Understanding Fertilizers

During the growing season, vegetables use up a huge amount of the soils nutrients, so it’s important to replenish depleted nutrition.

Fertilizers are derived from animal or vegetable matter and can be organic or synthetically produced with chemicals.

The advantage of using organic fertilizers is they are slow acting and more gentle on the plants than the synthetic fertilizers. Organic fertilizers are not produced in a form that can be immediately absorbed by the plant. Soil is a living organism, so it’s important to consider what we put into it. Organic fertilizers help maintain healthy soil. This is achieved as the micronutrients in the organic fertilizers must first be broken down by soil bacteria or fungi before they can be converted into a form that can be utilised by plants.

Whilst synthetic fertilizers have plenty of micronutrients, they also contain acids, sulphuric and hydrochloric acids, which dissolve the soil crumb. Synthetic fertilizers also adversely change the soils pH level, which impacts the microorganisms living in the soil. These microorganisms provide the plant with natural immunity to disease. Synthetic fertilizers also prevent the plant from adsorbing trace elements from the soil.

Synthetic fertilizers tend to be water soluble, meaning they are easily washed through the soil.  So any nutritional benefits maybe short lived. Whilst it’s less likely that the nutrients resulting from the use of organic fertilizers will be easily washed away during heavy rainfall.

Fertilizers are made up of three key micronutrients:

N – Nitrogen

P – Phosphorous

K – Potassium.

By law, any fertilizer sold must state, on the packaging, the quantities of nitrogen, phosphorous and potassium contained in that fertilizer.

For example: If the packet is labelled 6:4:4 this means there is 6 grams of nitrogen to every 100 grams of fertilizer. And 4 grams each of phosphorous and potassium to every 100 grams of fertilizer.

The remainder of the fertilizer will be made up of vitamins and other minerals plus organic matter, if organic, or fillers if not.

Every packet of fertilizer will display the ratio in the same order, N – P – K. This will help identify the type of fertilizer.

In order to identify the best fertilizer for use, we need to understand what effect each micronutrient has on plants.


Nitrogen helps plant foliage to grow strongly.

Nitrogen is a component of chlorophyll. Chlorophyll not only gives plants it’s green colour, but is also a very important biomolecule. It is the site for carbohydrate formation or photosynthesis. Photosynthesis is the function that enables a plant to absorb energy from the suns light.

A plant with sufficient nitrogen will show vigorous foliage growth.

Blood meal fertilizer is an excellent source of nitrogen.  However, as it’s an animal derivative, ensure the fertilizer is sterilised to prevent any potential contamination.

Blood meal fertiliser is a fast release fertilizer. Blood meal is highly soluble and therefore fast acting.


Blood meal fertilizer is typically labeled

N 13.25% : P 1% : K 0.6%.

Planting legumes is another method for increasing nitrogen levels in soil. Legumes have the ability to fix atmospheric nitrogen in their root system, and, in a form that is accessible to plants. The roots develop nodules where the nitrogen is stored. To maximise nitrogen levels in the soil, allow legumes root system to break down in the soil once their productive life is over.

Chicken pellets are also a good source of nitrogen. It can be slightly alkaline, it has a pH range of 6.5 to 8.0.


Chicken pellet fertilizer is typically labeled

N 4% : P 2% : K 1%



Phosphorous encourages root and shoot growth, and aids the overall development of the plant. It also plays a key part in photosynthesis, the development of flower, fruit and seed production as well as the plants ability to use water efficiently.

Phosphorous is a component of the complex nucleic acid structure of plants (DNA), which regulates protein synthesis. Phosphorous is important for cell division and the development of new tissue.

Although there may be phosphorous present in the soil, a large proportion of it may not be in a form that is accessible to plants, this is known as fixed phosphorous. This may remain in the soil for years. Any phosphorous in the soil or applied to the soil must be converted to the orthophosphate (soluble reactive phosphate) form before plants can utilise it.

A deficiency of phosphorous results in stunted growth.

Bone meal fertilizer is a good source of phosphorous and calcium, it also has a small amount of nitrogen, and this is of little use to plants. Again it’s an animal derivative so ensure the fertilizer is sterilised to prevent any potential contamination. Use bone meal sparingly. There is risk of burning, so apply directly to the soil and avoid contact with the plants foliage.

Bone meal fertiliser is a slow release fertilizer. The nutrients from bone meal are insoluble in water and therefore must convert in the soil in to a form that can be utilised by plants. This takes time so the results won’t be immediate. It takes between two to three months to activate.

Plants can only get phosphorous from bone meal fertilizer if the soil pH is below 7, according to a recent Colorado study.

Bone meal fertilizer is alkaline but it’s use will have a negligible impact on the soils pH level. Literally tonnes would have to be used.


Bone meal fertilizer is typically labeled

N 1% : P 13% : K 0%



Potassium encourages flower and fruit production. It also improves the plants resistance to disease by helping thicken cell walls in stalks and stems. It is also vital for longevity and aids the plants hardiness against harsh winters. It can also help increase crop yield and improve crop quality.

Potassium does not form part of the chemical structure of a plant but it is key to many important regulatory roles in the plants development. Potassium activates around 60 enzymes involved in plant growth. For example, plants require potassium to regulate the opening and closing of stomates/stomata. Stomates are pores through which the plants leaves exchange carbon dioxide, water vapour and oxygen. Proper functioning of stomates is vital for photosynthesis. Potassium is also responsible for activating enzymes that regulate water and nutrient transportation, protein synthesis, starch synthesis, and transportation and storage of energy.

Potassium is plentiful in soil, especially clay soil, but very little of it is readily available to plants in a form they can utilise. Approximately 90% to 98% of potassium found in soil is ‘unavailable’ potassium. Typically only potassium that has been dissolved in water can be used by plants.

Potash or wood ash can be a good source of potassium. Wood ash contains approximately 3% potassium. However, the levels of potassium depend on the age of the wood and the combustion temperatures. Young wood from pruning has a higher level of potassium than older thicker branches.

Wood ash has a liming effect, it can be used to make soil less acidic.

Vegetables grow best in soil with a pH value of 6.5. Wood ash/potash can raise the soils pH level. It is therefore important to check the soils pH level before applying potash. However, where club root is present, wood ash can be used to raise the soils pH level to 7.5 or above. The disease isn’t able to survive

Fruit trees perform best in slightly acidic soil. As potash reduces the soils acidity, it may not always be suitable for the fruit garden.

Seaweed fertilizer

Most commercial seaweed fertilizers are low in nitrogen, phosphorous and potassium.

Seaweed fertilizer is typically labeled

N 0% : P 0% : K 1%

Whilst low in micronutrients, seaweed fertilizers contains approximately 60 trace elements, growth hormones and disease control properties. Seaweed is also an excellent food source for beneficial fungi in the soil. It is a gentle fertilizer that has many benefits and is ideal for organic gardening.

Soaking seeds in a weak solution of seaweed can aid germination, root development and give the seedling extra vigor!

Liquid seaweed is an excellent way to give seedlings a healthy start. Diluted in water and sprayed on to foliage. It promotes strong root growth and helps protect young plants from disease. This is also true of mature plants. Spraying a weak solution on the plants foliage during the growing season will help give the vegetable plants a boost

It can also be used as a foliar spray on flowers. If sprayed a couple of days before cutting, it will give the cut flowers

Posted in Fertilisers
2 comments on “Understanding Fertilizers
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March 2014
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