On our farm, we feed our plants by feeding our soil. Specifically, we feed the life in the soil, including fungi, bacteria, and macroinvertebrates like worms. Those animals eat organic matter (dead plants and animal poop), and they eat each other. IN then end, when the life in the soil dies or poops, chemical nutrients are released for plants to use.
On a conventional farm, the farmer will apply chemical fertilizer to feed the plants directly. Excess fertilizer will run off or filter down into the ground water, creating a nitrate-rich groundwater or algal blooms and dead zones in surface water.
On our organic farm, we will use some composted turkey manure to indirectly feed the plants. This releases slowly and has less chance for run-off or filtration to groundwater.
But the long term plan is to not feed the plants at all. We want to feed the soil life, which can feed the plants.
In a natural forest or grassland, there is an annual resupply of the soil ecosystem when leaves drop or grasses die back for the winter. We try to mimic that by either applying animal manures and compost or growing cover crops like buckwheat.
Different cover crops have different chemical nutrients that they make available to the life in the soil. For example, clover adds nitrogen to the system. Buckwheat takes mineral phosphorus and makes it more available to the life in the soil.
The most important chemical that I think about for soil-building is carbon.
Cellulose from dead plants and compost is made up long chains of carbon, linked together to make woody fibers. When we have stable fibrous carbon in the soil, we call that humus. Stable carbon breaks down slowly, and it is what makes soil black or dark brown. If you dig down, you will find a layer of soil on top that is darker than the soil underneath.
The humus in the top soil binds with clay to form crumbles (or peds) known as the clay-humus complex. Those crumbles make the soil loose for air, water, and roots to easily penetrate. At the microscopic level, those crumbles look like honeycombs, with incredible amounts of surface area. That surface area is key, because many key plant nutrients are water soluble, but they are ionically charged - meaning if there is oppositely charged surface for them to stick to, they will not wash away.
A soil without carbon will be either too sandy (and will not have enough microscopic surface area to hold nutrients - like a beach) or too clayey (and will not have a crumble texture to allow air, water, and roots to penetrate).
The topsoil requires regular additions of more carbon from dead plants and/or animal manure because even the most stable carbon will be degraded as the processes of life in the soil release methane and carbon dioxide.
So when I talk about "soil-building" buckwheat, it is on two fronts. On the one hand, when I mow and till the buckwheat into the soil, I am feeding the life in the soil and adding carbon to the system. On the other, the buckwheat is scavenging phosphorus and making it bioavailable to the next year's crop. Buckwheat also provides sources of pollen and nectar for predatory insects like tiny wasps, ladybugs, and lacewings. And it suppresses weeds. It is a great summer cover crop.
As a side note, one of the biggest drivers of climate change that no one talks about is that conventional farming is slowly removing more and more carbon from the soil because they do not deliberately replace the carbon lost due to biological activity. This creates a vicious cycle because without carbon, their soil requires more chemical fertilizer.
We know how to replace the carbon, but it would take returning many crops to the soil instead of harvesting them. I think soil could become a huge carbon sink if we could figure out a way to incentivize carbon sequestration in the soil for all farmers. The advantage is that as carbon content increases in the soil, the fertility increases because the ionic holding capacity increases (it is a virtuous cycle).
Long reply. Sorry.
