I was reading about gardening and it made me wonder: why is animal manure used when presumably the animals are just eating plant matter that could be composted? Is plant compost just as good as manure? Or does animal digestion change the nature of their diet such that it is a better fertilizer? Granted, if you have animals the manure is “free” 
Are you saying that the food you eat is indistinguishable from your shit?
You’ve obviously never tasted my cooking.
Animals break down the plant material far faster than the bacteria on the open ground do. Thus ounce for ounce there is more available chemicals for instant use by the growing plants.
a big fertilizer contribution are nitrates, they are abundant and available quicker in manure.
if you are producing meat, dairy and eggs you will have manure to dispose of.
there are plants, sometimes called ‘green manure’, which can add nitrates to your soil. you can rotate these with other crops.
But what about composting vegetable matter? Will that not break it down for quicker release?
One advantage of using animal manure is that it’s a by-product of producing other things, such as meat, eggs, wool, or even transportation (from horses).
Why is doggy n kitty poo not good for fertilizer? Is it 'cause they eat meat?
Also, what happens if you put meat scraps in compost? Is that bad?
Hope this doesn’t count as a “hi, jack!”, but the thread made me curious but it didn’t seem like enough to start a new one.
the microbes that eat plant material are not the same as those that eat meat scraps. the meat smell (or eggs or dairy) could attract rodents or dogs while just plant compost will not.
manures aren’t good for direct application fresh. they need to age so that they will not cause fertilizer burn.
A compost pile is still slower than an animal digestive tract, and it requires lot’s of care to get plants broken down in a couple weeks instead of months. Let a cow feed and you get fertilizer.
Nitrogen, the first number in the N-P-K rating for fertilizers, is the main plant growth nutrient. So called ‘green fertilizer’ plants are those that are able to fix nitrogen from the air into themselves and hold it. So when you plow them under the nitrogen is available to the bacteria in the soil. There are also many chemical sources of nitrogen for fertilizer, such as urea.
In organic gardening the nitrogen comes mainly from protein sources. That is why things like fish meal or blood meal are common in organic fertilizers, they are mostly protein. The protein is broken down by bacteria in the soil.
When an animal eats plant matter bacteria break down and consume it. Some of the nutrients are used by the animal and the bulk of it is used by the bacteria. The manure contains a large amount of it’s mass as dead bacteria bodies and their excretions. It is this bacterial mass that supplies the protein/nitrogen part of the manure.
Composting is the same process as animal digestion, but slower. The plants in the compost are NOT being broken down so that your garden can use the nutrients in the plants. The plants are being consumed by bacteria, and when the process is done it is the bacterial bodies and excretions that make the compost useful to your garden. The fiber and other partially composted material that take longer to break down are useful in increasing the texture of the soil, but it is the bacteria that grows the new plants. If you have made your own compost you may have seen steam rising from it on a cool morning or when you turn it. The compost gets warm enough to feel with your hand and has even been known to catch fire. The heat comes from the millions of bacteria busy eating and dying. Worm castings in compost are another source of poop/protein/nitrogen.
Basically, manures and bacteria break long protein chains into short protein chains that are soluble and can be used by the plant.
So, just the decaying leaves of a nitrogen fixing (right expression??) plant or tree (like neem trees?) are not enough or just mildy effective as fertilizer? I was thinking about a booklet I read years ago when I was in Haiti, about using rows of neem trees that are pruned to be bushy as terrace plants, not only to hold in the soil but (I think) as fertilizer. Is this possible?
Yes, decaying plants alone are good to compost for use. Putting bark or shreaded organic material directly into the soil is fine too. Some plants like alfalfa are high in protein that converts to nitrogen. But how do the plants ‘decay’? Without something eating them they won’t.
You can’t get plant nutrients directly back to plants without the enzymatic digestion that breaks things down to a soluble form that the plants can use.
You seem to be asking if plant nutrients can go directly back into other plants. If that is the question, the answer is no. Inside the cow or outside on the ground, there is an intermediate stage of digestion that has to take place. This decay or digestion is due to bacteria. The neem trees you mention could be nitrogen fixers drawing nitrogen from the air, but in order to make the nitrogen available to the plant the bacteria will need to turn it into soluble form first. Most legumes are good nitrogen fixers, like chickpeas. Soy beans are great to plow back under, they are high in protein which translates to nitrogen.
If you were able to grow a completely sterile soybean plant and plowed it into sterile soil the nitrogen would never become available to the plants you sow.
Chemical fertilizers can come in soluble form that can the plants can take directly up, but any organic form need the intermediate step of digestion.