Are you dealing with soil that is high in clay or sand? Most gardeners need to amend soil to improve it, and here are some of my favorite organic soil amendments that can help.
Posts may contain affiliate links, which allow me to earn a commission at no extra cost to you. This helps keep costs down so that I can continue providing high quality content to you for free. I appreciate your purchase through the links! (full disclosure)
When a gardener talks about the soil in his garden, he is likely to discuss his challenge in dealing with it, “My soil is rock hard clay, and I can’t get anything to grow.” Or, “Nothing grows in my sandy soil.” We tend to think that our plight as a gardener is totally unique and difficult compared to all of those *lucky* gardeners out there who naturally have perfect soil. Truth is, the majority of gardeners have challenging soils that require improvement for cultivation. We are all special by not being unique!
Soil Texture vs. Structure
Soil TEXTURE refers to the proportion of sand vs. clay. Does it feel course or fine? Sandy soil has a course texture, while clay, comparatively, has a fine texture. Fine textured soil tends to be more nutrient rich and is able to hold more water because there is more surface area to grab ahold of the nutrients and water and hang on to them.
(The sandy soil gardener is now saying, “Ha! I knew I had it worse!”) Not so fast! 🙂
Soil STRUCTURE refers to the arrangement of soil particles. While clay soil has a finer texture, the particles stick together forming solid sheets and clumps. While relatively nutrient rich, clay soil can become waterlogged and low in oxygen, which is a tough environment for plant roots. The particles of sandy soil tend not to stick together, and stay loose, causing too much aeration, and the loss of water.
Ideal soil for gardening will seek to bring these two spectrums into balance. We want to seek a loamy soil, which balances clay, sand, and organic matter. As we improve soil with organic matter, we will also improve the “tilth”, or the workability/structure of the soil. Loamy soil that is rich in organic matter will have a more granular structure, like cookie crumbs, and this is what we’re after.
Improving Soil Using Soil Amendments
When we use soil amendments properly, they can increase beneficial soil organisms, organic matter, and improve moisture retention. The following list contains a variety of amendments from animal, mineral, and plant based sources. Some items are free or inexpensive and some are biodegradable. Some are easy to find locally or make yourself, while others are products that will need to be purchased.
In general, soil amendments are best added in the fall or spring before the garden is planted.
Soil amendments can be divided into three categories: Animal-, mineral-, or plant-based amendments.
Animal-Based Soil Amendments
Some animal-based soil amendments can increase beneficial soil organisms in addition to improving soil structure. Untreated animal products are most safely applied nine months before harvest, or at a minimum of two weeks before planting.
Mineral-Based Soil Amendments
Mineral-based soil amendments are most often used to correct mineral deficiencies, but I’ve included one mineral based amendment that can also benefit soil structure. Because mineral-based amendments do not break down easily in the soil, it is essential to get a soil test before applying them so as not to over-apply them.
Plant-Based Soil Amendments
Plant-based soil amendments can also be used to improve soil structure. It is important to source herbicide-free plant-based amendments in order to avoid contaminating the soil. The result of herbicide contamination is low germination rates and curled/yellowing leaves.
Here are a few of my favorite amendments from each category:
1. Bat Guano:
Bat guano—or bat poop—is a fast-acting, organic fertilizer high in nitrogen and phosphorus, which promotes strong and healthy plant growth. It can also improve the texture of soil, improve drainage in heavy soils, and help to neutralize soil contaminants. By increasing beneficial bacteria in the soil, it helps to protect plants against disease.
Bat guano is highly concentrated, so although it can be expensive to purchase, a little will go a long way. As with other animal manures, it is best mixed into the soil in the fall, or at least two weeks before planting, to allow time for the nutrients to break down into a form that plants can absorb.
Livestock manure is used mainly as a slow-release fertilizer, because it contains most of the elements required for plant growth, including nitrogen and many other nutrients. It can also condition the soil, increasing beneficial soil organisms and moisture retention. The manure can come from nearly any livestock animal, NOT dogs or cats.
I often find horse manure at local farms where it is given away for free if I’m willing to pick it up myself. When finding livestock manure locally, look for farms that pasture-raise their animals and feed them organic feed, since manure from other types of farms can include herbicide residues that can stunt plant growth.
Fresh manure should be spread at least 3-4 months before a crop will be harvested in order to avoid potential pathogens, and is therefore often spread in the fall, or one month before planting. This will also prevent it from burning plants. The aged manure will contain less nitrogen than fresh manure, but it will make an exceptional soil conditioner.
Manure should be turned into the soil within 12 hours of the time of spreading, which ensures that more nitrogen is captured in the soil rather than leaching away. Spread fresh manure on ground that isn’t frozen or oversaturated by a recent rain. If a heavy rain is in the near forecast, wait it out.
These are common agricultural prohibitions included in many state laws that are helping to reduce runoff from farm fields to keep our waterways clean. Plus, you don’t want all of your hard work and valuable nutrients to wash away!
3. Worm Compost:
Worm castings—or worm poop—are the richest fertilizer known to humans, made up of 50% humus (the organic component of loamy soil). Worm castings are high in minerals as well as nitrate, a more bioavailable source of nitrogen than that found in other fertilizer sources. Worm castings help plants regulate water usage, improve soil structure, and increase plant vigor. They can even be used in place of potting soil and to filter out contaminants.
Worm castings are so safe that there is no upper limit to how much you can apply at one time. I can’t say enough good things about this soil amendment.
To learn how to create your own worm castings, see my article about worm composting.
Greensand is a slow-release soil conditioner. It is largely composed of glauconite, a mineral harvested from ancient forest floors. Greensand is considered high in potassium and trace minerals such as iron and magnesium, but its main benefit is that it loosens clay soil and improves moisture retention.
Greensand should not be confused with regular sand, which when mixed with clay soil, can produce a cement-like mixture. Apply in early spring before planting.
Comfrey is a perennial herb with large green leaves and purple, pink, or white flowers that grows in hardiness zones 3-9. Comfrey’s deep roots condition and mine the subsoil for nutrients and accumulate those nutrients in its leaves. Its nutrient levels rival those of animal-based amendments.
Comfrey can be used in many ways to fertilize soil. It can activate a compost pile due to its high nitrogen content. The chopped leaves can be used as mulch around fruit trees and mature fruiting vegetable plants. Comfrey leaves can also be used as a green manure—spread on garden beds in the fall and turned under in the spring before planting. Comfrey powder can be used to fertilize a garden bed in the spring before planting.
6. Compost (Homemade):
Homemade compost made from food scraps and yard waste is an inexpensive, slow-release fertilizer and soil conditioner for the garden. It’s also a great way to keep household waste out of the waste stream. Be sure to use only compost that is completely decomposed in garden soil, because partially decomposed compost contains bacteria that will compete with vegetable plants for nutrients and substantially reduce germination rates.
As a soil conditioner, homemade compost will improve the structure of soil by aerating existing soil, improving drainage as well as moisture retention. Add 3-4 inches of compost to garden soil each spring before planting and work it in with a digging fork. For perennials, spread compost annually around trees and shrubs without working it into the soil.
To learn more about composting, see my article Building the Right Compost Bin.
7. Cover Crops:
Cover crops are plants that are seeded in empty garden beds in the fall, about four weeks before the frost date. The main reasons for using cover crops are increasing soil fertility, improving soil texture, and increasing beneficial soil organisms and fungi, all of which help to reduce pests and disease.
Cover crops will overwinter, and by springtime they will have grown full and lush, outcompeting early spring weeds. Just when they are flowering or setting seedheads, they are cut back just above the soil line. After a couple of days, the “green manure” is incorporated into the soil with a digging fork, breaking up roots. Many micro-farmers use livestock, such as chickens, to help turn cover crop residue into the soil. Wait three weeks before planting in the bed.
There are many kinds of cover crops, and which mixture you use will depend on your local climate and your goals. If your garden is a no-till garden, avoid grass-type cover crops since they will be a challenge to hand-turn into the soil. Your local extension office can help you choose appropriately.
When cover cropping, alternate keeping a few garden beds for overwinter vegetable production, while planting the rest in cover crops.
8. Leaf Mold:
Leaf mulch that has aged for two to three years is called leaf mold, and it can benefit the garden in many ways. Its consistency lies somewhere between shredded leaves and leaves that have composted completely into humus. It has been reported to hold up to five times its own weight in water, making it an effective, water-retaining mulch or soil conditioner.
When the hot gardening season strikes, lay leaf mold over the garden as mulch, keeping it away from the stems of plants. It will have a cooling effect on the soil, and as the mulch breaks down over the course of the year, it will attract beneficial soil organisms while transforming into humus.
To make leaf mold, shred the leaves first by running over them with a lawnmower, or by using a leaf mulcher. To make “quick” leaf mold, make a rectangular pile of shredded leaves that is 5 feet square by 5 feet high. Turn the pile monthly, and you might be able to make leaf mold in as little as 12 months, though the process usually takes a couple of years.
As a soil conditioner, add finished leaf mold to garden soil in the fall, then mix it in with a digging fork in the spring before planting.
9. Wood Chips:
With the growing popularity of the film Back to Eden, wood chips are being added to gardens at an accelerated rate. However, it is important to know how to use this soil amendment correctly. Wood chips are beneficial to the garden because as they break down, they create humus, the organic component of soil.
Adding wood chips is like mimicking the forest floor, where leaves and twigs naturally decompose on top of the soil. Wood chips improve the nutrient levels of the existing soil as they break down, and they increase the numbers of beneficial soil organisms. They hold in moisture, reducing irrigation needs. Covering the ground, they reduce weeds. Wood chips create a stable growing environment by insulating against the hot summer sun and freezing winters.
The way wood chips were meant to be used is as mulch, not tilled into the existing soil. Tilling them in causes them to bind to nitrogen in the soil and temporarily prevent it from being available to your plants. To use wood chips in the vegetable or perennial garden, age them for two or three years before mixing them into garden soil as an amendment, and add a teensy amount of blood meal with them to make up for lower nitrogen availability. Or lay fresh wood chips on top of the soil as mulch without mixing them in.
I prefer to use wood chips in the pathways of my vegetable garden, where I can reap some of their benefits in the beds themselves. Beneficial soil organisms and fungi will enjoy plowing through your beds as they create connections between the pathways.
Tree trimmers will often deliver wood chips for free if they are working in your area. I have access to an arborist who delivers a trailer-load for a $20 fee.
In the end, focusing more on increasing healthy soil life as well as soil structure and texture will help our overall growing conditions. Focus first on those amendments that you can make for free. They will often have the biggest impact. This will also improve your soil so that later, if you want to add a store-bought amendment such as bat guano or greensand, you can purchase less of it. This will maximize the efficiency of your efforts and reduce your micro-farming costs, too.
For more information about improving soil, see my other articles:
- The Nature and Properties of Soils by Nyle Brady
- Start with the Soil by Grace Gershuny
- Teaming with Microbes by Jeff Lowenfels
- Soil Biology Primer by Elaine Ingham
Whether your challenge is sandy or clayey soil, these amendments will help improve your gardening success.
What type of soil do you deal with and what amendments have helped?