The Dirt on Soil

comments (12) May 4th, 2009

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mswift mswift, member
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When I was a little girl, one of the rhymes I remember best is the old standby, “Mary, Mary, quite contrary, how does your garden grow?”  I’m sure it wasn’t because I was contrary (at least not back then!).  These days, I find it an easy ditty to remember that important question – how does a garden grow?  The fundamentals of how begin very simply and at the lowest level, literally – the ground. 


In my training, I have been taught that “dirt” is a four-letter-word, best used to describe what is under your fingernails when you finish harvesting potatoes.  “Soil,” on the other hand, is the essential pot-of-gold that can make or break a garden’s success.  In a perfect garden world, the soil would be loose and thoroughly aerated (never compacted); it would be perfectly balanced with nutrients, alkalinity, and moisture, and be full of all the right organisms to help keep the soil “perfect” and plants healthy.  More often, however, the soil we have is imperfect and needs amending regardless of the reasons the soil is imperfect. 


Different types of soils occur as a result of the interaction of several factors:  the parent material (think basic ingredients), organisms, climate, topography, and time.  After hundreds or thousands of years of interactions, the soils left behind are sand, silt, clay, or a combination of the three because of how the ingredients come together during the interactions.  Perfect garden soil is medium-textured soil that is balanced, contains an equal mix of all three particles, and is commonly referred to as loam. 


Soil science is a broad subject but, for our purposes, it is important to learn about three key soil characteristics that need to come together to make the perfect garden soil – soil texture, structure, and pH and to learn that adding organic matter to any garden soil will have beneficial effects.


Soil Texture:  Soil texture is important because it gives you a baseline from which to create perfect garden soil.  The term describes the relative volume of solid soil particles (sand, silt, or clay) that you have as well as the air spaces between those particles.  Sand is the largest particle, silt is the texture of talc, and clay particles are microscopic.  Dampen a small amount of soil and feel it between your fingers to get a pretty good idea of its texture – sandy soils feel gritty, silty soils feel silky or smooth, and clay soils are sticky and feel almost putty-like.  Knowing what you’re starting with will help you get to that “perfect” soil.  Soil texture is important because it affects how easily the soil can be worked, how well it holds water, and how quickly it warms. For example, sandy textured soils like those typical of Northern Michigan allow water to pass through more quickly, warm up faster, and can be tilled more easily than finer soils – the downside is they dry out more quickly and are lower in nutrients which leach more readily out of the soil.


Soil Structure:  Structure is the way all the particles and air spaces come together.  A well-structured soil is a soil with varying sized particles, like in a loamy soil, because it allows maximum space for air and water and yet still holds together.  By contrast, pure sand has very little structure – it doesn’t hold together, while clay has poor structure – there are few spaces for air and water.  Structure is critical to plant health because it affects root development.  If the soil doesn’t have good structure, one of the best ways to improve it is to encourage earthworms to take up residence.  Worms eat their way through the soil and expel nutrient rich ingredients (castings) that make the soil coarser.  These castings create spaces and the worms create tunnels that allow air and water to flow freely and improve soil structure.  You can make worms feel welcome in your garden by adding lots of soil organic matter.


Soil pH:  The pH scale is a measure of the degree of alkalinity or acidity in the soil.  Knowing your soil pH is the most important soil factor because plants need certain pH conditions to take up essential nutrients.  For example, maximum nitrogen availability is between pH 6.0 and 8 because it is the most favorable range for the soil microorganisms that mineralize the nitrogen in organic matter and those organisms that have nitrogen-fixing abilities – activities necessary for healthy plants.  Other nutrients have similar favorable ranges and too acid or too alkaline soils prevent nutrient uptake by plants.


The pH scale ranges from 1 to 14.  Acid soils are at the lower end of the scale (1 to 6.9) and alkaline soils are at the higher end of the scale (7.1 to 14).  Neutral soils which are neither acidic nor alkaline have a pH of 7.0.  Increments between the numbers on the scale (e.g., from 4.0 to 5.0) represent a ten-fold increase so it is a geometric scale – a pH at 4.0 is ten times more acidic than one at 5.0.  Most vegetable do best with a soil pH of between 6.0 and 7.0 but there are some exceptions.  Carrots, eggplant, sweet corn, and potatoes grow well at 5.5, while cabbage and cauliflower can tolerate a pH of 7.5. Soil pH can be altered with soil amendments like lime (overly acid soils) and sulfur (overly alkaline soils) depending on your starting pH.


Perfect Soil

First and foremost, how do you know which soil characteristics you are starting with?  Don’t guess; soil test.  (See sidebar).   Soil tests identify the characteristics that are most important to manage for “perfect” soil – pH, nutrient availability, and organic material.  If you weren’t lucky enough to find perfect garden soil, don’t despair.  You can fix it relatively easily with soil amendments (organic matter).  If your soil already is healthy, these same amendments will keep it that way.  Poor soil cannot be made right with fertilizer alone.  Organic matter (like mature compost) promotes stabilization of the soil particles and acts like a glue to hold them together.  If you build and maintain soil organic matter through the addition of organic amendments, the quality of the soil will be improved by increasing drainage and water holding capacity, inviting necessary organisms (bacteria, fungus, earthworms, spiders, nematodes, etc.) to take up residence, and reducing compaction. Plants will be healthier, more resistant to pests and diseases, more tolerant of dry spells, provide more produce, and use less fertilizer because the organic matter holds on to soil nutrients.   It’s almost like magic.  The same amendments are used to fix every soil -- sandy and clay soil alike.  In sandy soils like those that are typical in Northern Michigan, the organic matter increases water and nutrient holding ability of the soil.  In clay soil, the same organic matter helps break up the tiny particles to increase drainage and air spaces. 


Many of you already have an interest in food, especially if locally grown.  Maybe you even do some growing of your own or you would really like to grow edibles but for any number of reasons you haven’t started yet.  With the uncertain economic situation, let this be the year to start, or to expand, your food growing.  If you remember only one thing from reading this, remember:  Feed the soil and the soil will feed you (and get your soil tested).  In our lifetimes, soil will not renew itself quickly without our help.  Your health comes from soil health.  Bon appetit!


Science Sidebar:  If you were to dig down through the earth from ground level, you would pass through different layers of materials (called horizons).  Simply, the ground level is where plants grow and animals live. The next horizon would be the topsoil where decomposers recycle dead plants and animals into organic matter and humus. The subsoil horizon would be the next layer and you would find it lower in organic matter and where most of the nutrients are found. The next horizon would be weathered parent material (the basic ingredients of soil).  This layer contains rock particles but no organic matter and can be a very deep layer.  Finally, you would reach bedrock – solid rock formed before the soil above it.  It will stay there until something (e.g., earthquake, erosion) brings it to the surface where time and weather will change it into parent material and the cycle of making soil begins again. 


Soil Test Sidebar: 

Soil tests tell you the current fertility status of your soil so it is a “point-in-time” analysis.  Standard soil tests include tests for pH (alkalinity), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg), with liming/sulfur and fertilizer recommendations based on the analysis.  Testing does not include nitrogen (N) levels because they fluctuate too rapidly for testing to be meaningful.  Although tests for soil organic material can be done, accurate measurement of soil organic material is difficult.  Usually, the standard soil test is sufficient but additional tests can be conducted as desired.  The best time to test your soil is in the fall or spring.  This gives you time to make adjustments before you plant your garden and allows the soil amendments to become effective.  A soil test every two to three years is usually adequate, depending on whether you grow plants that are heavy feeders and want to monitor fertility levels more closely.  Soil test kits are available at garden centers if you want an immediate analysis of your soil’s pH.  However, your local agricultural extension office (Michigan State University Extension) will test your soil sample for pH and nutrient levels for a small fee.  The analysis usually takes a few weeks to get back to you and includes detailed results and suggested amendments specific to your site and the types of plants you want to grow.  You can pick up a soil test kit from your local MSU- Extension office and it comes with instructions on how to take the soil sample.  Be sure to talk to the Extension agent if you have any questions.


Organic Matter Sidebar:

One of the best ways to improve soil texture, structure, and fertility is to add organic matter.  To build soil organic matter levels, more has to be added to the soil than is lost through decomposition and erosion.  Organic means it came from a once living organism and is capable of decay.  When it decays to the point that it is no longer recognizable, it is soil organic matter.  It continues decaying until it becomes stable (no further decomposition) and then is called humus. You can increase the organic matter by adding mature compost, organic mulch (chopped leaves, grass clippings, leaf mold, small wood chips, etc.), cover cropping (green manures), and organic fertilizers.  Applying organic matter increases the amount of carbon in the soil and helps the growth of beneficial bacteria which leads to healthy plants.  An important consideration to note is that turning over the soil, or tilling, adds oxygen to the soil which increases microbial activity – meaning, the organic matter is getting used more quickly by the bacteria – so, limit the number of times you disturb the soil. 


Mature compost is well-rotted plant materials or animal manures (typically cow or horse manures but also most other farms animals and bat guano).  Compost is very similar in composition to soil organic matter.  It breaks down slowly on its way to becoming humus while improving the soil condition.  Wise gardeners add organic matter to soil as often as possible, anyway they can.  The more unrecognizable the once-living material, the better for increasing soil organic matter.



“Whoever could make two ears of corn or two blades of grass to grow upon a spot of ground where only one grew before, would deserve better of mankind, and do more essential service to his country than the whole race of politicians put together.” - Jonathan Swift, Gulliver’ s Travels, 1726


“The nation that destroys its soil destroys itself.” - Franklin Delano Roosevelt


“Essentially, all life depends upon the soil … There can be no life without soil and no soil without life; they have evolved together.” – Charles E. Kellogg, USDA Yearbook of Agriculture, 1938



posted in: compost, soil, Grow

Comments (12)

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Posted: 2:47 pm on May 7th
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