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Thursday, April 18, 2013

Soils; Notes from Week Three

(This, in general, describes the Week 3 lecture. As usual, many directions were taken inside this framework in response to questions. Get notes from other students if you were not able to attend.)

We know more about the movement of celestial bodies than about the soil underfoot. Leonardo da Vinci

We will move in and out of three different scientific disciplines: physics, chemistry and biology. We can only talk about one subject at a time, because that is how we learn, but I want to stress to you from the very beginning that these are interconnected in a very intricate dance. Whatever you do to one will affect the other two as sure as cutting up beets will give you red fingers. Remember that and you will go a long way towards mastering the soils you garden with.

Air and water share spaces in the soil. After a rain, an event that has happened here once in a while, as much as 100% of the soil pore space may be filled with water; this same pore space may be 100% filled with air in the event of an extended drought – in which case, all the plants in that soil would be dead. Therefore the percentage allotted to water and air is always in flux. Approximately half of the volume of soil is pore space and can be taken up with water or air depending on the current weather conditions.

Except for a precious small number of you, most of you will garden in soils that have 5% organic matter. Maybe even less. These figures represent an ‘average’ soil. There are variations from place to place, but this representation is close enough for an average number through out.

Soil Formation

Soil forms over thousands of years and is an ongoing process. Soils in California are relatively young soils and haven’t, for the most part, developed any great depth.

The following factors inform the process:
  1. Climate – including temperature and rainfall
  2. Organisms – from the itty bitty (microscopic) to the biggies (macroscopic)
  3. Topography – (the book calls relief) – land surface
  4. Parent material – the original rock
  5. Time – the factor that weathers us all.


Soil forms from the parent material. Climate participates in this process in many guises:
A mild climate forms soil more slowly than a non-forgiving climate


Organisms from lichen growing on a rock to a tree that sends its root hairs down into crevices of the rock and fissure it.


Soil forms more easily on a level surface. Look at the sheer face of a cliff and you’ll see the extreme proof of what I’m saying.

Parent Material

The rock underneath your garden. Granite becomes soil less rapidly than sandstone.


Because time ages everything.

Soil Composition

Sand/Silt/Clay – the physical sizes
Sand – from 2mm to 5 hundredths of a mm
Silt – from 5 hundredths of a mm to 2 thousandths of a mm
Clay – smaller than 2 thousandths of a mm

Characteristics of Soil Components

Water holding
Medium +
Drainage rate
Slow/Very slow
Soil organic matter
Medium +
Decomposition of organic matter
Speed of warming
Storage of nutrients
Resistance to pH change

Notes on Clay Soil

Clay particles, though tiny, have a much larger surface area – clay particles are hollow with an interior that looks very much like Marina del Rey from the air, jetty and boat slip like interior contours creating a much larger surface area than would seem possible.

More particles fit into the same area (less pore space between them)

Clay particles are electronically charged and bind water (and therefore nutrients) to the particle while such water (nutrients) are washed away easily in a sandy.

Soil Texture

Is determined through the proportion of these differing components found in a given soil.

An ideal soil is a mix of all these different components, sand, silt and clay. While it is possible to have a soil that is composed of one or the other component, the likelihood is that it will be a combination of all three. The proportion of one to the next determines how you call your soil.

Textural Triangle Exercise

Activity: Using the Textural Triangle

Sand, silt, and clay are the three particle sizes of mineral material found in soils. The percent of each of these in a given soil is called the "particle size distribution" and the way they feel is called the "soil texture". Soil Scientists have created classes which break these textures into 12 categories. The textural triangle is a diagram which shows how each of these textures are classified based on how much sand, silt, and clay is in each.

To get the sample you will use for this exercise, choose a spot in the area you wish to plant. If it is a large area, you may wish to take several samples to work with in different jars or you may take several samples and combine them together in one jar. In the first case, the soil might have several diffrerent textures that you wish to account for while the second case, the soil would be rather homogenous and consistent throughout the planting area. In this class, we will only work with one sample.

Using a pint jar, add soil to fill the jar about 3/4's full. Add water, leaving about ½ to one inch of headspace. If you have Calgone Bath Beads, or alum at home, add one teaspoon to your jar of soil and water; if not, wait until class where will have alum on hand. Shake the mixture up as thoroughly as you can. Allow to settle. The ideal amount of time the mixture should settle out for the most clarity is about 24 hours. We will not have that luxury in class.

Once it has settled, observe the layers. There will be three distinct layers of soil. The first will be sand, as it is the heaviest. The second will be silt and finally, on top, clay. Sometimes the difference from one to the next will be color. Other times, it is all the same color, but the texture is different. There can be a certain amount of art to finding which is what. Assign percentages to each layer, based on your best guess. Remember that the total of your three percentages MUST EQUAL 100. The soil in the water is the 100%.

The following directions assume you start with the sand. You do not have to; use the one you feel most comfortable with, always remembering to total to 100.

1. Place the edge of a ruler at the point along the base of the triangle that represents the percent of sand in your sample. Position the ruler on or parallel to the lines which slant toward the base of the triangle.

2. Place the edge of a second ruler at the point along the right side of the triangle that represents the percent of silt in your sample. Position the ruler on or parallel to the lines which slant toward the base of the triangle.

3. Place the point of a pencil or pen at the point where the two rulers meet. Place the top edge of one of the rulers on the mark, and hold the ruler parallel to the horizontal lines. The number on the left should be the percent of clay in the sample.

5. The descriptive name of the soil sample is written in the shaded area where the mark is located. If the mark should fall directly on a line between two descriptions, record both names.

Feel the texture of a moist soil sample in your classroom. Sand will feel "gritty", while silt will feel like powder or flour. Clay will feel "sticky" and hard to squeeze, and will probably stick to your hand. Looking at the textural triangle, try to estimate how much sand, silt, or clay is in the sample. Find the name of the texture that this soil corresponds to.

Practice Exercises:

Use the following numbers to determine the soil texture name using the textural triangle. When a number is missing, fill in the blanks (note: the sum of %sand, silt and clay should always add up to 100%):

sandy loam












We will do this in class, together, next week. BRING YOUR SOIL SAMPLE & TRIANGLE TO CLASS.

Carbon Sequestration

Wetland restoration
Wetland soil is an important carbon sink; 14.5% of the world’s soil carbon is found in wetlands, while only 6% of the world’s land is composed of wetlands.


Globally, soils are estimated to contain approximately 1,500 gigatons of organic carbon, more than the amount in vegetation and the atmosphere.
Modification of agricultural practices is a recognized method of carbon sequestration as soil can act as an effective carbon sink offsetting as much as 20% of 2010 carbon dioxide emissions annually.
Carbon emission reduction methods in agriculture can be grouped into two categories: reducing and/or displacing emissions and enhancing carbon removal. Some of these reductions involve increasing the efficiency of farm operations (i.e. more fuel-efficient equipment) while some involve interruptions in the natural carbon cycle. Also, some effective techniques (such as the elimination of stubble burning) can negatively impact other environmental concerns.
In practice, most farming operations that incorporate post-harvest crop residues, wastes and byproducts back into the soil provide a carbon storage benefit. This is particularly the case for practices such as field burning of stubble - rather than releasing almost all of the stored CO2 to the atmosphere, tillage incorporates the biomass back into the soil where it can be absorbed and a portion of it stored permanently.

Enhancing carbon removal

All crops absorb CO2 during growth and release it after harvest. The goal of agricultural carbon removal is to use the crop and its relation to the carbon cycle to permanently sequester carbon within the soil. This is done by selecting farming methods that return biomass to the soil and enhance the conditions in which the carbon within the plants will be reduced to its elemental nature and stored in a stable state. Methods for accomplishing this include:
  • Use cover crops such as grasses and weeds as temporary cover between planting seasons
  • Concentrate on perennial food production vs. annual production
  • Cover bare soil with hay or dead vegetation, protecting soil from the sun and incorporating much more compost into the soil so it holds more water and is more attractive to carbon-capturing microbes.
  • Restore degraded agricultural land, slowing carbon release while returning the land to agriculture or other use.
Agricultural sequestration practices may have positive effects on soil, air, and water quality, be beneficial to wildlife, and expand food production.
Typically after 15 to 30 years of sequestration, soil becomes saturated and ceases to absorb additional carbon, implying the extistence of a global limit to the amount of carbon that soil can hold.
Governments such as Australia and New Zealand are considering allowing farmers to sell carbon credits once they document that they have sufficiently increased soil carbon content

Organic Matter

The end process of compost is: humus

Humus is a complex organic substance resulting from the breakdown of plant material in a process called humification. This process can occur naturally in soil, or in the production of compost. Humus is extremely important to the fertility of soils in both a physical and chemical sense (see below). Physically it helps the soil retain moisture and encourages the formation of good soil structure. Chemically, it has many active molecules that can bind to plant nutrients, making them more available. It is difficult to define humus in precise terms because it is a highly complex substance, the full nature of which is still not fully understood. Physically humus can be differentiated from organic matter in that the latter is rough looking material, with coarse plant remains still visible, while once fully humified it become more uniform in appearance (a dark, spongy, jelly-like substance) and unstructured in structure; which is to say, it has no determinate shape, structure or character, it is not square, round or triangular.

Plant remains (including those that have passed through an animal and are excreted as manure) contain organic compounds: sugars, starches, proteins, carbohydrates and organic acids. The process of organic matter decay in the soil begins with the decomposition of sugars and starches from carbohydrates which break down easily as detritivores initially invade the dead plant, whilst the remaining cellulose breaks down more slowly. Proteins decompose into amino acids at a rate depending on Carbon: Nitrogen ratios. The humus that is the end product of this process is a mixture of compounds and complex life chemicals of plant, animal or microbial origin which has many functions and benefits in the soil as outlined below;

The process that converts raw organic matter to the relatively stable substance that is humus feeds the soil population of micro-organisms and other creatures which helps in maintaining high and healthy levels of soil life.

Effective and stable humus are further sources of nutrients to microbes, the former providing a readily available supply whilst the latter acts as a more long term storage reservoir.

Humification of dead plant material causes complex organic compounds to break down into simpler forms which are then made available to growing plants for uptake through their root systems.

Humus can hold the equivalent of 80-90% of its weight in moisture, thus increases the soil's capacity to withstand drought conditions.

The biochemical structure of humus enables it to moderate- or buffer- excessive acid or alkaline soil conditions.

During the humification process microbes secrete sticky gums- these contribute to the structure of the soil by holding particles together, allowing greater aeration of the soil. Toxic substances such as heavy metals, as well as excess nutrients, can be bound to the complex organic molecules of humus and prevented from entering the wider ecosystem.
The dark color of humus (usually black or dark brown) helps to warm up cold soils in the spring.

Humus which is also capable of further decomposition is referred to as effective or active humus. It is principally derived from sugars, starches and proteins and consists of simple organic acids. It is an excellent source of plant nutrients, but of little value regarding long term soil structure and tilth. Stable humus consisting of humic acids on the other hand, are so highly insoluble (or tightly bound to clay particles that they cannot be penetrated by microbes) that they are greatly resistant to further decomposition. They add few readily available nutrients to the soil, but play an essential part in providing it's physical structure. Some very stable humus complexes have survived for thousands of years.

Humus should not be thought of as 'dead'- rather it is the 'raw matter' of life- the transition stage between one life form and another. It is a part of a constant process of change and organic cycling, thus must be constantly replenished- for when we are removing prunings and crops for the kitchen we are depriving nature's cycle of potential humus. This is why we need to substitute compost and other sources of organic matter to maintain the fertility of our productive land.

Organic matter placed on the soil is called: mulch.

Organic matter dug into the soil is called amendment.

Some of either can be called humus, but not all.

Organic matter in the soil mitigates any negatives of that soil:

Too much clay is opened up by adding OM.

Too much sand is cohered by adding OM.

Micro and macro organisms live on OM.

There are several different schools of thought on how to get OM in to and used by the soil: from double digging, to using a tiller to sheet composting.

Soil Water

Nutrients enter a plant via soil solution.

Water coheres to itself (describe the miniscus).

Roots take up water one molecule at a time. Water molecules cohere throughout the plant – form the water column. That water molecule pulled into the plant root will pull along one behind it and one behind it.

Discuss water pulled across a moist soil and watering away from the plant’s base. Water not making across differing soil types.

    Each shovel of soil holds more living things than all the human beings ever born.


Nutrients Available (via atmosphere or water)


Primary Nutrients


Secondary Nutrients





NPK – a ‘complete’ fertilizer and what do the numbers mean..

Nitrogen; Phosphorous; Potassium
The difference between fertilizers and amendment.

A Soils Bibliography

Out of the Earth: Civilization and the Life of the Soil; ©1992 University of California Press , Hillel, Daniel. Hillel has written one of the most beautiful books on soil that has ever been published. This book introduces a little of soil science to the reader, but more than that, it fosters a love of the soil and an understanding about the magnitude and gravity of misuse and degradation; civilizations have paid little heed to the soil underfoot and it has cost them dearly. A delightful read!

Soils and Men, Yearbook of Agriculture 1938, © 1938, United States Department of Agriculture, The Committee on Soils. A government publication, I challenge you to read from beginning to end! It is referenced here because it clearly shows the US government knew about the soil food web and chose to ignore that information in favor of more commerce in chemical based fertilizers. We are at a point where ignoring the soil food web is too costly to continue.

Teaming with Microbes: The Organic Gardener's Guide to the Soil Food Web, Revised Edition, © 2010 Timber Press, Lowenfels, Jeff and Lewis, Wayne. This is the second edition of the book that blew my eyes open on the biology of the soil and how we cannot ignore that biology plays at least as big a part of soil fertility as chemistry. We ignore biology to our own detriment and destroy our soils.

The Rodale Book of Composting, ©1992 Rodale Press, Martin, Deborah and Gershuny, Grace Editors. This is the only book to read on composting. Everything else is compostable. Only.

The Soul of Soil; A Guide to Ecological Soil Management, 2nd Edition, ©1986; Gaia Services, Gershuny, Grace. This fabulous and passionate book is injured by being targeted to farmers (only) and therefore all recommendations are written in “pound per acre,” when we need ounces per 100 square feet. When I used this book, I wrote up a formula in Excel to convert all these into a usable figure.

The Worst Hard Time, The Untold Story of Those Who Survived The Great American Dustbowl © 2006; Mariner Reprint Edition, Egan, Timothy. Not strictly a soils book, but a real eye opener that shows how we are repeating many of the same mistakes today as what lead to the disaster we call the Dustbowl. This book is gripping reading and is not fiction. It really happened and it happened on a scale unprecedented in modern times. We can do it again if we fail to heed these words. A VERY good read on soils and man's relationship to them.

Also note, not included in the lecture, because I didn't see it until the day following, look at this article on perennial grains and the work currently being done on them.


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