Secrets of Soil
Updated: Feb 13, 2021
Its that stuff beneath our feet .. "earth" it is sometimes called, the source of the name for our home planet "Earth". I suppose, when it comes to naming things, humans aren't always very imaginative, but we come by it honestly: Even the name "human" is derived from "humus", which is yet another term for earth. Wouldn't it be odd to meet an alien, and introduce yourself by saying "Hello, I am a dirt man from the planet dirt. Pleased to meet you!" You might be clever, and refer to yourself as a "Terran", but really, that is a Latin name, derived from "Terra", which simply means "Earth"
All humor about humus aside, it is time to dig in, dish the dirt on dirt, tell the truth about turf, and study the secrets of soil.
What is soil? Travel around the world, and ask that question of one hundred people, and you are liable to get one hundred different answers, but what most of those answers seem to have in common is this: Soil is the upper layer of earth, consisting of a mixture of organic matter and minerals, that together support life OK, lets start there. My definition, and I'm not alone in this opinion (because that is what it is .. my opinion) is that, in order to be considered soil, that mixture of organic and inorganic matter must not only support life, it must also contain life. More on that thought later.
WHAT IS SOIL?
In the picture above is some potting mix .. some might call it potting soil, but it is composed of a mixture of peat moss and perlite. Perlite is a volcanic glass that has been 'puffed up' by water vapor during its creation. Although it is certainly made of minerals, it has no value as a mineral source, and neither does the peat moss. Potting mix is mostly sterile, has no nutrient value, and is primarily good for starting cuttings and seeds. Finer (small) seeds may also be started in a mix of peat moss or coconut coir, and perlite or vermiculite .. once again, no mineral value, no nutrients, but those mixes have the advantage of being sterile, so less chance of fungal or bacterial activity harming sensitive, fussy young plants. Now, lets talk about soils.
Highly academic soil scientists will tell you about twelve different types of soil, from the volcanically formed Andisols to the expansive clays found in Vertisols .. but for our purposes, lets just stick to five broad categories for now, divided by their ratio of organic to inorganic constituents, and the physical characteristics of those components. Our five soil types are Sand, Clay, Silt, Loam, and Muck. These descriptors can be applied to almost every kind of soil one is likely to encounter.
SAND Sand is the product of weathering rocks. It is composed mostly of silicon dioxide, and is carried down to the oceans by rivers, which tend to drop a good deal of it on their banks and bottoms along the way. Sandy soil is great, because it is loose enough to allow roots and water to penetrate into it easily, and drains readily. Sandy soil is terrible, because it has little mineral value (it is mostly glass, after all!) and it does not hold on" to minerals or nutrients much at all.
CLAY Clay is also produced by the chemical weathering rocks, most notably feldspar and igneous rocks. Clay particles are very fine, and have the ability to form close bonds with water, becoming hydrates of silica, in combination with an assortment of other minerals that are important to growing healthy plants. Clay soil is great to have, because of those important minerals, and clay's ability to retain water - up to nearly twice it's own weight! Clay soil is terrible because the particles are so fine that they squeeze tightly together, impeding water and root infiltration, and being slow to drain.
SILT Silt is a product of rivers, mixing weathered quartz and feldspar or igneous rock together as it makes its way down to the sea. The particles are more coarse than clay, but finer than sand, and it is what gives a river its 'muddy' appearance. Rivers deposit plenty of this material on their banks and bottoms, and it can usually be found resting on top of sand, owing to its lighter weight. Silt is great to have because it drains better than clay (although not as well as sand) and actually does have some minerals available for your plants. Silt is terrible because it does not store nutrients very well.
LOAM Loam is a soil that has a fairly even distribution of sand, clay, and silt .. and often a fair amount of organic matter as well. Loam is great to have because it retains moisture well, but drains adequately, is loose enough to permit roots and water to infiltrate reasonably well, but has enough density to hold on to minerals and organic matter. Although loam is not terrible, it can often be improved, and we will explore some ideas in that direction in a minute.
MUCK Muck soils are composed primarily of decomposed organic matter, which can be from animal manures, vegetation, or both. Muck soils are great to have, because they retain moisture remarkably well, feed the fungal and microbial life necessary to promote healthy plant growth, and supply one of the most important macro nutrients, Nitrogen, at a ratio of about 10:1. Muck soils are terrible because they tend to be mineral poor, drain poorly and resist oxygen infiltration.
Before getting too much farther, it might be a good idea to examine how soils are formed, and here we are going to look at three more broad categories: In Situ, Alluvial, and Moraine soils.
In Situ means "in place" .. the soil was formed right were you found it. This is where exposed rock is weathered, producing sand, silt, and clay, and pioneer species like grasses and small trees began the process of further breaking up the rock and laying down organic matter. This is the sort of soil that you will find in mountains and on plateaus that were once nothing more than exposed rock, and tend to have a thin layer of organic matter on top, followed by clay, then rock underneath. Finer particles of sand and silt will often be carried downhill by wind and water, eventually depositing those materials in stratified layers .. that is alluvial soil. Finally, moraines are formed when glaciers slowly grind across the landscape, picking up bits and pieces of everything along the way, eventually melting, and depositing all of that material in an unorganized jumble.
Due to the relative mass of soil particles, they will eventually stratify - that is, form layers, with the lightest layers on top. Even moraine soils will stratify, given sufficient time. Another term for these soil layers is 'horizon'. You might hear me, or someone else referring to processes that are taking place above, at, or below the soil horizon .. this is what that means. Working down from the top:
The first horizon is the organic layer, and it is composed of decomposing and partially decomposed organic matter. In a healthy ecosystem, Nitrogen is tied up here temporarily as decomposition occurs - dead plants, animals, and manures composting in place, but there is no need to be concerned, as the roots of your plants don't grow much in this layer at all: The roots of plants extend down into the surface horizon, and all of that decomposition is occurring above it.
The second horizon is the soil surface. If you have seen a freshly ploughed field, you have seen the soil surface horizon exposed - no organic layer above it. This is where the bulk of fungal and microbial activity occurs, and is also where most of a plant's roots are found. The soil surface will contain a mixture of organic matter and minerals, but this is where the bulk of soil organic matter is found. Heavier minerals and particles will work their way down from this point, passing into the subsoil horizon.
The third horizon is the subsoil. Some organic matter may be found in this layer, but it is primarily composed of heavier particles: clay, and minerals like Iron, Calcium, and soluble minerals. Some plant roots extend down into the subsoil .. certain plants are quite adept at putting down deep tap roots to mine the subsoil for minerals that might be unavailable in the soil surface horizon. Minerals continue to work down from here at a much slower rate, sometimes entering into the substratum.
The fourth horizon is the substratum. Very little if any organic matter infiltrates to this point, and the migration of minerals is very slow. Plant roots may grow down into this horizon with some difficulty; clay becomes much more dense at this point, and partially weathered rock is common.
The fifth horizon is bedrock. Minute fissures may provide space for adventurous roots to explore, as those cracks will often become the repository of finer soil materials that have either worked their way down from the surface horizon over time, or be present as a result of the bedrock slowly weathering.
Getting a professional laboratory test of your soil will provide you with detailed information that exceeds the limits of simple observation: Soil organic matter %, macro nutrient levels (Nitrogen, Potassium, Phosphorous) Soil Ph, and levels of other essential minerals: Calcium and Magnesium being the most common, but some labs will include other minerals in the basic test as well: Zinc, Sulfur, Manganese, Iron, Copper and Boron. Shop for a lab that can provide you with as much information as possible, for the most reasonable price - there is something to be said as well for accessibility: I can walk into the OSU extension just a few miles from me with my test in hand and ask a real living human to explain it to me, if I see something puzzling in the results.
Organic Matter is that portion of the soil that is composed of once-living material: the remains plants and animals. OM levels should be higher than %2, with %5 considered 'good'. High quality organic soils can have OM as high as %20, but don't be surprised if that number is much lower; the percentage is determined by weight, not volume, and organic matter is less dense than the inorganic components of soil.
Macro nutrient levels will be expressed as a range from "very Low" or "deficient" to "very high" or "excessive", and are often accompanied by a numeric value, depending on the lab that is doing the test. Since the exact presentation of the data varies from one lab to the next, I am not going to attempt to detail all of them; that would require several volumes of information, with only a tiny fraction of it being useful to any one person - another reason to select a lab that is willing to explain what their particular test results mean, should you have questions. Generally, nutrient levels in the "moderate" to "optimal" range are good, and there is no benefit to adding additional nutrients. "Excessive" or "very high" levels are typically not harmful to plants, with the exception of Aluminum and Manganese in low Ph (acidic) soils, where those metals can reach concentrations that are toxic to plant life.
Soil Ph is a measurement of how acidic or alkaline your soil is, with a score of 7 being neutral. The higher the score is, the more basic, or alkaline, and the lower the score is, the more acidic the soil is. Each point on the scale represents an increase of ten times the level of alkalinity or acidity. The ability of plants to uptake nutrients is affected by the soil Ph, with most nutrients being readily absorbed at a range between 6 (slightly acidic) and 7 (neutral) No doubt you have heard that ericaceous plants enjoy acidic soils, and they do - but keep in mind that even your blueberries and cranberries need to be able to uptake minerals like Calcium, so lowering soil Ph below 5.5 may not be a good idea especially if your soil has an excess of Aluminum or Manganese.
Not all soil testing needs to be done by a laboratory - in fact, there are several practical tests that can be done with little or no equipment that can provide you with a lot of information about your soil. Let's get hands-on! Pick a day where the temperature is above 55 degrees (10C) and the soil is moist, but not wet. Bring along some Ph test strips, a shovel, a 5 gallon bucket full of water, a one gallon clear glass jar, and a tarp. Begin by digging a hole one foot in diameter, and one foot deep, and put all of that soil on to the tarp, grass roots and all. Fill the hole with water, and let it drain. While the hole is draining, pick up a handful of soil, and give it a good squeeze, hard enough to make it stick together when you relax your grip. If the soil simply wont form a cohesive ball when squeezed, it is a good bet that you have sandy soil - and you most likely heard the sound of sand particles scraping on your shovel when you dug the hole. Once you have formed a ball, pinch it slightly. If the ball crumbles apart readily when pinched, you have loam. If the ball 'smears' or sticks together stubbornly, you are likely to be looking at a soil with a high clay content.
Spread the soil out on your tarp, and begin looking for signs of life: earthworms are a good indicator of soil organic matter, but there may be other things living in your soil as well. If there aren't at least ten living organisms in a one cubic foot sample of soil, you will need to add some organic matter. Have a good look at the roots of any vegetation that you dug up. Can you see white, hair-like or fuzzy growth on the roots? If so, then you have fungal activity in your soil, and that can be a very good thing! While examining the roots, look to see if there are any small lumps or knobs, nodules on the roots. If nodules are present, and the plant that you are examining is a member of the fabaceae family, there is more good news - rhizobium bacteria have colonized your plant, and are busily working away, taking free nitrogen from the air, forming nitrates, and exchanging it with the plant for sugars. If the plant in question is not a leguminous plant, the presence of lumps or knobs on the roots may be an indicator of a different sort: root nematodes, that can adversely affect the health of your plants. It may be soil life, but not necessarily of the type that you want. There is a list of plants in the fabaceae family in the blog post "The Internet of Trees", about midway through, on page 4 of "A permaculture Designer's Quick Guide to Mycorrhizal Associations".
Fill your one gallon glass jar 1/4 full of soil from the tarp, and fill it the rest of the way up with water, and stir the contents vigorously until as much of the material as possible has become suspended in the water. Un decomposed organic matter may float to the surface .. go ahead and scoop that portion out and set it aside. Place the jar somewhere where it will be easy to look at the contents from the side - a table or shelf will do just fine. Wherever you choose to put the jar, make sure that it can remain un disturbed for several hours. Remember that bit about how alluvial soils are formed? That is what we are doing here, just on a small scale. The lighter fine particles of organic matter and silt will remain in suspension for a much longer time, but will eventually settle out on top of the sand, clay particles, and any small rocks that may be in your sample. Take a small portion of soil, and mix it with water until you have formed a soft mud. Dip your test strips in the mud to get a quick estimate of your soil's Ph.
Has the water in the hole drained yet? If so, re fill your bucket, and go ahead and fill it up again. Make a note of the time; you will want to come back and check the rate that the hole drains every hour or so. If it takes longer than four hours for the hole to drain, you have poor drainage, almost certainly heavy clay soil. Sandy soil will likely drain in under an hour. When four hours have passed, it will be a good time to check in on your jar, to see the layers that have settled out. If the water is still murky, leave it be for a while longer. Using a strong flashlight, backlight your sample: it should be possible to make out the different layers of particles in the jar. Rocks and dense clay particles will be at the bottom, followed by sand, then silt, and then fine particles of organic matter. The more narrow your sample jar is here, the better .. a broad jar may make discerning the layers more difficult. This test can provide you with a rough estimate of the ratio of clay to sand to silt in your soil.
Once we have determined what kind of soil we have, and identified any deficiencies, we will want to improve it. Our main goals here are to provide adequate drainage, organic matter, and soil nutrients to optimize growing conditions for our plants - but where do we start? Well, lets begin with a little formula: We know that the average weight of one cubic foot of soil is about 40 pounds. For every pound of soil amendment, we can cover 2.5 square feet of surface, and increase the soil by 1% in that particular amendment to a depth of one foot - about the depth of the average shovel. Now, all we need to do is figure out what the ideal soil composition should be, and amend accordingly. *ahem* That is approximately 1 Kilogram to 1.5 meters, for anyone using the metric system. (edit) Something that I neglected to mention previously: finished compost is about 40-60% water .. so it takes 2 pounds of finished compost to provide one pound of organic matter.
The ideal soil organic matter percentage is an interesting subject .. I've often heard it said that "one cannot have too much organic matter", and although it is true that most soils can greatly benefit from having more organic matter, the ideal percentage for a good, rich organic soil is about 15-20% If the organic matter content gets too high, we are heading into muck soil territory, and although some plants enjoy that sort of thing, remember that although organic matter is a champ at retaining moisture, it is also terrible at drainage .. and we want water to be able to move through the soil, which I will explain a bit more in a moment. If one is doing the initial amendments on their soil, and the soil does not already contain a decent amount of charcoal, consider adding up to ten percent charged bio char as an amendment. This can account for up to half of the ideal organic material content, retains water well, provides lots of surface area for microbial activity, does not impeded drainage in the same way as muck will, and once established in the soil, will remain there without decomposing into atmospheric carbon for a long, long time.
Why does soil need to drain? Well, there are a number of reasons. The first reason is to permit air flow through the soil .. plant roots and microorganisms need air to breath. Without the microorganisms to break down and convert nutrients into forms that the plants can absorb, the plants starve. Water sitting on roots for too long can promote rotting of the roots, and finally, plants cannot take in nutrients if water cannot flow readily through the soil. The way that a plant moves water through its cells is similar to drinking from a straw: up in the leaves, plants release water vapor into the atmosphere, creating a zone of low pressure within the plant, which draws in water from the surrounding soil. Imagine trying to drink a very thick milkshake through a narrow straw .. now imagine how frustrated your plants must be, if they are immersed in an environment that has plenty of moisture, but no way to bring it up through the roots.
Do you recall the five broad categories of soil from earlier? The reason that loam is preferable to silt, sand, and clay alone is that it has a mixture of different particle sizes. Different sized and shaped particles don't stack together evenly, they leave gaps between them where air and water can flow. Aside from increasing organic matter, one way to improve on loam is to add even more diverse particles to the mix: Coarse sand and small rocks. Coarse sand, about the size of what you might find in a standard aquarium provides more gaps, and pea-gravel, even up to pieces nearly the size of your thumb improve drainage even more. And here I thought that we wanted to remove all of the rocks from out soil!
If soil has been sitting about for too long, with inadequate water, or living roots to create and maintain channels for water, nutrients, microbial life and so forth, the natural settling of particles will eventually seal up the spaces within the soil - it becomes compacted. Presuming that at this point, there is a diversity of particle sizes, and adequate organic matter available, it is time to introduce plants to the system that have a habit of growing deep taproots, to force open pathways and channels once more. There may even be circumstances where it may be desirable to engage in some major disruption: mechanically loosening heavily compacted soils with machinery before re-introducing plant species that can maintain a healthy, well draining soil structure.
At the very beginning, I stated my opinion that soil could not be properly called soil unless it contains life. Bacteria, protozoa, and fungi are responsible for taking minerals and nutrients from the soil structure, and converting them into forms that can be absorbed by plants. This process is only possible when the soil Ph is within acceptable parameters: 5.5 at the very lowest (and then only when attempting to grow acid loving plants, like members of the Ericaceae family) to 7.5 at the highest. If soil Ph is too far towards one extreme or the other, amendments may be needed to bring it back to proper health. Because there is a lag between when a Ph adjusting amendment is added, and when that amendment has worked its way through the biological components of your soil, it is best to make adjustments slowly. You can always add more later if needed, but it is far more difficult to correct an imbalance if you change Ph too quickly.
To lower Ph, adding organic mulch above the soil horizon will gradually lower Ph as the organic matter decomposes. For faster results, a common method is to add elemental Sulfur. Soil bacteria consume the Sulfur, secreting Sulfuric acid, and lowering Ph. Aluminum Sulfate is advertised as a means of lowering Ph, but that leaves the problem of Aluminum toxicity. When in doubt, always alter Ph slowly. Raising Ph can be accomplished by adding cru