ARTICLE - What is Garden Soil

First it should be pointed out that dirt is always called soil in gardening. Soil is arguably the most important component in a successful garden, so not calling it dirt is a show of respect. However, it is still dirt when it gets on your clothes.

Soil is often viewed as the boring part of gardening. While garden soil will never be glamorous or even as interesting as choosing plants, there is a whole world under our Wellingtons that literally and figuratively is the foundation for our gardens. New gardeners are cautioned to put money and effort into improving their soil before they even consider planting, but few appreciate the wisdom in what they are hearing until they watch their new plants struggling for survival and demanding more and more food and water. In organic gardening, you learn to feed the soil and let the soil feed the plants.

The soil found in a typical yard will be about 90% mineral residue and only about 10% decayed organic matter. Yet it sustains a community of insets and microorganisms. The reason for adding additional organic matter to your soil is to provide food for the beneficial microorganisms that release nutrients into the soil as they decompose the organic matter. Earthworms and other soil dwelling insects aerate the soil as they move through it and contribute still more organic matter with their waste and decomposition. This makes for what is called healthy soil.

Pesticides sprayed on the plants will make its way into the soil and can kill the insects and microorganisms living there. Synthetic fertilizers contain salt, which can also kill the soil's residents as well as build up in the soil and cause harm to the plants you are feeding. And synthetic fertilizers add nothing to the soil's fertility.

What is Healthy Soil?

When discussing soil, we generally focus in on five things: texture, structure, pH, organic matter and fertility.

1. Soil Texture

Soil texture refers to the size of the soil particles.

  • Sand has the largest particles and they are irregularly shaped. This is why sand feels course and also why it drains so well. Sand doesn't compact easily.
  • Silt particles are much smaller than sand, but still irregularly shaped.
  • Clay has microscopic sized particles that are almost flat. Clay packs very easily, leaving little to no room for air or water to move about.
  • Sandy loam is considered the ideal garden soil and consists of a mix of the three basic textures. However, don't run out to buy sand to add to your clay soil or vice versa. Mixing sand and clay will give you cement. There's more to the equation than just balancing soil textures.

2. Soil Structure

Soil structure refers to the way soil clumps together. You can usually determine what your texture is by testing your structure. Squeeze a handful of damp soil into a ball in your hand. If you poke the ball lightly with your finger and it breaks apart, it is probably sand. If a bit more pressure breaks it, you're dealing with silt. If it sits there despite your poking, you have mostly clay. Do determine a more accurate reading of the percentage of each texture in your soil, try this easy experiment.

A good soil structure is crumbly. This allows plant roots to work their way through it, air can pass through and water drains, but not so quickly that the plants can't access it.

There are two basic ways to improve soil structure and they work in tandem.

  • Soil dwelling insects move about in the soil help to aerate the soil and they add small amounts of organic matter, the second structure improver. Organic Matter.
  • Organic matter improves any type of soil. Compost, leaf mold, manure and green manures are all decaying organic matter. They loosen and enrich soil and provide food for the soil dwelling insects.

You can loosen soil structure by tilling and sometimes this is necessary. But tilling can over crumble soil and it kills the insects living there. So regular tilling is not the best option.

3. Soil pH

Soil pH is a measure of your soil's acidity (sourness, a measure of below 7.0) or alkalinity (sweetness, a measure higher than 7.0), with 7.0 being neutral. Most garden plants prefer a pH in the neutral range. Some plants are more specific in their requirements. Lilacs and clematis thrive in sweet soils. Rhododendrons and blueberries like a lower pH. You can adjust the pH in different parts of your landscape.

Generally speaking, if your plants are growing healthy and well, your pH is probably fine. If your plants are having nutrient problems or are not growing vigorously, it's worth it to test your pH. If the soil's pH is not within an acceptable range for the plants you are growing, the plants will not be able to access the nutrients in the soil, no matter how much you feed them. You can buy many types of pH testers in a garden center.

You can also bring a sample into your local Cooperative Extension office, to be tested for a nominal fee. Once you know what your pH is, you can begin to adjust it slowly. You add some form of lime to raise pH and a form of sulfur to lower it. What type and how much depends upon your soil and test results. Your Extension report and most testing kits will tell you what to do once you get your results.

Adding lime or sulfur to alter soil pH is not a quick fix. It can take months to register a change in the pH and you will need to periodically retest your soil to insure it doesn't revert to its old pH. It is sometimes easier to simply change your plants to suit your pH.

4. Organic Matter

Organic matter does so many wonderful things for a garden, it's just silly not to take advantage of it. There would be no organic gardening without organic matter. Decaying organic matter is how plants are fed in nature. Unfortunately we've become very tidy landscapers and we tend to remove any dead plant material that falls onto our lawns. It would be so much more beneficial to allow the fallen leaves to blow off into the bushes, where they will not only feed the soil, they also prevent erosion and mulch the soil.

Organic matter added to garden soil improves the soil structure and feeds the microorganisms and insects. The more beneficial microorganisms your soil can support, the less bad organisms will survive. The good guys feed on harmful microbes like nematodes and certain soil born diseases. They also release their nutrients into the soil when they die. So the more beneficial microorganisms that are in the soil, the more nutrients will be in the soil. And many types of organic matter add still more soil nutrients to the mix.

Organic matter also contains acids that can make plant roots more permeable, improving their uptake of water and nutrients, and can dissolve minerals within the soil, leaving them available for plant roots.

Types of Organic Matter

Compost is the poster child of organic matter. Compost is any kind of decayed organic matter. You can make your own or buy it by the bag or truckload. Finished compost looks like rich soil. It's dark and crumbly with an earthy smell. By the time the compost cooking process is complete, weed seeds, fungus spores and other undesirable elements that may have gone into your compost bin, should no longer be viable. Compost can be added to your gardens at anytime, either turned into the soil or used as a mulch or top dressing.

Aged Animal Manure is an organic material with an added bonus of soil nutrients. Animal manure must be aged for 6 months to a year, before it is applied to the garden. Fresh manure will burn your plants, may contain bacteria that can cause illness from contact and it stinks. You can add fresh manure to a compost heap and let it age there.

Green Manures are basically cover crops that are grown with the intention of turning them back into the soil. Obviously this would be more useful in the vegetable garden or in a newly created bed where tilling will not harm existing perennial plants.

Different green manures offer different advantages. Some, like alfalfa, are grown for their deep roots and are used to breakup and loosen compacted soil. The legumes, clover and vetch, have the ability to grab nitrogen from the air and eventually release it into the soil through their roots. If allowed to flower, clover especially is attractive to pollinators and beneficial insects. All green manures will suppress weeds and prevent erosion and nutrient runoff in areas that would otherwise be unplanted. And they all assist with creating good soil structure and food for the microbes, once they are tilled in and begin to decompose.

Popular choices for green manure include: annual ryegrass. barley, buckwheat, clover, winter wheat and winter rye.

5. Soil Fertility

The nutrients in your soil are the final component in building healthy soil. Just like people, plants need certain nutrients to grow and to fend off disease. Organic fertilizers can be made from plant, animal or mineral sources and are basically returning what was taken from the soil. Organic fertilizers are released slowly, which means that plants can feed as they need to. There is no sudden change in the makeup of the soil which might harm the microbial activity.

The three primary nutrients used by plants are nitrogen(N), phosphorus(P) and potassium(K) .

  • Nitrogen is largely responsible for healthy leaf and stem growth. In the soil, nitrogen is made available to plants by nitrogen fixing bacteria which convert nitrogen into nitrates, a form plants can use. Nitrogen does not remain in the soil for long. It gets used up by your plants and by decaying matter in the soil. It is also water soluble and can wash out of the soil rather quickly. Even so, an excess of nitrogen will cause a lot of foliage growth at the expense of flowers and fruit.
  • Phosphorus is very important for root growth. Flowering bulbs and root crops can always use some phosphorous. That's why bone meal is often recommended for fall bulb planting. It also is crucial for producing flowers and you will sometimes see fertilizers with a high phosphorus content advertised as flower boosters.
  • Potassium is needed for overall plant health. It keeps the plants growing and aids their immune systems. Like nitrogen, potassium is also water soluble and needs to be replenished from time to time. Besides the three primary nutrients, there are several trace elements that are necessary for good plant health like: calcium, magnesium, zinc, molybdenum, etc.

Fertilizers are labeled with three numbers that indicate the strength by percentage of each nutrient. For example, 23-6-8 means that the product contains 23% nitrogen, 6% phosphorous, and 8% potassium.

Building healthy soil is an ongoing process. By making healthy soil a focus at the start of making a garden, you will have a head start on creating a sustainable organic garden.