eli5 : If plants can make their food with water and carbon dioxide using photosynthesis why do they need manure and fertilisers ?
In: Biology
The same reason you can’t just survive on bread or rice alone. Photosynthesis only make sugar which is basically pure energy in biology sense but you need more than that to survive. Like how human need proteins, vitamins, minerals to survive, plants need those too and they can’t make them from thin air.
The building blocks plants need to make these nutrient (which is the same nutrient we eventually get from eating vegetable) can’t be found in the air so it need to get these from the soil. The most important of them are Nitrogen, Phosphorus and Potassium which is exactly what we add using fertiliser.
Normally, they wouldn’t need added manure or fertiliser, naturally-provided dead plants and animal already provide enough nutrients but we human plant so many plants in so little space and time that the nature can’t renew these soil nutrient quick enough so we need to artificially add them.
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