For much of human history, agricultural needs were catalysts for technological and scientific progress.  In fact, it was only with the spread of the Agricultural Revolution that humans were able produce enough food to stay in one place and develop cities, and thus human civilization. 

Soil is not dirt, but instead is composed of many types of living or organic matter. 
One teaspoon of soil typically contains:
5 Billion bacteria
5 million amoeba/protozoa
6 Beetle Mites
5,000 Nematodes (Round Worms)

    Single-celled bacteria are the oldest living things and they power the entire ecosystem.  Thousands of species fill the soil and most are decomposers who recycle the energy stored in dead organic matter back into plant food. Other bacteria make nutrients with the help of host plants.  For example, bacteria living in the roots of plants like peas and clover turn nitrogen from the air into fertilizer. A few bacteria can get energy from minerals instead of organic matter. Some of these can even feed on chemical pollution. Bacterial colonies make slime that helps them stick to soil particles. The slime also sticks soil particles to each other. These goopy clumps help hold water in the soil. Plant roots can easily grow through the spaces between these clumps. The familiar, earthy odor of rich, damp soil is the smell of bacteria at work.
    Soil amoebas belong to a diverse group of one-celled animals called protozoa. They roam the film of water coating each soil particle. This water layer is so thin that an amoeba can still survive in even very dry soils.  Its tentacles surround a food item and bring it inside its body. A single amoeba can eat thousands of bacteria each day. Amoebas eat so many bacteria that they compete with nematodes for food. Soils with lots of protozoa have fewer nematodes.  Amoebas belong to the soil recycling crew. They can't use all of the nutrients in the bacteria they eat. So a lot of the nutrients go back into the soil. Plants use these nutrients to grow.
    Nematodes (Round Worms) are the most common multi-cellular animals in soil and they live all over the world. Roundworms also live as parasites in animals, including people. They aren't related to earthworms at all. Nematodes feed at every level of the soil food chain. This species hunts for bacteria, swallowing them whole and digesting them in its gut. Other kinds of soil roundworms eat dead organic matter. Some prey on protozoa and other nematodes. Still others eat fungus. Roundworms that eat and live in plant roots can badly damage farm crops and flowers. Nematodes wriggle their way through the thin film of water between soil particles. Some kinds of fungus actually trap nematodes in little nooses. Once the nematode is stuck, the fungus digests it.
    Cousins of the spiders and ticks, Beetle mites are just one group of thousands of kinds of mites that live in the soil. Like most soil organisms living in a world of darkness, beetle mites have no eyes. Mites are one of the first links in the chain of decomposers that break down organic matter into humus. Armies of mites swarm through leaf litter and the air spaces between soil grains. Each kind looks for its favorite food. Beetle mites eat nothing but fungus. They turn the fungus into pellets. Bacterial colonies coat the pellets and feed on them. The bacteria make plant nutrients from the pellets. The leftovers become part of the soil.  Beetle mites also carry fungi and bacteria with them, helping to spread these organisms through the soil. When the die, their bodies join the chain, adding minerals back to the soil.
    Fungi, unlike plants, have no chloroplasts, so they can't make their own food like plants do. Many soil fungi are decomposers and get energy from the organic matter they eat. But this food supply isn't always reliable. A very special group of soil fungi actually grow into the cells of plant root and use sugars that the plant makes. This sounds like it would harm the plant, but the it also benefits. The fungus extends its hyphae out into the soil beyond the plant's roots. This brings plants extra water and nutrient minerals. The fungus also protects the root from grazing nematodes and other, harmful fungi. In return, plants feed the fungi. This give and take is a true symbiosis. It helps both partners. It's so important to plants that just about every plant in the world has some of these little fungi growing in or around its roots. In grasslands, the hyphae even connect different kinds of plants. This allows plants to trade the minerals they need to grow. Above ground, a meadow may look like separate plants. Underground, they all belong to a single web of living things. A square meter of soil contains up to 20,000 km of hyphae (threads) , which is enough to stretch from Seattle to Miami five times!
    Earthworms have no teeth. That doesn't stop them from eating just about any dead organic matter they come across. They also swallow soil as they burrow. The microscopic animals in each bite of soil become worm food, too. An earthworm's gut is a great place for bacteria to grow. Earthworm castings add more bacteria back to the soil than the worm eats. More bacteria mean healthier soil. Together, all the worms in an area the size of a football field can eat about four tons of earth a year! All this adds to the humus in the soil. The lifespan of an earthworm is between three and six years!
    Moles use their webbed feet and strong shoulder muscles to literally swim through the soil. Unlike their cousins the shrews, moles are almost blind. They use their sense of smell and the ability to feel vibrations to find food and navigate.  Gardeners blame moles for ruining plants. But they don't actually eat plant roots or bulbs. Instead, they're hungry for earthworms and insect larvae. Moles help control populations of harmful insects. Digging is hard work. So an active mole must eat about half its entire body weight each day.


Soil is divided into layers, called horizons, which form the soil profile:

Ground Level/ Topsoil: Plants grow and animals live here on this level. Plants keep the soil cool and from drying out. Decomposers recycle dead plants and animals into humus. Topsoil is also referred to as the organic layer.

Subsoil: Mix of mineral particles and some humus near the top. Low in organic matter compared to topsoil, although most of the soil’s nutrients are found. Deep plant roots reach this level looking for water. 

Weathered parent material: This horizon is often very deep and there is no organic matter at all, just rock particles, full of minerals. The entire soil profile used to look like this at one time up to the surface, but physical weathering broke the parent material into small pieces. This layer may contain rock particles different from the bedrock below because a river or glacier might have deposited it from somewhere else.

Bedrock:  Solid rock formed before the soil above it. Can be exposed by erosion or an earthquake, where it will be weathered and broken down into the next batch of parent material and the soil making process starts all over again.

Did you know that scientists at Harvard have created a bacteria powered battery that uses bacteria found in African soil? The battery uses a layer of sand as the ionic membrane, mud with manure as the bacterial substrate, and a graphite cloth as the anode.