Beyond Sand, Silt and Clay – Organic Matter, Loam, Soil pH and other magic 

Last month I wrote about how to determine your soil texture. This month I’d like to discuss some of the other components of soil and what they mean for growing native plants (or other plants, for that matter).  

Organic Matter 

Organic matter (OM) in the soil refers to that part of the soil that is derived from decomposing or decomposed plant or animal material. This may be in the form of animal feces or simply the byproduct of microbial action on once-living materials.  

Organic matter is important in the garden soil because it is a key source of nutrients. Most soil organic matter originates from plant tissue. Most plant residues contain 60-90% moisture but the dry matter that remains is made up primarily of carbon, oxygen, hydrogen and small amounts of sulphur, nitrogen, phosphorus, potassium, calcium and magnesium. Although these nutrients are found in small quantities, they are very important for plant growth and health. 

In fact, because it is primarily from plant tissues, organic matter contains pretty much all the nutrients that new plants will need, effectively recycling those nutrients. In addition, organic matter can hold onto moisture after a rainfall (or spring thaw), keeping that moisture available for new plant growth. Decaying plant and animal material can also help the soil warm up faster in the spring.  

Certain plants (often shade loving plants that evolved on the forest floor) require high levels of organic matter in the soil. They have evolved without the capacity to easily extract nutrients and moisture from soils that have little or no OM.  

Adding mulch to your garden, whether it be from tree leaves you rake onto the flowerbeds in the fall or from wood chips or chopped straw you spread through the garden, is a great way to maintain nutrient levels and keep the soil moist. A good layer of mulch can also help roots to stay cool in the heat of the summer and prevent the plants from drying out too fast.  

(Although I have no technical support for my assumptions, it is my gut feeling that the type of organic material you add should be appropriate to the type of plants you are growing. For instance, wood chips and decaying leaves are perfect for a shade garden. Those plants evolved with the nutrients that come from fallen leaves and branches and the organisms that break them down. Plants that evolved in prairies – usually most of our sun-loving garden plants – on the other hand, evolved with grasses and using chopped straw would, in my mind, be a much better option as a mulch. I use this strategy in my own gardens and so far it seems to be working well.) 

Loam 

In last month’s article we saw that loam is the term given to a soil texture that is a well-balanced mix of sand, silt and clay. If you recall, Sand is made up of ground minerals and rocks that range in size from 2 mm down to 0.05 mm in diameter, Silt particles are between 0.05 and 0.002 mm in diameter and Clay particles are extremely small – less than 0.002 mm in diameter.  

Having loam soil, a nice mixture of these particle sizes, is often thought of as the “ideal” garden soil because the sand particles provide air space for the roots and allow excess water to drain away, the clay helps to hold on to water better so that plant roots won’t dry out, and the silt fills in the rest of the space. Most plants, except for some that evolved and adapted to extreme soil textures, will do well in loam soils. It’s kind of the “one size fits all” of soil textures.  

That doesn’t mean that if your soil is sandy-loam, or clay-loam or one of the other soil textures that are on the periphery of the loam classification (see the soil triangle, below), that these are bad. Many plants will do quite well in these soil types, too. The only time you may find an issue is if your soil is pure sand, pure clay or pure silt – then you will be somewhat restricted in what you can grow.  

Amending your clay with sand, or sand with clay, can sometimes help, especially if your garden area is small and your soil isn’t too extreme, but this can be an expensive proposition. Sometimes it’s better to accept what you have for soil and, rather than strive for an ideal loamy soil, simply grow the plants that are adapted to your soil type.  

Soil pH 

Soil pH is a measure of the acidity or alkalinity of your soil and is measured on a scale from 0 to 14, with 7 being neutral. Lower numbers indicate more acidic soil, while higher numbers indicate more alkaline soil. Most plants do well when the soil pH is between 6.2 and 6.8. 

NOTE: pH, which is always written with a lower-case p and an upper-case H, stands for “potential of Hydrogen” and is a unit of measurement that indicates how much Hydrogen (H) is available and how active the H ions are. pH is measured on a logarithmic scale, but that level of chemistry is way beyond the scope of this article. 

The more acidic your soil, the more difficult it is for plants to absorb nutrients such as phosphorus, nitrogen and potash. If your soil becomes too acidic, some elements – such as aluminum and manganese – become readily available and can reach levels that are toxic to most plants (usually at pH levels below 5.0). 

Whether your soil is acidic or alkaline is often a factor of the rock from which your soil was formed, and changing soil pH is possible, but usually not practical on a whole-garden scale. 

If you have soil that is acidic, you can raise the pH by adding ground agricultural limestone (or wood ash, or ground eggshells, etc.) to the soil. The calcium and magnesium in these elements will help to neutralize the soil’s acidity. But this is not a once-and-done option – you will need to monitor your soil’s pH and repeat the process as necessary.   

To lower the pH for plants that like acidic soils (blueberries, for instance), you can add elemental sulphur, aluminum sulphate or even vinegar or sulfuric acid. But do your research first – the choice of which material you use will depend on how fast or extreme of a change in pH you need. But because of the nature of soil and the effects of rainfall and other factors, this is also a process that requires constant repetition. If you have acid-loving plants, you may be better off planting them in large planters with special acidic soil instead. You can buy such soil, often labeled as soil for azaleas and rhododendrons, in most garden centers.  

Summary 

The amount of organic matter in your soil, along with your soil texture and pH, can have a great influence on which plants you grow and how well they will do. Next month, in the final part of this series I’ve called “The Dirt on Dirt”, I’ll discuss the role of water and nutrients in your garden, and we’ll look at whether fertilizing your native plants is necessary, or even a good thing. 

Until then, happy native plant gardening. 

Let me preface this piece with the caveat that I am not trained as a soil scientist (though I did take a couple of university soils courses many years ago). Nevertheless, as a gardener I am always working with soil, and I’ve been curious enough to do some more research over the years.  

In any good book on growing plants, you are likely to come across terms such as clay-loam, soil alkalinity, humus, topsoil, etc. Understanding what some of these terms actually mean, and how they relate to your gardening, will help you to be more successful at growing your native (or other) plants. 

First off, some definitions. 

Texture: refers to the size of the soil particles (this is not the clumps of soil you may see in your garden, but the elemental particles that make them up). Soil scientists have determined that there are 3 basic soil particle sizes – Sand, Silt and Clay – and these combine to form 12 soil types that you would typically see in gardens (more on these later).  

Sand is made up of ground minerals and rocks that are rounded or irregular in shape. These range in size from 2 mm down to 0.05 mm in diameter and feel gritty when rubbed between your fingers. 

Silt particles are between 0.05 and 0.002 mm in diameter and feel a bit like flour when dry. 

Clay particles are extremely small – less than 0.002 mm in diameter. In fact, they are so small that you can really only see the individual particles through an electron microscope. We often refer to clay soils as “heavy”. (One suggestion for this term is that plowing clay soils takes 2 to 4 times the tractor power to pull the plow through it as in “light” sandy soils.) 

The Implications of Texture 

If your soil is mostly sand, you cannot form a ball of it when it is wet – it will just crumble and fall apart. This is because the large particle size of sand results in large air gaps between the particles and thus water doesn’t have a strong enough bond to hold the particles together. Although these gaps allow water to enter easily, it also lets the water flow right through. As a result, sandy soils dry out quickly.  

Some plants have adapted to sandy soils – Big Bluestem (Andropogon gerardii) and Prairie Smoke (Geum triflorum) are just two examples – and thus will typically not do well in wetter areas with heavier soils. By the same token, plants that have evolved in wetter areas, especially in clay-rich soils, are unlikely to survive in sandy soils. This is, in part, because without moisture retaining mechanisms, their roots will simply dry out. 

At the other end of the spectrum, clay particles are so small that the air spaces between them are extremely tiny. Often, water will just puddle on top of clay soils and take ages to drain away. (This is why they use clay to line ponds and landfills.) However, water will eventually soak into the pores and because of the physics of water molecules and soil particles, the clay will actually “hold onto” the water for a much longer time.  

Anybody who has walked through wet clay can attest to the fact that wet (even damp) clay sticks tightly to boots, shovels, etc. One impact of this characteristic is that clay soils readily compact and compacted soils do not allow roots to penetrate easily. 

To test if you have clay soil, wet a small lump of it and press it between your finger and thumb and try to form a ribbon of soil. In pure clay, the small clay particles will stick tightly together and you should be able to press out a ribbon a few centimeters long. This Ribbon Test is used by soil scientists to get a quick idea of the soil texture. 

Just as some plants have evolved in sandy soils, so too have some evolved in areas of heavy clay. Cardinal flower (Lobelia cardinalis) and Swamp Milkweed (Asclepias incarnata) are two that come to mind. 

Everything in Between 

Soil scientists have developed a Soil Texture Triangle which, once you learn how to read it, will allow you to identify which of the 12 soil types you have based on the clay, silt and sand composition of your soil. 

Let’s say you get your soil tested and it comes back that you have 45% sand, 20% clay and 35% silt. Where these 3 lines cross on the triangle indicates your named soil type – in this case, loam. (The angle of the numbers indicates which line to follow.) 

If you want to determine your own soil texture, one method you can use is what is called a jar test. 

• Start by getting a shovel full of soil from your garden (you can take samples from different areas, if you want, to get a general feel for your entire yard though it is unlikely that there will be significant differences in an area as small as a town lot). Make sure you take your sample at least six inches (15 cm) down – not just off the surface – as you want to get a complete picture of your soil.  

• Sift your soil to remove debris, rocks and large organic matter and fill a clear jar about 1/3 full of this “cleaned” soil.  

• Add water almost to the top of the jar and add a small amount of water softener or liquid dish soap. Put the lid on the jar and shake vigorously for a few minutes until the soil/water mixture is a uniform slurry. Let the mixture stand for 48-72 hours for the soil to settle out (it will separate out into layers with sand at the bottom and clay at the top).  

• After the mixture has set for a couple of minutes, you can mark the side of the jar with a black marker. This represents the sand component. 

• After the mixture has set for about two hours, mark the side of the jar. This represents the silt layer. 

• Finally, after 2-3 days, the clay will finally have settled out and you can mark that level. 

• Using a ruler, measure the depth that each layer makes up. To calculate the percentage of each element, divide the height of that element by the total height of the soil in the jar.

With this information, you can now determine which plants will like the soil you have in your garden. After that, you can determine light and moisture levels to narrow down your selection. 

Values from my yard’s soil sample: Total soil = 42 mm

Sand = 21/42 = 50%, Silt = 15/42 = 36%, Clay = 6/42 = 14%

Reading these values of the Triangle, we find that my soil is classified as loam (and very close to being a sandy loam).

Next Month – Beyond Sand, Silt and Clay – Organic Matter, Loam, Soil pH and other magic