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Soil Structure
by Dan Clost
by Dan Clost


First serious garden earned 25 cents from the Kemptville Horticultural Society when I was 12. Have been poor in horticulture ever since but rich in spirit.

Went to work writing the Good Earth column (over 500 articles published in newspaper, magazine, website and journal.) and learned that what was printed wasn't what I wanted to say and certainly not what Gentle Reader understood me to say. Subsequently have developed a certain clarity and economy of words.

Day job- nursery and production manager for a large nursery/garden centre
Side job- Garden restoration and renovations, design consultations, remedial pruning.
Night job- garden writer and communicator (overnight success in another 20 years)

Dan gardens in Canadian Zone 5b

August 5, 2001

Hopefully, we've stopped thinking of soil as dirt, unless it is on the bottom of our footwear as we walk into our homes. Soil has three functions.
It provides a means for the plant to take in nutrients and this is why the term "growing medium" is often used instead of "soil." Secondly, it provides the nutrients. Usually, excepting nitrogen, potassium and phosphorous our soils have sufficient quantities. ( The other four essential substances are sulphur, magnesium, calcium and iron.) If they don't, our plants look abnormal and this is where I suggest you talk to the friendly professionals at your favourite garden centre. Thirdly, soil provides an anchor to keep plants from being uprooted before their time. We sometimes use stakes and trellises to provide extra support. I once tried to use sunflowers as natural scaffolding for scarlet runner beans. The beans died. Seems that some varieties of sunflower exude a toxin into the soil killing the competition. I believe my sunflowers were Helianthus x hemlock, cv Borgia. I have since learned that Morning Glories - and by extension other ipomeas- are not bothered by such things.
So, what makes a good garden soil? Soil has five components:

  1. rock weathered into gravel, silt, sand, clay, salts and minerals, 

  2. organic matter which is the decaying remains of plants and animal,

  3. air,

  4. soil water; and,

  5. organisms ranging from the relatively gigantic earthworm to the microscopic protozoa, bacteria, and fungi. These must be present in the proper proportions to function as a good growing medium.

The proportion of these components gives soil its structure and ability to support plant growth. An ideal composition is called loam. It is a combination of 45% soil particles- (20% sand, 20% clay, and 60% silt), 5% humus (organic matter), 25% water, and 25% air. If there is more clay, it is a clay loam; more sand and it is called a sandy loam. As for humus, think of it as the glue that holds soil particles together. 
Here's a quick, easy and surprisingly accurate method of determining the constituents of your garden's soil. Fill a glass jar 3/4 full with the soil and then top it off with water. Pour the water in gently, displacing as little air as possible. Immediately, use a marker and draw a line on the glass showing the "top" of the water. Put on a lid and shake the container until the soil and water are completely mixed. Take the lid off and let the goop settle out. The humus will float on top. The heavier particles will sink to the bottom, etc., with clearly defined separations between each component. Use a marker and draw lines across the separations. You will notice the solution level is not as high on the glass as when you started. This is because the water has displaced most of the air. The current high mark less the previous high gives you the air space within your soil sample. Boffins tell us that the math is simple but five out of four people have trouble with fractions so take your time. If your glass jar was filled up to 8" but now the contents stand at 6 ½" then your soil air content is [1½/ 8 x 100] 18.75%.
Once we've measured the proportions we can now start to modify our soil if necessary. Is there too much clay? Add sand. Too much silt and clay, add humus such as leaf mould, peatmoss or compost. Too sandy, add clay. Does water stand on the surface too long? Add coarser materials or landscape with ditches and subterranean drains.
Once we have the soil structure squared away, we can look more closely at the nutrients within the soil and how to make them available for the plants. 

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