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human
Why does river sediment create fertile soil?

As a river rushes past rocks, it erodes them, and some salts dissolve. When it slows down, the bits of rock sediment, and I have always read that this creates very fertile soil. However, isn't the sediment mostly sand, as anything that could be useful to plants must be water soluble and therefore wouldn't sediment?
gpt
There's also lots of life in the river: algae, fish, fish poop, dead leaves falling in from trees nearby. All of these decompose into organic compounds (e.g. Humid acid), and those organic compounds also sediment out when the water slows down.

Organic mater makes very fertile soil (it's a similar process to making compost).

Why does OM make such fertile soil? Well, as you said, the nutrients that plants need are water soluble. Let's take phosphorus as an example. OM acts like a chemical sponge. When there is a lot of phosphorus in a sandy soil, the plants suck up some of it, but a lot of it gets dissolved into the rain water and flows downstream.

But, if the soil has a lot of OM, then a lot of the phosphorus temporarily binds to the OM. Remember, it's a 'chemical sponge.' Now those phosphorus atoms won't get washed out with the rain. This keeps them around for longer. Then, plant roots can start using up the phosphorus from these OM molecules.

This property is called 'cation exchange capacity,' and is one of the standard tests that farmers have done on their soil samples to judge it's fertility.

OM isn't plant food, but it keeps plant food around in the soil for longer so the plants can keep growing.

Where does the phosphorus come from? Well it comes from dissolved rocks. When you splash some acid on a rock, it dissolves some of the minerals in the rock. You can dissolve 1kg of rock a lot faster if it has more surface area, since the acid can only make contact with the surface. So the fine sand in riverbeds has a lot of nutrients in store with a lot of surface area available for exposure to acid in the soil.

Soil bacteria also chew up OM and make carbonic acid, which is very good at dissolving minerals out of rock.

So In river sediment soils:

Some of the OM gets eaten by soil bacteria to make carbonic acid.

That carbonic acid dissolves those perfectly sized sand grains to release minerals.

The released minerals are consumed by the plants over a longer period of time, because the remaining OM particles bind to them, 'sponge them up,' temporarily to take them out of solution.

This makes a soil with a relatively constant and steady supply of plant food, i.e. a fertile soil.

Over time, if you are not adding OM, this process will eventually use it up. This isn't a problem in natural settings (e.g. Rainforest) because the plant life above ground eventually dies and falls to the ground to be decomposed. Or it gets eaten by animals and pooped out onto the ground and the manure works to add the necessary OM.

But if you're growing crops and harvesting all of the plant life and shipping it for consumption off the land, then you will use up all of the OM in the soil. Farmers for many years had this problem of 'tiring out' a farm. We now have strategies for retaining OM, although not everyone is using them.

This is also why regularly flooding river valleys were great cradles for civilization. Not only was the soil good fertile soil, but the annual floods brought in new OM, so the early farmers didn't have to learn about soil OM management. They got a fresh supply every spring thanks to Mother Nature.

It's also important to note that rivers are long (duh). This means that a lot of organic material gets collected. Think of all the trees along the whole river dropping their leaves. And then all of that OM gets deposited at one place. So the river collects OM from a wide area and deposits it in a small place. This process gives that small place an unusually high concentration of OM.

Rivers also flow through lots of geography. Maybe they start in the mountains, then travel through the hills and plains and then a valley. This means those sandy particles come from lots of different geological strata. So the sand that's releasing minerals into the soil is more likely to have a good diversity of mineral content. Maybe calcium from the mountainous rocks, phosphorous from the hilly rocks, iron from the plains, etc.

There are other factors too, like soil crumb structure, Ph, and drainage, where plants like to grow in a Goldilocks zone of not-too-much not-too-little. River sediment tends to hit those sweet spots.

Lastly, it is important to note that different plants thrive in different soil types. You won't grow a cactus in river soils. So when we talk about 'fertile soils,' we are slightly biased towards soils that are great for food crops. Because those are the plants we like most.