eli5 : If plants can make their food with water and carbon dioxide using photosynthesis why do they need manure and fertilisers ?
In: Biology
Photosynthesis makes sugars, starch, fibers, fat, etc. But if you know your diet recommendations you also need protein. Plants are no different from humans in this regard. And photosynthesis is not making any protein. The plant get a lot of help from the bacteria in the soil to make its protein. A big challenge when making protein from scratch is that you need to split nitrogen molecules from the air to make nitrates which is one of the core components. This process is called nitrogen fixation. There are bacteria that can do this, some plants like the clover specialize in working with these bacteria. But we do find a lot of remains of old proteins in manure, and we can do nitrogen fixation using natural gas in a chemical plant. So by adding this directly to the soil we give the bacteria an easier starting point, plants might often just absorb the nitrates directly and make its own protein.
In addition to nitrates there are a number of minerals that a plant need. Things like potassium, calcium, phosphorus, magnesium, etc. are all important to the plant. For example the chlorophyll molecule which does photosynthesis requires a magnesium ion as part of its structure. So without magnesium there is no chlorophyll which means no photosynthesis which means the plant can not get any more sugar or fat. Again the plant get help from bacteria in the ground which produce acids that dissolve sand and clay particles to release these minerals, and fungi to transport these minerals from deep in the ground to the plant roots. But there are also lots of minerals in manure and we can mine these minerals ourselves and refine them. So by adding these to the soil directly we make sure the plants always get enough minerals.
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