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Sunday, October 13, 2013

Soil Lecture Notes

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

Some of you will have questions about your own situation – and that’s how it should be. Many of these questions will be excellent for demonstrating a concept or principle to the class, however, some of you live in simply impossible conditions that conform to no known laws of man and what you need is not an answer in a class room setting but a consultation with someone first hand with your situation as the sole focus – or perhaps a miracle, I don’t know – in those cases, I will defer your question and ask you approach me out of the classroom setting in order to answer you completely. Let’s be fair to your fellow students, this is their time too.

In teaching soils through the years, I have found that we will move in and out of three different scientific discipline: 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. So this 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 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

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 – see the chart on page 32. How does this happen?
  1. More small particles can fit into the same area.
  2. The shape of clay particles.
This could be illustrated by coffee beans, ground and whole – if I could think of a container to show this. I.e. a test tube.

Soil Texture

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

Ref. Chart on 33
An ideal soil is a mix of all these different components. 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.

Do the texture triangle procedure

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




Symptoms of deficiency of N vs. Fe

PH chart on p. 54 and the differences of pH on nutrient uptake…


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

Nitrogen; Phosphorous; Potassium

The difference between fertilizers and amendment.

Discuss lead uptake in soils…


Hillel, D. J. 1992, Out of the Earth: Civilization and the Life of the Soil, Reprint Edition, Berkeley, CA; University of California Press

Logsdon, Gene, 1975; The Gardener’s Guide to Better Soil, Emmaus, PN; Rodale Press

Gershuny, Grace, 1986; The Soul of Soil; A Guide to Ecological Soil Management, 2nd Edition, St. Johnsbury, VT; Gaia Services

Kohnke, Helmut, 1995; Soil Science Simplified, 4th Edition, Prospect Heights, IL Waveland Press

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