eli5 : If plants can make their food with water and carbon dioxide using photosynthesis why do they need manure and fertilisers ?
In: Biology
Photosynthesis is like a factory. A factory needs raw materials to turn into finished products. Without them, a factory is useless.
Plants take nutrients in from soil and fertilizer, and the photosynthesis factory turns them into things the plant needs.
They work better in soil that has those trace chemicals. Agriculture isn’t about leaving plants alone to do their thing, it’s about maximizing the yield produced by the plants.
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.
Plants make a special kind of sugar by photosynthesizing. Just like we need certain vitamins, minerals, and amino acids to grow and let our bodies function in a healthy way, plants also need minerals and other nutrients to stay healthy. They need things like nitrogen and phosphorous to grow. Usually they are able to get the nutrients they need from the soil in their environment. Because humans have altered the environment by building cities and farming which have changed how water and nutrients usually cycle through the environment, soil often doesn’t have the right balance of nutrients plants need. That’s where manure and fertilizers come in. They help supplement the nutrients the plants need but aren’t getting from the soil they are in. They are like foods we eat that are enriched with vitamins and minerals or vitamins and other nutritional supplements people might take to help balance their diet.
Imagine you’re trying to build a nice big barn. You make it out of wood. It’s entirely made out of wooden planks. But you need something to hold the planks together, mount the doors on, etc. so you use some metal nails and hinges. So the barn isn’t entirely made out of wood is it? That’s what’s happening here. All of their “food” (the energy they use) comes from the sun and photosynthesis which turns that water and CO2 into sugars, which only have oxygen, hydrogen, and carbon, and then they also use that carbon for the bulk of the structures, but some things need to be iron, phosphorous, sulfur, etc. almost the entire mass of a plant (and actually you too) is carbon, oxygen, and hydrogen. Nitrogen is next usually, and that typically comes from the atmosphere indirectly (bacteria in the root system “breathe” it), or comes from fertilizer. The rest are trace elements that come from the soil.
water and carbon dioxide are just three atoms: Carbon, Oxygen, and Hydrogen. A plant is more than those three and those come from the soil.
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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