cells, constantly taking in nutrients that have attached to soil particles or organic matter. Over time though, this continuous feeding can deplete even the most fertile soil making it necessary for gardeners/growers to replenish the nutrient reserves. Fertilisers are applied to the soil in order to add nutrients and encourage strong, vigorous plant growth.

Plant Essential Nutrients

The theory of plant essential nutrients is a fundamental aspect in plant nutrition today. There are nutrients essential for all plant growth: nitrogen, phosphorus, and potassium are the major nutrients; calcium, magnesium, and sulfur are the secondary nutrients; the micronutrients are boron, chlorine, copper, iron, manganese, molybdenum, and zinc. They all have specific roles within the plant; sometimes the roles overlap, sometimes they are different. If any of the nutrients are deficient plant growth will be affected in some capacity.

Over time the levels of plant essential nutrients found in the soil profile deplete for a variety of reasons: plant uptake, water runoff, soil erosion to name a few. As these levels are reduced plants will show deficiency symptoms. Hence the importance of reintroducing nutrients to depleted soils through the addition of fertilisers other plant foods such as manure or compost.

Fertiliser Basics

Fertilisers come in many forms but in simple terms they are chemicals or natural substances that are added to soil or land to increase fertility. They add nutrients back to the soil providing food for plants.

N-P-K Ratio

It’s important to understand that fertilisers are labeled based upon the three main elements they may contain. Of the plant essential nutrients nitrogen, phosphorus, and potassium are needed in higher amounts than the others. This universal method of labeling fertilisers makes it easier for consumers to compare products to one another.

To make it easy for consumers to quickly understand the components of a of fertiliser and compare products to one another, all fertilisers are labeled with an N-P-K (nitrogen-phosphorus-potassium) ratio. This ratio tells the percentage, by weight, of the three main nutrients in the bag.

In a 10-10-10 formulation, a 10 kilogram bag of fertiliser would contain 1 kilogram of nitrogen, 1 kilogram of phosphorus (in the form of P2O5), and 1 kilogram of potassium (in the form of K2O).

Nitrogen takes center stage and listed first on the fertiliser label as it is needed in the highest amount within the plant. The main responsibility of nitrogen is to regulate vegetative growth but It performs many other functions too: it is assimilated into amino acids to build proteins, it helps to keep foliage green as a building block of chlorophyll, and nitrogen helps to drive enzymatic reactions to completion.

Phosphorus is listed second being a critical component of root growth and flowering. It is an important structural component in the genetic building blocks found within plant cells.

Compared to nitrogen and phosphorus, potassium plays a different role in plants cells. Instead of being an integral structural component phosphorus activates many of the vital enzymatic reactions that occur in plants, thus making it critical for overall plant health.

Types of Fertiliser

Since it was first understood that plants need nutrients to grow, a plethora of products have been developed for agricultural and horticultural use. While it may seem daunting to try to choose one for your given application, having many options allows you to customize your fertiliser regime to meet your gardening philosophy.


Conventional, or inorganic, fertilisers are made from synthetic, or manmade materials. Their specifically blended nutrients are quickly available for the plants, meaning plant deficiencies can be fixed quickly, minimizing long term effects on the plant.

Inorganic fertilisers are usually less expensive to buy, and readily available for purchase because of their mass production. The big drawback to conventional fertilisers is the quickly available nutrients can leach out of the soil easier, causing possible environmental problems.


Unlike conventional products, organic fertilisers are derived from all-natural ingredients. They are typically made of the broken down remains of organisms, or a byproduct (i.e. waste) of the organisms themselves.

Due to their composition, some of the nutrients found in organic fertilisers are not readily available for plant uptake.the nutrients and adding organic matter to the soil. Because of labeling laws, the N-P-K ratio may only include readily available nutrients; organic fertilisers appear to have much lower nutrient contents than their inorganic counterparts when in reality the nutrients are there, they just take longer to be released.

Common Fertilisers and Plant Foods

There are many options available for feeding your garden plants. You can choose to buy commercial products, or you can opt to apply natural ingredients to help enrich the soil.

Blood meal is an excellent source of organic nitrogen, made from the dried blood of slaughtered animals. It can also help to increase the acidity of soil and deter garden pests such as squirrels and deer. The slow release product typically has a nutrient ratio around 12-0-0.

Bone meal is a finely ground powder -- used as an organic fertiliser -- that is made up of ground animal bones. It has a fine, powdery consistency making it easy to apply. Typically beef bones are used but it may contain the bones of any animal commonly slaughtered. Most of the commercial products available have a ratio close to 3-15-0, making them an excellent source of slow-release phosphorus fertilizer without adding an abundance of nitrogen or potash (i.e. potassium).

Compost tea provides garden plants and soil with a helping of beneficial microorganisms and soluble nutrients. The liquid is produced by “brewing” compost, hence the name. Compost teas should be applied directly to the leaves or to the soil once a month during the active growing season.

Fish meal has been used throughout the history of growing plants for food. It is high in micronutrients, macronutrients, amino acids, and a range of vitamins. Fish meal is ground, dried fish and is best applied prior to planting to allow nutrients to become available slowly throughout the season.

Grow More fertiliser products offer gardeners a wide range of water-soluble formulations designed to provide balanced N-P-K and chelated micronutrients that are ready available to plants. They offer general purpose formulas, high nitrate, high phosphorus/potassium, and even products with extra calcium, sulfur, or magnesium.

Manure, when composted completely, provides an inexpensive, excellent source of nutrients and organic matter for garden soils. Kilogram for kilogram, the nutrient content may be lower than other products but the addition of organic matter to the soil helps to improve water retention and soil structure. The key to using manure is to make sure it is no longer “hot” to prevent the nitrogen from burning plants. Different animal manures will provide different benefits to the garden.

Chicken manure is the highest and nitrogen, making it beneficial for green, leafy plants. It needs to be composted at least 6 months prior to use.

Cow manure is low in nitrogen but has a good balance of nutrients overall. Because of the nature of bovine digestion it contains little weed seeds as well.

Horse manure can be a good, all purpose plant food but is high in weed seeds due to equine diets. Ensuring the compost reaches high enough temperatures to kill the weeds will reduce seed germination after application.

Rabbit droppings are one of the best manure sources available for use. It is high in nitrogen and phosphorus and if you have your own bunnies, it is produced prolifically.

Seaweed fertiliser is an eco-friendly, organic product that contains magnesium, potassium, zinc, iron, and nitrogen plus many other vitamins and enzymes. Liquid seaweed extracts should be applied directly to the foliage for the best results.

Tomato Feed encompasses a group of plant foods specially designed to provide the nutrition needed for tomato plants. They are high in potash (i.e. potassium) to promote flowering and fruit formation. Some formulations contain extra calcium sources to prevent blossom end rot.

DIY Fertilisers and Plant Foods are being used more often as gardeners are trying to lessen their environmental footprint. Gardeners are using aquarium water to nourish plants, coffee grounds to provide nitrogen while acidifying the soil, egg shells to increase calcium levels in the soil, and epsom salts for magnesium and sulfur.


As plants grow, they constantly consume essential nutrients from the soil, often making it necessary to replenish soil nutrient reserves. Commercially available fertilisers allow gardeners to add “food” back to the soil while tailoring a fertiliser program to fit plant needs and growing philosophies. Knowing the basics of fertilisers and plant foods will help ensure you are feeding your plants correctly, encouraging strong, vigorous growth.