Fertilisers: Grade 9 understanding for IGCSE Biology 5.3
Fertilisers is the term used for “chemicals or natural substances added to soil to promote the growth of plants”.
Key point: in spite of what it says on this packet of fertiliser, fertilisers are not food for plants. (Just adding this photo to the post makes me feel slightly sick inside: how could MiracleGro be so happy to confuse generations of people who visit garden centres…..?)
Plants are autotrophic: they make their own food molecules in the amazing process of photosynthesis. Plants use carbon dioxide from the air plus water from their roots to produce a whole range of organic molecules powered by the energy from sunlight.
But remember that in order to make amino acids, proteins and DNA plants will also need a source of nitrogen atoms. Carbon dioxide and water do not contain any nitrogen atoms and yet nitrogen is needed for building amino acids, proteins and DNA.
Where do plants get this nitrogen from?
Well the key idea is that they do not take it from the air. Nitrogen gas in the air is very un-reactive and cannot be fixed in the plant. But the soil contains nitrate ions and plants can absorb these by active transport in their root hair cells. Nitrate ions are transported up the plant in the xylem and can be used to make amino acids etc. in the leaf cells.
Nitrates are not the only mineral ions taken up by plants in their roots. Plants absorb phosphate (for making DNA), magnesium (for making chlorophyll), potassium (for a wider variety of cellular processes) amongst many others.
So fertilisers are a way of replenishing the concentration of these essential minerals in the soil. More fertiliser, more minerals, faster plant growth as more proteins/DNA etc. can be made in the leaves…. Simples!
The commonest type of inorganic (chemical) fertiliser are called NPK fertilisers. (Nitrogen, Phosphate, Potassium). These can be bought in handy 50kg sacks (see picture above), stored and then spread easily over fields.
Farmers can also use manure which is an organic fertiliser. Here are some advantages/disadvantages of organic fertilisers in case you are interested…. It is smelly, bulky and difficult to store.