The Story of Soil

As winter becomes spring, I am thinking more about growing food. One of the things I have been studying is soil. Last spring, I wrote a post on 'Food (Soil and Seeds)' (5/13/09). In some respects, this post is a continuation of that one. Here I am going to concentrate on soil science.

The story of soil is the story of the earth, so I will begin with the cosmic view. After the big bang, the universe slowly formed into lots of space with giant 'clouds' of hydrogen (and a little bit of helium) floating about. As the hydrogen atoms began attracting each other, they began condensing and getting denser and denser, until the atoms in the center got so squished that they began fusing and giving off massive amounts of heat and forming stars. All elements we know, other than hydrogen and helium, have been formed in the fiery furnace at a star's core, and thus Joni Mitchell is right, we are all 'stardust'--and so is almost everything else on earth. When these stars went nova, they flung their contents out into space in giant explosions. Slowly these fragments formed new stars and planets (including earth).

Earth cooled to a huge mass of rock, with an atmosphere, and lots of water. Life emerged, and modified the atmosphere and the rock. (See my post on 'Gaia', 1/3/10, for more on how life modified the atmosphere.) The atmosphere and water also modified the rock, breaking it down into pebbles, and then sand and dust. The surface of the moon, and many of the planets, is like this.

But life crawled out of the oceans, and died. And more life crawled out, and died. And this happened again and again and again. And as more life crawled onto the shore, it began feeding on the remains of earlier life, and transforming it. Slowly, the debris of life became humus, the main component of soil.

Soil science is truly an interdisciplinary venture. It combines geology, chemistry, and many branches of biology: botany, zoology, microbiology, and ecology. The rock of the earth is buried in most places under the soil and called bedrock. Above that is a section of loose rock called 'regolith' or the 'parent material'. Above that is the subsoil and the topsoil. Soil is a mixture of three types of rock particle (in order of decreasing size: sand, silt, and clay) and humus which is the decomposed remains of plants and animals, now an amorphous, almost gell-like, substance. Sand, silt, and clay in the right proportions is called loam and loam plus humus create a healthy soil structure called tilth.

The chemistry of soil is where things get interesting. Rocks are made up of many elements combined into minerals. The earth's crust contains roughly 47% oxygen, 28% silicon, 8% aluminum, 5% iron, 4% calcium, 3% sodium, 3% potassium, 2% magnesium, and less than 2% everything else (combined). The chemical needs of plants (and really most living things) are (in roughly decending order): carbon, hydrogen, oxygen, phosphorus, potassium, nitrogen, sulfur, calcium, iron, magnesium, boron, manganese, copper, zinc, molybdenum, chlorine, nickel, and cobalt.

The first thing that seems to join these two lists is high placement of oxygen--but, as it turns out, plants don't get much of their oxygen from the soil. It's also obvious that although much of the rock, and indeed the soil, consists of what is called aluminosilicates (minerals made of silicon, oxygen, and aluminum), living things don't use silicon or aluminum (except in reconstructive surgery). (Okay, I take that back--I just found out that corn, as well as some other plants, benefit from silicon.)

Plants get their carbon from the carbon dioxide in the air, the hydrogen and oxygen from water, and the nitrogen from the atmosphere--but indirectly. It's everything else on the plant list that comes from the rock via the soil. The minerals--calcium, iron, potassium, phosphorus, sulfur, magnesium, etc--come from the breakdown of rock due to weathering. This releases ions which are attracted to the clay and humus and passed on from there to the plant via its root system. For example, phosphorus comes from a mineral called apatite and magnesium comes from minerals like serpentine and dolomite--all part of the rocks.

But, as I mentioned in my previous post on soil, soil is more than just rocks and chemicals. It is more than even sand, silt, clay, and humus. It is a living entity, filled with living creatures. Take for example how plants get nitrogen. Unlike carbon dioxide and oxygen, which it can breath in directly, all the nitrogen in the atmosphere is not in a state that can be used by plants. However there are bacteria that can take nitrogen from the air and use it directly. Some of these bacteria (called rhizobia) live in a symbiotic relationship to certain plants, particularly the legumes, as part of their roots. Other bacteria change the nitrogen from protein to ammonia which other bacteria change to nitrite ions and still other bacteria change to nitrate ions. It's the nitrate ions that are absorbed by the plants after all the work of the bacteria.

Soil is just teaming with life, some of it microscopic (besides bacteria, there are algae, a specialized bacteria called actinomycetes, protozoa, and fungi with its microscopic mycelia), some of it worm-like (including the earthworms and the nematodes), some of it arthopod (including springtails, mites, and insects such as ants and termites), and some of it is full fledged mammals (like moles and mice and groundhogs).

At the end of my post 'Food (Soil and Seeds)', I closed with a quote from Elaine Ingham which I will repeat here: "Agricultural soil should have 600 million bacteria in a teaspoon. There should be approximately three miles of fungal hyphae in a teaspoon of soil. There should be 10,000 protozoa and 20 to 30 beneficial nematodes in a teaspoon of soil. ...
"There should be roughly 200,000 microarthopods in a square meter of soil to a 10-inch depth. All these organisms should be there in a healthy soil." When I used this quote, I had no idea who Elaine Ingham was. Elaine Ingham runs a group called Soil Foodweb Inc that may be the premier group looking at life in the soil. She wrote the Soil Biology Primer, which is an incredibly detailed overview of soil life and is available to read for free on the web courtesy of the Natural Resources Conservation Service. She also wrote the The Compost Tea Brewing Manual, a classic about how to add life to the soil.

This is just a tidbit of what soil is all about. Think of all that complexity the next time you garden--or even just walk on the earth.


Sources:
Milo Harpstead, Francis Hole, and William Bennett, Soil Science Simplified
David Lambert and the Diagram Group, The Field Guide to Geology
James Nardi, Life in the Soil
Elizabeth Stell, Secrets to Great Soil
and, of course, Wikipedia

Quote of the Day: "I've never outgrown a child's simple love of dirt. It has continued to fascinate me since those early days of mud pies and simple earthworks. After studying it at the university level I learned to respect its complexity and call it soil. Years of gardening work have taught me how resilient it is. The more I learn about soil, the more marvelous and magical it seems." - Elizabeth Stell