All plants evolved in different parts of the world with wholly different environments, and therefore have wholly different requirements. Obviously temperature and light will be much different in the tundra than the tropics, but the soil composition will also be very different. When it comes to planting plants, the soil type is one of many inputs you should consider for optimal results. You have two choices when it comes to your garden soil. You can:
1. Determine your native soil type and choose plants that will thrive in that soil.
2. You can change the soil in your planting beds to match the requirements of your desired plants.
If you’re going to go the second route, you still need to determine the quality of your native soil so you can make the appropriate adjustments. Here's how to go about doing that...
First, look at your soil’s texture.
Go get a shovel and move a few scoops around. Is it: Sandy? Clumpy and mostly clay? Rich and full of organic material? Have a lot of rocks? Really hard packed, lifeless dust? Next, dig a small hole and fill it with water. How does it drain? Does the hole fill with water? Does the water just sit on top and not go anywhere? Or does water sit there for a bit but is then slowly absorbed as you watch it? Now that you have some wet soil, take a handful of it and pick the rocks and roots out. Try to squish it with your fist and roll it into a ball. Is it just grit that falls apart? Could you model little figurines out of it? Somewhere in the middle?
By now, you are probably starting to get a feel for what your soil is like. You might be thinking that this is all fine and good, but you would like more technical definitions. In general, Clay is made up of very tiny particles. It holds moisture well (sometimes too well). It doesn’t have many air spaces. It sticks to your shoes, and is slow to warm up. When it dries it forms that apocalypse movie, cracked-dirt landscape. Sand has large particles, lots of air pockets, drains quickly, and warms up quickly. Very sandy soil looks like, well, the beach. Silt has medium soil particles and is in-between the other two. It has medium particles, medium air pockets, drains water quicker than clay, but not as quickly as sand. It looks like what you see in garden supply store commercials with happy people idyllically digging in their yard. Silt is a gardener's friend (for most traditional garden types).
When you have the right mix of sand, silt, and clay (about 40%, 40% and 20% respectively) you have what is often called Loam. Growing recommendations for plants will often mention loamy soil. Don't forget the additional substances that happen naturally in your soil -- water, air, and organic matter. The chart below shows the best distribution of each.
Next, determine your soil pH.
You will need a little bit of equipment here. Most garden centers sell little kits to test your soil pH. You will either have acidic soil, neutral soil, or alkaline soil.
Most plants including many flowers and veggies prefer slightly acidic soil (pH 6.0- 7.0). Some plants like azaleas, and blueberry bushes for instance, prefer really acidic soil (4.5-5.5). Most common garden plants don’t require alkaline soil (pH 7.5 and higher), but many will tolerate it. Plants like lavender and lilac will do fine in neutral to alkaline soil.
Special Note for food gardens: If you’re planning on planting an in-ground veggie or herb garden, you may want to consider heavy metal testing, especially if you live in an urban area. Sometimes soil is contaminated with lead or cadmium. It goes without saying that you don’t want to eat plants that were grown in contaminated areas. If you find that your soil is contaminated, consult a professional for remediation. In the meantime, grow your veggies in raised beds or containers with soil purchased from a garden center.
Now, decide if you want to make changes to your soil to plant particular plants, or if you want to find plants that fit your soil. Shopping for plants native to your area is a good way to match plants to your existing soil. Look for descriptions like “requires well drained soil” or “tolerates clay”.
If you decide to amend, try the following if you have:
Too much clay: You might think to add sand to make clay soil better, but that actually isn’t a great plan. Clay plus sand makes really hard cement. Instead, mix in compost. This will put some space between the tiny particles. Don’t work clay soil when it is wet, this will make difficult dirt clods. Avoid compaction by not walking on your soil -- standing on a piece of piece of plywood while gardening can help with this. Adding gypsum will help break up clay soil, but it takes time to do so and will raise the pH.
Too much sand: Again, add compost. This will add more medium-sized particles to fill in some of the spaces, and allow the soil to hold onto more moisture. You can also add peat moss. It will act as a binding agent to the large particles in sand.
Slow draining: Mix in compost and small-sized gravel. Be cautious when mixing in sand, and only do so if there isn't much clay. If that still doesn’t fix the problem, you may have to deeply dig your garden. You may have a thick layer of clay somewhere that is acting as a water proof barrier that isn't allowing the water to go anywhere. If this is the case, you can amend A LOT, or you can roll with it and buy some bog adapted plants that like “wet feet”.
Drains too quickly: Mix in compost.
If soil is too Alkaline (aka sweet soil): Add garden lime to lower soil pH.
If soil is too Acidic (aka sour soil): Add garden sulphur to raise pH.
Follow directions on the product you purchase regarding dosage per square foot. Once you’ve worked in the right ingredients, you’re ready to plant! The graphic below explains the process of how to amend your soil.
But what if you have existing plants in your bed? That’s okay, you can still amend your soil with a process called side dressing. Carefully mix in your amendment ingredients to the top two or so inches of soil starting about 2 inches away from the base of your plants. This will require a little bit of careful trowel or hoe work, as you don’t want to damage your existing plant’s roots. This process will allow your amendment ingredients to trickle down into the root area of the plant. Adding compost every year is a great idea, and will replenish your soil with organic matter and nutrients.
This may seem like a lot of work, but one Saturday of digging and a little maintenance side dressing is a wonderful investment for a beautiful garden this year and every year.