Tuesday we talked about when you can start working your soil. (Click here for more on that topic.) Today we’re going to talk about the different types of soils and the kinds of amendments you can use to improve soil structure. In our next post we’ll finish up the topic by telling you what amendments to use to improve fertility.
First, why should you use amendments and how do you know if your soil needs them? Amendments help soil retain the right amount of moisture and provide a good flow of nutrients and air to the plant’s roots. If your plants don’t thrive, or the soil is always wet, or always seems dry, you should think about adding amendments.
Next you’ll need to identify the type of soil you have. The Sunset Western Garden Book suggests thoroughly wetting a patch of soil, then letting it dry out for a day. Then grab a handful and squeeze it. If it feels gritty and falls apart, it’s sandy soil. If the soil has formed a tight ball and feels a bit slippery, it is mostly clay. If it kind of holds together, yet is crumbly, it’s probably loam.
While sandy soil has good drainage, it won’t hold water or nutrients long enough for plants to take them up. This type of soil needs lots of organic matter.
Clay soil is the most difficult to work with. It bakes hard in the heat and holds too much moisture when it’s wet. The one virtue of clay, or heavy soils, is that they are very mineral rich. These soils need lots of organic matter to open them up. One thing to note is that there are times when it’s just too hard to fix this type of soil. In this situation the best you can do is to build some raised beds, fill them with nice loamy soil and take it from there.
Loamy soil is the best type of soil for growing healthy plants and it’s the soil structure we’re aiming for when we add amendments. It has a good balance of sand, silt and clay — the three main elements of soil. Loamy soil doesn’t need much in the way of amendments for improving structure, but might need amendments for fertility.
Here’s a list of organic amendments for improving soil structure:
- Composted wood shavings
- Commercial amendments
- Kelp and seaweed
- Lawn clippings
- Peat moss (some controversy as to whether or not this is a renewable resource)
- Sawdust (you’ll need to add a high nitrogen fertilizer such as aged horse manure)
- Straw (don’t use hay, it will give you lots of weeds)
You can use any one of these or a combination. Some people recommend amending with sand, but we don’t think it’s such a good idea, especially with heavy, clay soils where you could end up with something resembling cement.
When amending poor soil for the first time, you can use as much as 30% by volume of any of the above. Soil should be neither too wet, nor too dry when working it. Dump the amendments on top of the soil and dig in to a depth of 12″ to 18″.
Some final notes:
- Be sure to ask for organic amendments.
- Don’t use sewage sludge-based or “biosolids” amendments. They claim to use “natural, organic” ingredients and they’ll say “natural organic fertilizer” on the package, but they are NOT what any organic gardener would consider to be truly organic. Worst of all they contain heavy metals which are toxic even in very small amounts. Absolutely NEVER use them in a vegetable garden.
- If you use horse manure make sure it’s well-aged, at least three months. Otherwise it can be too “hot” and burn your plants, plus it might contain live seeds if it’s not aged properly.
If you are still unsure of what kind of soil you have, call your local Cooperative Extension or go to your local nursery — you might even bring a small baggie of your soil with you. The folks there will know about the soil in your area and will be able to recommend the appropriate amendments.
There are lots of other amendments out there; things like rice hulls, bean straw, coir, apple and grape pomace, and they are all good but less readily available. If the local experts recommend something that works in your area and it’s easy to get ahold of, use it.
Good soil texture, or structure, is just half of the equation. You’ll also want to add amendments to improve fertility and we’ll talk about that next week.