Organic and Inorganic Fertilisers

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Alison McEvoy

Organic and Inorganic Fertilisers

A fertiliser is a chemical or natural substance added to soil to increase its fertility. (From The Concise Oxford English Dictionary.)

Organic fertilisers are derived from animal or plant remains that decompose on or in the soil, slowly releasing mineral ions. Inorganic fertilisers are manufactured and consist of mineral ions and are usually sprayed onto soil in solution. Fertilisers are needed because in natural ecosystems, decomposition recycles mineral ions whereas with crops the plants are removed at harvest and therefore the minerals are removed and not replaced.

All growing crops require nutrients to stimulate photosynthesis and growth throughout the growing season. (Arable Handbook brochure from Kemira Grow-How.) Some of which are more important than others. The major nutrients are nitrogen, phosphate and potassium.  The secondary nutrients are calcium, sulphur, magnesium and sodium and the trace elements are manganese, iron, selenium, copper, cobalt, boron, zinc and iodine. (Multi-choice leaflet from Carrs Fertilisers.)  

Proportions of elements required for healthy plant growth

Of the elements required for healthy plant growth, oxygen, carbon and hydrogen account for 96% - 45% carbon, 45% oxygen and 6% hydrogen.

Fig 1: - A graph to show proportions of the elements required for healthy plant growth

(The Organic Garden Book by Geoff Hamilton)

If plants become deficient in these elements fertilisers can be used to replace them.

If a plant does become deficient in one of these elements it may be best to use an inorganic fertiliser, as there is a fast increase of mineral ions and are therefore immediately available to plants, whereas organic fertiliser must be converted by micro-organisms in the soil before it can be used.  This can also be seen as an advantage though as the slow release of mineral ions means that there is a steady supply to plants. (www.gardenseeker.com/fertilizers). Not all organic fertilisers are slow at releasing ions though. For a nitrogen deficiency dried blood is very fast acting and is good at providing a quick-short term boost.  (The Organic Garden Book by Geoff Hamilton) Some Inorganic fertilisers also release nutrients slowly over time; they are called ‘slow-release fertilisers’. Slow release fertilisers are inorganic fertilisers that have been coated in a special resin so that the nutrients are slowly released over time, releasing more nutrients when the soil is warm and moist like organic fertilisers.  This seems a good idea but overall costs as with all other inorganic fertilisers are higher. (Gardening techniques by Jonathan Edwards.)

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Inorganic fertilisers are immediately available to plants because they are soluble in water. This can be a big disadvantage though because the fertilisers can easily be leached from the soil by heavy rains. Nitrates from inorganic fertilisers are very soluble and easily leached; if it is washed below the root zone of the plants, it will not be available for plant use, this is why nitrogen is the most common element lacking and most often needing replacement. (Gardening techniques by Jonathan Edwards.) There is very little leaching from organic fertilisers, which is a big advantage as leaching can be a ...

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