I'm sure we've all either used fertilisers or been around your parents when they've been using fertilisers. Fertilisers provide crops with chemicals needed for growth, like nitrates. This is because crops use up minerals as they grow and the soil can't usually supply enough minerals to fulfil demand.
There are two main types of fertilisers - natural and artificial. Natural fertilisers are made up of organic matter - like manure and sewage sludge. These don't have a set combination of minerals - there's no 'recipe' pretty much just because they're natural.
Artificial fertilisers are man made using inorganic material - unlike natural fertilisers there is a 'recipe' and the recipe depends purely on what you're growing and where you're growing it. These contain pure chemicals like ammonium nitrate in the form of powders or pellets.
Whether they're artificial or natural, they provide crops with the minerals they need for growth, like nitrates. Over time, this level needs to be replenished because they use up the minerals as they grow. The job then falls to the farmers who decide how much to use, too much is too expensive, too little and it would get washed away before it had an affect.
When fertilisers are washed off of fields (leached), this causes the problem of Eutrophication. Excess fertiliser causes an algal bloom, this blocks light, in turn, it reduces photosynthesis. This means that submerged plants can't carry out photosynthesis, they can only respire so die. Saprobiotic microorganisms aerobically respire, over time the oxygen supply is depleted and fish also die.
This article specifically relates to AQA A2 Biology Unit 4
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