The Components of Good Soil
There are four basic ingredients in soil. Good soil has them in these percentages: 45% rock/mineral, 25% air, 25% water, and 5% organic material.
Healthy soil is half solid material (organic and rock) and half porous (air and water)
A lot of people are surprised to learn organic matter is a very small percentage of good soil, while half is air and water. That’s because the organic matter isn’t food for plants, it’s food for soil microorganisms like bacteria, fungi, microscopic worms, and other tiny critters. They use oxygen and nitrogen from the air along with minerals in the soil to break the organic materials into simple proteins and nutrients. Those dissolve in the water trapped in the soil and plants absorb them via their roots.
So, when you’re making good, rich garden soil, what you’re really doing is creating an environment in which the microorganisms will thrive, which in turn, ensures your plants will thrive.
Rock and minerals
The bulk of soil is decomposed rock, which generally contains several different minerals. This part of the soil is not only important because of its chemical contribution to the mix, but because it also provides space to trap air and water, and anchors the plant in place.
There are three basic types of decomposed rock in garden soil:
- Sand – fairly coarse rock fragments that don’t stick together and therefore allow water and nutrients to easily pass through.
- Silt – fine rock particles, about 10-times smaller than sand, that do a better job of holding water, but often at the expense of the ability to hold air.
- Clay – very, very fine rock particles so small they can stick together and form a barrier that can prevent water and air from passing through it.
Ideally, you want a balanced combination of all three. This is called “loam.”
If you’re lucky, you’ve got loamy soil so you don’t have anything to fix. But it’s more likely your soil falls either at the sandy or clay-like end of the spectrum, which means it either has problems holding water (sand) or getting rid of it (clay). You can fix this by adding clay or sand (both of which are available from the garden center), but dry organic matter like leaves or plant clippings will solve the water problem as well. PLus it’s cheaper and easier.
This is pretty much any dead or cast-off matter from living organisms. This includes spent plants, leaves, coffee grounds, old fruit and vegetables, paper, skin, scales, feathers, manure, and so on.
Organic matter is high is carbon, which is the essential building block for all life here on Earth. It’s also a source of proteins and nutrients that soil-dwelling insects, worms, fungi, and microorganisms consume and convert into other nutrients and compounds as waste. Plants ingest those nutrients and use them to grow.
Organic materials are lower density than rock and minerals, so they not only provide a nutrient source, but also help improve soil structure by adding space to trap air and water in the soil. They can be added as is and nature will process it in place (as done in the “lasagna garden” technique), which will release nutrients slowly.
It can also be pre-processed via composting before going into the soil, which will release nutrients more quickly.
There’s not much to explain about air and water other than they are required for the growth and decay cycle. Air provides oxygen which must be present in the soil because aerobic (e.g., air-breathing) microorganisms require it to live. It also provides nitrogen which is used by certain bacteria as an ingredient in the proteins they produce that become foodstuffs for other soil-born organisms (these bacteria are known as “nitrogen-fixing”).
Plant also require air because their roots “breathe.” Healthy development requires pulling gasses from where they are trapped in the soil and releasing them into the atmosphere as part of the respiration process. During photosynthesis plants absorb carbon dioxide, split off the carbon to make sugar, and “exhale” the extra oxygen back into the air.
The role of water in the soil is both a transport mechanism, and a source of hydrogen and oxygen, both of which are required to make sugar (aka: “food”).
Soil nutrients dissolve in the trapped water which makes them available for absorption by the plant’s roots. The plant sucks up this “nutrient smoothy” and uses it to build roots, stems, leaves, flowers and fruit. Photosynthesis splits the water into hydrogen, which is used to make sugar for energy (see the CO2 part above), and oxygen, part of which is used to make the sugar, the rest of which is “exhaled” back into the air.
It’s important to note that water and air must be balanced in the soil. Too much air and the plant will starve. Too much water, and the plant will drown.
How to tell if you’ve got good soil (and fix it if you don’t)
You don’t need to spend money on soil testing to find out if you’re garden soil is in good condition or not. With hands, eyes and a nose, you can do all the basic tests you need.
The Squeeze Test
A day or two after a good watering, grab a handful of soil, squeeze it and release. If it falls apart, the soil is too sandy. If it sticks together like a mudball, it’s got too much clay. If you squeeze it and it mostly sticks together, but crumbles easily, you’ve got a good balance.
The Worm Test
Again, a day or two after watering, use a trowel to dig up a couple of scoops of soil from about six inches under the surface. Count the number of worms. If you’ve got eight or more worms, your soil is good. If it’s less than that, it’s probably low on organic material and you need to add more.
This soil definitely passes the worm test
The Sniff Test
Take a handful of soil and give it a good sniff. Good soil should smell earthy and fresh, without any off dead/decay or ammonia/urine-like smells. That’s the scent of a healthy group of microorganisms happily converting the soil into nutrients.
If you detect dead or decaying smells, your soil is probably too dense or wet so the organic matter is rotting rather than being consumed by healthy soil organisms (this is why bogs smell the way they do). Try adding sand and/or more organic matter like dry leaves or straw to correct that.
If it has an ammonia/urine smell it’s because it’s high in nitrogen and is reacting with an alkaline material in the soil like lime or calcium and creating ammonia. Ammonia reduces the amount of nitrogen in the soil available to plants. Adding dry organic materials to the soil will fix that.
If your soil seems alright but your plants are still looking off – yellow leaves, gangly stems, or generally weak and scrawny – you can try one other test as well.
By default, good garden soil should have the right balance of all the nutrients and minerals your plants need. Sometimes, however, things can get out of whack and you can end up with soil that’s too alkaline or too acidic. So, even though the nutrients are available in the soil, the soil pH is interfering with the water’s ability to dissolve the nutrients and make them available for uptake by the plants.
To find out if this is the case, you can do a quick and easy pH test using some vinegar, baking soda and water.
Most plants are fine with soil that’s slightly acidic or alkaline (though most prefer slightly acidic). If you find that yours is too far one way or another, it’s easy to correct.
For soil that’s too acidic, you can bring it back in balance quickly with some wood ash (use sparingly), or gradually with lime, gypsum or crushed oyster shell (calcium carbonate).
For soil that’s too alkaline, application of organic material that’s acidic like dry pine needles or oak leaves will work. For something that’s fast acting, try sulfur, which is available from garden centers.
Good soil is naturally fertile and low maintenance
With healthy soil you don’t need to add fertilizers or other types of chemical amendments. The soil organisms will produce the right balance of nitrogen, potassium, and phosphorus, as well as trace minerals like magnesium and iron, all on their own. Better yet, because healthy soil is home to many types of predatory and parasitic insects, it also helps keep your pest problems to a minimum with no effort from you (isn’t nature great?).
Once you’ve achieved the right balance of minerals, organic materials, air and water in your soil it’s easy to keep it healthy too. Just add fresh organic matter at the beginning and end of the growing season (or at the end of each harvest if it’s a vegetable garden). That will give your soil critters the food they need to restore the nutrients depleted during the season. And the humus they make as a by-product will help maintain your soil’s air and water retention capabilities.
Keep to these simple steps, and your garden soil will be amazing year in and year out, and your plants will be healthy, lush and, for the most part, pest free. It’s really that simple!.