A little earlier on, I wrote a post or two about why soil is critically important when you’re growing your own organic food. Basically, aside from the sun, air, and water, everything else that makes a vegetable or fruit tasty and nutritious comes directly from the soil! It’s pretty amazing if you think about it—plants take solar energy (an energy form we’re just beginning to be able to harness directly as humans, and even then, not very efficiently), along with some rainwater, and the minerals and organic matter from the soil, and then these amazing plants turn it into stuff that we can eat, and derive our own energy from. I, for one, am extremely glad for edible plants! In addition, healthy soil helps plants keep themselves healthy, which improves their resistance to pests and diseases, overall yield, and their nutrition content.
So I suppose the natural question to ask now is, “Okay…well, what exactly makes for good soil?” It’s an excellent question, and I’ve learned that there are actually a handful of factors, and they all interact with one another to determine the overall capacity of your soil to grow abundant, healthy food. So I’ll share a bit about what I’ve learned about each of the major factors that contribute to soil quality below.
Compost from the worm bin, complete with eggs and baby worms. Great for the garden!
Three basic nutrients that plants need in order to flourish are nitrogen (N), phosphorus (P) and potassium (K). All three are found in all-purpose fertilizers in some form or another, whether organic or otherwise. The amount of each that you may need to add to your garden soil depends somewhat on the particular plant species. For example, some plants are really heavy feeders when it comes to nitrogen (like corn) while others require much less.
According to my well-worn copy of The Vegetable Gardener’s Bible by Edward C. Smith, other macronutrients that plants need in considerable amounts are carbon, hydrogen, oxygen, calcium and sulfur. Some of these elements are readily available from the air and water, while others will occur in sufficient amounts naturally. Still, some others may need to be added to the soil through various amendments.
In addition to the nutrients above, there are a handful of other minerals that plants need in smaller quantities in order to carry on the business of growing and thriving—namely, iron, boron, manganese, copper, zinc and molybdenum. Like the macronutrients, these elements will be present in soil in sufficient amounts most of the time. But if they aren’t, your plants may show it through different disease-like symptoms like wilting, yellowing, small fruits, or stunted growth. That being said, it’s definitely worth keeping tabs on your garden soil’s nutrient content from year to year. And keep in mind that too much can be just as bad as too little.
3. pH Levels
pH is a scale that goes from 0 to 14 which measures the relative acidity of a substance. A pH of 7 would indicate a neutral substance, while a lower pH would indicate acidity, and a higher pH would indicate that the substance is basic in nature. To get a bit more science-y with it, the pH scale actually measures hydrogen ion concentration. (Low pH means high hydrogen ion concentration, and a high pH indicates lower ion concentration.)
That’s pretty neat and all, but what sort of soil pH do garden plants like? There’s isn’t one magic pH level that will work the best, but there is an optimal pH range for each type of plant. The vast majority of plants will grow well in the 6-7.5 range, including artichoke, beans, beets, brussels sprouts, cabbage family plants, cantaloupe, peas, spinach and squash. Asparagus has a wider range, tolerating soils with a pH of up to 8, while potatoes like fairly acidic soils that range in pH from 6 down to 4.5.
Most garden stores have DIY soil test kits that you can bring home and check your soil with. The one I got this year cost about $10, and I can do two tests of each for pH, potassium, nitrogen and phosphorous. There’s also a reusable electronic soil tester that you can get, which will tell you the soil pH as well as the overall fertility (which I have to assume is nitrogen, potassium and phosphorous combined). The problem with this sort of test of course is that it won’t tell you what particular nutrient is lacking, so you’ll have to go and get a chemical kit to check the individual nutrients anyway. I’ll admit it is handy for checking the pH, since you can use it indefinitely, and pH can vary quite a bit from one garden area to the next.
A basic soil test kit like one I've used in the past, available in garden stores and Amazon.com.
So how do you adjust the pH of your garden soil to best suit the plants that you want to grow in it? First bit of advice is to work on the pH before you put any plants in there. To make soil less acidic (to increase the pH for plants like asparagus), we use dolomitic limestone (garden lime) in Open Lotus Garden. To make soil more acidic (to decrease the pH for plants like potato and blueberries) we often use cottonseed meal, which also increases the nitrogen content in the soil at the same time. When adding amendments, you can expect to use more if you have clay soil, and less if you have sandy soil.
4. Organic Matter
Organic matter is pretty much what it sounds like: anything that was once living that breaks down and becomes a part of the soil. This includes leaves, branches, fallen trees, bark, grass, last year’s annuals, grass trimmings, and even dead animals of all sizes.
Everything that is living eventually returns to the earth, and becomes part of the all-important organic matter that’s found in the earth’s topsoil. Without organic matter, you’d basically have either rock, clay or sand (or a combination of those three)—and that’s it! Organic matter plays an important role in the texture of the soil, helping to maintain billions of tiny pockets of air that the microorganisms, sundry soil bugs, and plant roots all need in order to grow and prosper. Organic matter keeps the other elements of the soil from getting too compacted, which is important for soil drainage, water retention, and healthy root development in plants. Organic matter is a bit like a fiber supplement for constipated soil. (Apologies for the digestive analogy, but it’s completely fitting!) Most healthy soil should have enough “fiber” in it naturally, but depending on the history of your garden soil and your local geography and geology, it might benefit from a bit of help from you, the gardener.
A mixed cover crop in the garden of crimson clover and hairy vetch.
So how can you increase the amount of organic matter in the soil?
- Add compost (whether from a hot pile, cold pile or from vermiculture, all of it is a
great addition to garden beds. It’s pure, unmitigated organic matter goodness!)
- Turn some dry leaves into your veggie beds in the fall or spring. The worms will be happy to do the rest!
- Incorporate any mulch (straw, hay or lawn trimmings) into your beds at the end of the growing season.
- Plant cover crops in the fall, winter or early spring, and then turn them into the soil as “green manure” just before they go to seed. (Allow at least 2 weeks for partial decomposition before planting anything else. Allow more time if the weather is cool, as decomposition slows substantially in cool temperatures.)
- Add other amendments to the soil like peat moss or coir (coconut husk fiber.) The drawback here is that it’s rather expensive, it may require a lot of product to amend your entire garden, and peat moss can actually affect the pH balance of your soil (making it more acidic). I would only use this as a last resort if you’ve got some serious soil texture problems and you don’t have the means to implement any of the other methods mentioned above.
Screened compost and soil...headed for the garden beds.
5. Soil Vitality
Having living stuff in your soil is really important. Everything from earthworms down to microscopic soil bacteria and fungus can help by maintaining soil aeration and allowing plants to readily access nutrients in the soil. While soil vitality is definitely a component of healthy soil, it’s also a very reliable marker of healthy soil: if you have good soil, you’ll have lots of soil life; if you have poor soil, the number and variety of dirt critters will be much lower.
Take a hand trowel and go out and have a look into the soil of your garden beds, down to a depth of about 6-8 inches. What do you find in the area of a square foot? Are there a variety of different kinds of bugs and worms in there? How do the worms look? Are they healthy and happy? Or is the soil too compacted to serve as a good environment for macroscopic soil life? Microscopic soil life is nigh impossible to discern by visual observation, but if the soil texture is good and there are plenty of bugs that you can see, odds are that there are plenty of the other smaller, but necessary living components in the soil like mites (which often feed on molds), springtails, mycorrhizae, nematodes, and the like.
What’s the fix if there aren’t a lot of living creatures in your soil? Just gradually work on increasing your organic matter, and balancing your soil nutrients and pH as necessary. The eventual appearance of more abundant and diverse soil life will indicate you’re on the right track.
For more about alternative soil amendments (i.e. those that aren’t used typically in standardized industrial agriculture), have a look through this info on ATTRA’s National Sustainable Agriculture Information Service website by Preston Sullivan. Good stuff!