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Monday, October 15, 2012

Getting to Know Your Garden’s Soil: Simple Soil Tests

Soil is literally the bedrock on which a garden grows.  It can determine the types of plants that succeed and the best irrigation practices to use. Because soil is so important, it pays to take a little time to get to know your soil’s characteristics.   Fortunately, there are several easy, inexpensive soil tests that can tell you a lot about your soil.

Two key characteristics that effect plant health are soil drainage and soil fertility.  Interestingly, both characteristics are influenced by the types/sizes of the particles that make up a soil – the physical characteristics of the soil.  You may suspect that you have a sandy or clay soil, since both are common in our area.  In general, the following are true:
  • ·         Sandy soils - have larger soil particles that allow them to drain well.  They also dry out more quickly and tend to leach out their nutrients.  If your soil is sandy, you’ll need to water more frequently in summer.
  •  ·         Clay soils - have smaller soil particles that retain water and soil nutrients better.  They also tend to drain more slowly, which can create problems for some water-wise plants.  If your soil is clay, you’ll need to water less frequently to avoid over-watering.
  • ·         Loam soils – have a balance of sand, clay and intermediate size (silt) particles.   They are best for retaining water and nutrients – in fact, they are ideal soils.  Loam soils are more common than you think in our area.

You may already think you know your soil. But until you test, you won’t really know.  In fact, you may be surprised to find that your soil is less sandy – or clay-like – than you think.   Two simple tests can help you better understand the physical characteristics of your soil.   The Simple Sedimentation Test will tell you the percent of different sized particles – the soil texture. That will tell you whether you really have a clay soil – or actually have a loam.  The Percolation Test  will tell you whether your soil drains quickly, slowly – or somewhere in between.  If you are having problems with soil drainage, you may also want to test your Soil Depth.   If you are having trouble getting plants to grow – or if you want to grow plants with special requirements – you may want to test your Soil pH and the Basic Soil Nutrient Levels in your soil.

In our last posting on soils (September, 2012) we discussed urban soils.  Many local gardens contain a combination of native soils and ‘fill’.  The ‘fill dirt’ may be quite different from your native soil. If your garden appears to have several different types of soil, you should conduct at least the Sedimentation and Percolation tests for each soil type. 

Conducting a Simple Soil Sedimentation Test – to determine soil texture

Fill a large clear glass/plastic jar (1 quart or larger) 1/3 full with soil. Fill the jar almost to the top with water.  Cap securely and shake well; then let the layers settle out.  Mark the line of sediment that settles at 2 minutes (sand particles); 2 hours (silt particles) and 24 hours (clay particles).  The picture below shows a sedimentation test from Mother Nature’s Backyard compared to results from two other local gardens.

You may also see some darker brown material on top of the clay layer (or still floating on the water).  This is the organic (or humus) material, made up of old, partially decayed leaves, roots and mulch.   The darker the water color, the more organic material in the soil (in our area). Note that the soil from Mother Nature’s Backyard has less organic material than the Amended Soil sample.   A close-up picture of the sedimentation test for Mother Nature’s Backyard is shown below. 

Once you’ve conducted the test, measure the depth of each layer and convert it to a percent of the total.   In the soil test above, the total soil depth was 5 cm.  The sand depth was 2.5 cm. (2.5/5 = 0.5 or 50%), the silt depth was 2 cm. (40%) and the clay was ½ cm. (10%).   Now you can look at the picture below to see whether your soil is roughly classified as sandy, loam or clay.


You may want to go one step further and classify your soil using the U.S. Dept. of Agriculture (USDA) Soil Texture Triangle, below.  

The Soil Texture Triangle is easy to use.  The percent sand, silt and clay make up the three side of the triangle.  Each side has a set of arrows to tell you which direction to draw your line.  For example, the ‘sand arrows’ (along the bottom of the triangle) point up on a diagonal to the left, while the ‘clay arrows’ are horizontal.

In the picture above we’ve determined the soil texture for Mother Nature’s Backyard using the Soil Texture Triangle.  Start with the percent sand (50%) at the bottom of the triangle.  Then draw a line parallel to the ‘sand arrows’ through 50%.  Now we have our ‘sand line’.   Then draw the ‘silt line’ through 40% silt and parallel to the ‘silt arrows’.  Finally, draw in your ‘clay line’.   The intersection of the three lines (circled in the example above) is your soil type.   You can see that we have ‘loam soil’ in Mother Nature’s Backyard.  This was a surprise to us – we thought we had a clay soil!  

In general, loam soils (including sandy loams & clay loams) are good for growing just about anything.  Very sandy soils may require plants that are specifically noted to be ‘for sandy soils’.  Fortunately, many local native plants like sandy soils. We’ll talk more about gardening in sandy soils in another posting.

Clays and clay loams are often excellent garden soils, as long as they have adequate drainage. You’ll want to conduct a percolation test to check your soil’s drainage, especially if you have either loam or clay soils.   And you may need to use a few tricks to increase your soil’s drainage.

For more about soil texture testing see:


Conducting a Percolation (perc) Testto test soil drainage

Dig a hole 1 ft deep by 1 foot across. The photo above shows a perc test hole.  Fill with water – let it drain until there’s no more water.  Fill with water again (up to the top) and note how long it takes to drain completely. 

Soils that drain within ½ hour are very well-drained.  Native plants needing sandy soils will thrive in very well-drained soils.  Soils that drain in less than 3 hours are well-drained.  Well-drained soils are ideal for most water-wise plants, vegetables, trees and shrubs. You are very lucky if you have a well-drained soil.  Soils that take more than 6 hours have poor drainage.  You will need to monitor your soil’s moisture carefully so you don’t over-water.  If you want to grow plants that need good drainage, you may want to create berms or raised beds (see December 2012 blog posting) to improve drainage.  You’ll also want to check your soil depth to be sure you don’t have a dense layer that prevents drainage.

Soil depth – some poorly drained soils have a dense layer that keeps water from draining.  This may be a true ‘hardpan’ clay layer, a rock layer or just a compacted zone.  You can often detect a dense layer by digging down until you cannot dig further.  A depth of less than 20 inches means you have a shallow soil.  You may want to break up the impervious layer (or drill holes through it) to improve drainage (see December 2012 posting).

Soil pH – influences the availability of soil nutrients to your plants.  Most local garden soils have a pH from about 6.5 to 8.0.  An ideal soil for many plants (including vegetables) is around 7.0. But there are many native plants that like a soil pH between 7.5 and 8.0 (‘basic’ soils).  If you’re having difficulty getting plants to grow – or if you want to plant ‘acid-loving’ plants - you can test your soil pH with a simple test kit (available at most garden centers and plant nurseries).

Soil basic nutrients – Many California native plants have low fertilizer needs; other plants (including vegetables and some non-native plants) have higher nutrient needs.  Simple test kits that indicate levels of the basic nutrients (nitrogen; potassium; phosphorus) are available at most garden centers.  These kits are inexpensive and easy to use - just follow the directions on the kit. Most kits indicate whether the levels of nitrogen, potassium and phosphorus are low, adequate or high.  That’s usually good enough for most gardens.  We’ll talk about soil amendments and fertilizers next month (November, 2012).

If you want to learn more about your soil’s nutrients you can send a soil sample to a soil testing lab.  These labs can measure additional nutrients - and with greater precision than simple test kits.  One of the most reasonably priced labs is at the University of Massachusetts Center for Agriculture (http://soiltest.umass.edu/).   Visit their website to learn more about the tests available, prices, etc.


In summary, the key to successful gardening is knowing and working with the conditions in your yard.   A well-drained, loamy soil supports most water-wise plants, including those that need a ‘well-drained soil’.  If your soil is more sandy or clay, you can use some tricks we’lll discuss in the next few months (November-December, 2012) to increase the range of plants you can grow. Whatever your soil’s characteristics, don’t despair - there are likely plants that will thrive in it!

Learn more about soils and soil testing at:

You can e-mail your Garden Soil questions to: mothernaturesbackyard10@gmail.com 




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