Age-Old Wisdom meets Modern Tools
Identifying Soil Type and Improving Your Soil
Soil preparation is EVERYTHING when it comes to growing plants. You can improve your soil at any time of year, but the end of the growing season is an especially good time. Here are tips on how to identify your soil type and how to enrich your soil before you plant in the spring.
Why Does Having Good Soil Matter?
There’s a lot more to soil than just dirt and rocks! Soil is full of minerals, microbes, and other microscopic things that plants need to survive. Plants ;root into soil simply as a means to stay upright, but also soil is a plant’s primary source of nutrients and water. Having high-quality soil is critical for healthy plant growth. Compare it to the food you eat to live a healthy life!
All vegetables need soil that contains nutrients, but some soil needs a helping hand to get to an adequate point. Here are key things to keep in mind when building healthy soil:
Get a Soil Test
Before adding anything to your garden soil, test it to see what’s already there.
There are a few ways to get a soil test.
- First, you could buy an inexpensive soil test kit at your local garden store.
- Or, you could contact your local cooperative extension service office for a soil test (usually provided for free or a small fee).
For more reading, see this gardening blog about a resource that provides soil types around the country.
Why a Soil pH Matters
- Knowing your soil’s pH helps you to decide what to grow in it. For example, particularly acidic soil is great for acid lovers like blueberries, while soil with a higher, or alkaline pH is preferred by brassicas such as cabbage.
- Knowing whether your soil is acidic, neutral or alkaline will allow you to tailor your amendments or fertilizers based on what your soil actually needs, and prevent you from overloading it with any particular nutrient.
- Soil pH also affects the availability of nutrients and minerals in the soil, as well as how well a plant can absorb and regulate these materials. ;A very high or very low soil pH may result in nutrient deficiency or toxicity, leading to poor plant growth—or worse!
- While test results should reveal the soil’s pH, phosphorus, calcium, potassium, soluble salts, soil texture, it will not reveal insects, diseases, or chemical residues.
Ideal Soil pH
- The standard pH scale ranges from 0 to 14, with 7 being “neutral,” 0 being “extremely acidic,” and 14 being “extremely alkaline” (or “basic”). Generally, soil pH doesn’t reach the upper and lower limits of the pH scale; most garden soils will fall somewhere between 5 and 9 on the scale.
- For most plants, the ideal pH range is between 6.0 and 6.5 (slightly acidic). Microbial activity is greatest and plant roots access nutrients best when the soil pH is within this range (see chart, below). However, different plants are able to tolerate different pH ranges. Find a list of common garden plants and their pH preferences here.
Availability of Nutrients at Varying pH Values
This chart shows the availability of nutrients at different pH levels. Slightly acidic soil (6.0–6.5 pH) is best for most plants. Image by CoolKoom/Wikimedia.
Adjusting Soil pH
After testing your soil, you may find that the soil pH isn’t within the ideal range of 6.0–6.5. For example, perhaps your soil is too acidic for cabbage.You need to raise soil pH so it’s more alkaline. To get it into the ideal range, you’ll need to add “soil amendments” that raise or lower the pH. (Follow instructions on product packaging to know how much to use.)
- To raise soil pH, add lime (pulverized limestone) or wood ash.
- To lower soil pH, add sulfur, peat, or organic materials (such as compost).
Know Your N-P-K
Plants’ primary nutrients are nitrogen (N), phosphorus (P), and potassium (K). These are available in chemical/synthetic (non-organic) fertilizers or in the organic additives suggested here. On the package of a fertilizer, you’ll see these three values separated by dashes (N-P-K); the numbers of each nutrient indicate the percentage of net weight contained.
- Nitrogen (N) promotes strong leaf and stem growth and a dark green color, such as desired in broccoli, cabbage, greens and lettuce, and herbs. Add aged manure to the soil and apply alfalfa meal or seaweed, fish, or blood meal to increase available nitrogen.
- Phosphorus (P) promotes root and early plant growth, including setting blossoms and developing fruit, and seed formation; it’s important for cucumbers, peppers, squash, tomatoes—any edible that develops after a flower has been pollinated. Add (fast-acting) bonemeal or (slow-release) rock phosphate to increase phosphorus.
- Potassium (K) promotes plant root vigor, disease and stress resistance, and enhances flavor; it’s vital for carrots, radishes, turnips, and onions and garlic. Add greensand, wood ashes, gypsum, or kelp to increase potassium.
Learn more about NPK Ratio: What Do Those Numbers Mean?
Avoid applying excess chemical/synthetic fertilizer. It can damage roots and/or reduce the availability of other elements. Because organic fertilizers release their benefits more slowly, they are less likely to burn plants, although some, such as fresh manure, can do so. Once the nutrients are in the soil in an available form, plants cannot distinguish between synthetic and organic fertilizers. Organic fertilizers, however, tend to also improve soil structure and encourage earthworms and microorganisms that improve overall soil health.
When is a good time to fertilize your vegetables? See our Growing Vegetables Guide.
Soil Structure and Drainage
The structure and consistency of your soil plays a big factor in the success of your garden, too. Soil that hold too much water can promote fungal infections such as root rot, while soil that holds too little water can lead to malnourished and dehydrated plants.
Most soils tend towards one of four categories: sand, silt, clay, or loam (which has a balance of sand, silt and clay). Each soil type has its own characteristics.
- Sandy soils consists of large particles and drains quickly. Sand does not hold onto nutrients very well but warms up quickly in spring. Root crops, onions and asparagus will all grow well in sandy soil. Or, to amend sandy soil to grow other vegetables, add humus or aged manure, peat moss, or sawdust with some extra nitrogen.
- Silt soils have smaller particles than sandy soils, giving them a slightly slippery, floury feel. This type of soil holds onto moisture and nutrients for longer. If you have silt soil, add coarse sand (not fine beach sand), pea gravel and compost, or well-rotted horse manure mixed with fresh straw.
- Clay (or heavy) soils consist of very fine particles. Clay soil holds its shape when rolled into a ball. It is slow both to absorb moisture and to drain, which means soils like this can bake hard in summer then become waterlogged in winter. Well-cultivated clay soils are preferred by brassicas such as cabbage, as well as beans, peas and leafy crops like salads. To amend clay soil, add coarse sand (not fine beach sand), compost, and peat moss to add texture and drainage to the soil.
- Loam is the ideal soil type for growing fruits and vegetables. It’s fertile, drains well, is easy to work and contains plenty of organic matter that supports just about any crop.
Common Soil Amendments
Adding organic matter will generally move pH towards a level ideal for most fruits and vegetables. So, all soil types can be improved by adding organic matter to it. Organic matter can take many forms, for example leafmold made from decomposed leaves; farmyard manure that can be guaranteed to be free of all traces of herbicides; or good old-fashioned garden-made compost.
These soil amendments are commonly used to adjust the consistency and content of garden soil:
- Bark, ground: made from various tree barks. Improves soil structure.
- Compost: excellent soil conditioner that adds nutrients. May also lower soil pH.
- Leaf mold: decomposed leaves that add nutrients and structure to soil.
- Lime: raises the pH of acidic soil and helps to loosen clay soil.
- Manure: best if composted. Good conditioner.
- Peat moss: conditioner that helps soil retain water and can lower soil pH.
- Sand: improves drainage in clay soil.
- Topsoil: usually used with another amendment. Replaces existing soil.
How to Add Organic Matter
To add organic matter to your soil, pour enough on your ground in order to spread to a depth of a least two inches. Leave it on the surface over the winter. That’s it!
By the spring, worms will have done a great job of incorporating most of that organic matter into the soil.
Any remaining on the surface can always be forked in a few weeks before it’s time to sow or plant.
Now that you know the importance of high-quality soil, you’re ready to grow your best garden yet!
Got an over-spent or neglected field you want to turn into a garden? Read our article about reclaiming your garden soil.