Plants require more than proper amounts of water and sunlight. If you ignore their preferred soil type then they will simply die or hang on between life and death. Why waste garden space on them? Reputable nurseries have informative tags attached to their plants that indicate the best soil type for that particular plant. While I do recommend further research before you purchase any plants, these tags are a good point of introduction. Some tags say, “prefers medium to well drained soil”, “thrives in moisture retentive soil”, “sharp drainage necessary”, “will not tolerate heavy soils”, or “grow in soils with a pH below 6.0”. What does all of this mean?
Thanks to British explorers, our gardens are collections of plants from every corner of the planet. To ensure they reach their greatest potential we must try to replicate the growing conditions where they originated or create similar conditions. Let’s look at (Lavandula stoechas)(pronunciation: lah-VAN-dew-lah STOY-kas) or French lavender. These are Mediteranean plants. They prefer well drained but slightly fertile soil. If the soil in your garden is predominantly clay then you would be throwing money away if you planted lavender in your garden. You could grow it in a container but not your garden. This plant would die even if you met its other requirements of low humidity, mild winters and full sun for many hours per day.
Your soil’s texture is one of the greatest determining factors of success or failure in your garden and you must know your soil if you wish to succeed. Knowing your soil will save you money on plants and soil amendments. How do you determine your soil type or texture? We will look at clay, silt and sand (figure 1). These three determine a lot about your soil.
Sand feels grainy when wet or dry. It warms quickly, encourages sharp drainage, does not hold on to nutrients, is very dry, has good air circulation.
Sand originates from various sources of rocks, shells and minerals and is created through weathering.
Particle size: from 0.05-2.0 millimeters
Silt is slippery when wet but has the consistency of flour when dry. In ancient Egypt, silt was annually deposited on the riverbanks by the flooding of the Nile. Silt is easily carried by wind and water. Silty soil is fertile and commonly found in river deltas as well as wetlands. It promotes water retention while still allowing air circulation.
Silt originates from quartz and feldspar
Particle size: from 0.002-0.05 millimeters
Clay has the smallest particle size of the three. It is created through gradual chemical weathering. Soils with a lot of clay are sometimes referred to as ‘heavy soils’. Clay holds onto water very well and is HEAVY when wet. They are slow to drain and warm up. Clay holds on to nutrients for plants. It is strongly discouraged to walk on or work with clay soil while it is wet, as this will cause soil compaction. Once it is compacted, you should find a different hobby besides gardening. With a lot of work and amendments clay soils can provide a lush garden that will be the envy of your neighborhood. Clay feels sticky, dense and smooth when wet. These soils are easily formed into different shapes.
Particle size: below 0.002 millimeters
How do you determine your soil texture? The first thing you need to do is collect a soil sample. Different areas of your property should be sampled. I recommend drawing a small map of your property, including your house, and label different areas. Then mark spots in each area to take a soil sample.
1. Select a spot in your garden to take your sample
2. Brush aside any leaves or mulch.
3. Use your spade to dig a hole about 4-5 inches deep. Put that soil aside and use it to refill the hole later.
4. Take a trowel and return to the hole. Place the tip of the trowel’s blade about one inch from the edge of the hole and slice down. Carefully remove the soil.
5. Add the soil to a plastic bag. The bag should be labeled so you do not mix up samples. Remove any large gravels. You can break up the soil in the bag and start to mx it.
6. Add the soil to a jar. Add enough water so that it is about ¾ full. Shake the jar for about 10 minutes or until everything is broken down. Use your children for this part and tell them it is a game.
7. After everything is mixed you may place the jar down on a table. The sand, silt and clay will begin to settle. Gravels or stones will be first. Followed by sand, silt and clay. It can take up to 3 days for everything to settle and for the water to clear. Do not shake the jar during this time!
8. You will be able to easily distinguish the different layers (figure 2). Mark and measure the depth of each layer. Also, add all three measurements together to determine the thickness of the total deposit. Divide the depth of each layer by the thickness of the total deposit to find the percentage of each layer.
Thickness of sand deposit ____
Thickness of silt deposit ____
Thickness of clay deposit ____
Thickness of total deposit ____
Calculate percentages of sand, silt, and clay.
[clay thickness] / total thickness] = ___ percent clay
[silt thickness] / total thickness] = ___ percent clay
[sand thickness] / [total thickness] = ___ percent sand
Have all those percentages? Look at figure 3. This chart will help you determine your soil type. A soil with 45% clay, 25% silt and 30% sand has been used as an example. Once you determine your soil type you have a better understanding of which plants will thrive there as well as which organic soil amendments can be used. I added an extra chart that you can print out and use.
The other test is a little messy and faster (figure 4). Again, you should draw a map of your property and mark the spots where you took samples. Soil in a vegetable garden will show differences from soil under a lawn, even if they are not far from each other.
Besides soil texture, you need to know the pH. The pH of your soil is just a measurement of how acidic or alkaline it is. A measurement lower than 7 is acidic and a measurement higher than 7 is alkaline. Why do you need to know this? Some plants require acidic conditions, many plants cannot thrive in a garden that is too alkaline and important nutrients aren’t available to plants if the pH is too high or low. Why waste money on fertilizer if your plants can’t utilize it because the soil’s pH is too high or too low.
Nutrients such as nitrogen, phosphorous and potassium are sometimes unavailable to plants because the soil’s pH is below 6.0. If it is above 7.5 then phosphorus, iron and manganese are less available. A pH range from 6.0 to 7.5 is generally regarded as the best for the majority of plants in your garden. Soil pH is different in various parts of the country. Garden centers sell test kits for under $15. I would not use the probe types because they are less accurate than the other kits available.
The extension office also offers a soil sample service. If you collect the soil and give it to your local extension agent, they will send it to a university lab for testing.