There are very few gardens with naturally perfect soil for growing plants. But, as gardeners we have a wide assortment of garden soil amendments we can add to build soil, improve structure, provide nutrients, and promote healthy plant growth. I rely on amendments like compost, leaf mold, and aged manure digging them into my beds in spring, between successive crops, and in autumn to ensure I enjoy a bumper crop of homegrown vegetables. Read on to learn more about the pros and cons of the organic amendments you can use to improve your soil.
Why add garden soil amendments?
We often hear that soil is made up of particles like sand, silt, and clay, but that’s only part of the story. Soil is a complex ecosystem containing minerals, organic materials, microbes, and countless organisms which vary from region to region, and often from yard to yard. Soil anchors plants, but it also provides water and nutrients. New gardeners quickly learn the importance of building soil, and experienced gardeners prize the dark crumbly compost that comes out of their backyard bins.
Gardeners add soil amendments to their vegetable plots and flower gardens to grow better plants. But what do these materials really do for our soil? Here are a few of the many benefits of applying amendments:
- To increase soil organic matter
- To support the soil food web (read more about that HERE)
- To increase the moisture holding capacity of soil
- To improve the texture and structure of soil
- To improve soil aeration
- To promote healthy plant growth and reduce plant diseases
Choosing a garden soil amendment
With so many types of amendments to choose from, how do you know which ones are right for your garden? Start with a soil test. A soil test is a window into the health of your soil and provides information like pH, organic matter percentage, and general fertility. Once you know your soil’s quality, you can combine that with your plant needs to choose effective amendments. Perhaps your soil needs more nitrogen (add composted animal manures). If you’re looking to improve your soil quickly, like in a vegetable garden, choose an amendment like cow manure which breaks down fast. For a steady feed all season long (in a perennial border or with long-term vegetables like tomatoes), opt for a material like compost which takes several months to decompose.
Another factor in growing healthy plants is soil pH. Soil that is too acidic or too basic prevents plants from taking up nutrients. In my Northeast garden we have acidic soil, and I need to lime my vegetable beds every year. In regions where soils are basic, sulfur can be added to adjust the pH to ideal levels. For an in-depth look at soil pH, check out this article from Jessica.
How often should you test your soil? It’s a good idea to get a soil test every four to five years, even if your garden is growing well. It doesn’t cost much and helps you pinpoint which garden soil amendments should be added to your garden.
6 Types of garden soil amendments:
Head to any garden centre and you’ll likely find stacks of bagged composts, manures, and other amendments. Larger nurseries may even have bulk materials where you buy by the cubic yard. Here are six of the most common amendments available to gardeners.
Compost is a popular garden soil amendment that can be made in your yard (check out this easy DIY for a pallet compost bin) or purchased at a garden center. It’s typically made from decomposed plant materials like vegetable peelings, garden debris, and leaves. As a soil amendment compost is excellent, improving both clay and sandy soils, increasing water holding capacity, and enhancing plant growth.
I encourage gardeners to make their own compost. You can buy a compost bin, make your own, or just pile up organic materials and give them time to break down. It’s not an instant process, however and it may take several years for a pile to decompose into finished compost. Finished compost looks and smells like soil and is a lovely dark brown color. The speed at which compost decomposes depends on many factors including the materials included, temperature, the size of the pile, and whether it’s maintained (by turning and providing moisture). If you’re interested in learning how to make your own compost, check out this excellent how-to guide from Jessica. We also LOVE the book, The Complete Compost Gardening Guide by Barbara Pleasant and Deborah Martin!
Compost can be added to garden soils in spring, between successive crops, and in autumn. It also makes a good mulch around tomatoes, cucumbers, and squash with worms and other soil organisms working it into the earth. Compost takes several months to decompose and provides a steady soil enhancement to perennial beds and borders too.
Livestock manures are available bagged at garden centres and in bulk from farmers. I usually get a truckload of aged manure from a local farmer every two years, buying enough to amend my beds for several seasons. Common manures include cow, sheep, horse, and chicken. I suggest doing a bit of research first as quality and available nutrients vary greatly between the different types.
- Cow manure – Cow manure is the most common manure – bagged or bulk – for gardens. It provides plenty of organic matter and a balanced supply of nutrients.
- Sheep manure – This is a popular bagged manure because sheep manure is rich in nutrients like nitrogen as well as organic matter.
- Horse manure – This manure is often considered a weedy manure as horses don’t digest seeds as thoroughly as cows. That said, less digested manure also makes for a richer soil amendment so there are advantages and disadvantages of using horse manure.
- Chicken manure – Chicken manure is weed-free, but very high in nitrogen and should be well-rotted before it’s dug into a garden. It can also be added to a compost bin to speed up decomposition and enrich the final product.
- Rabbit manure – Often called ‘bunny berries’ because it looks like small roundish pellets, this is a great manure for the garden. It’s weed-free and low in nitrogen so it won’t burn plants. It helps build soil by adding organic matter and nutrients like phosphorous.
If buying bulk manure, ask the farmer about their herbicide and pesticide practices. I try to buy from an organic farm. Avoid fresh or partially composted manure. If you’re buying a truckload in the fall, you could buy half rotted manure and pile it up until spring. Using fresh manure on growing crops can burn plants as well as introduce dangerous pathogens to your food. One advantage of bagged manure is that it’s usually sterilized and contains no weed seeds. Buying in bulk has resulted in certain weed species being introduced into my garden beds and I always keep an eye on newly manured beds, pulling weeds as they appear.
Vermicompost, or worm castings, are also available for improving soil but they tend to be expensive. It’s not practical for me to use worm castings in my large garden. That said, I often use vermicompost in containers planted with vegetables and herbs as well as indoors for my houseplants.
Chopped leaves or leaf mold
Chopped leaves can be dug into garden beds in autumn or allowed to rot down into leaf mold. Leaf mold is one of my favourite amendments as it greatly improves soil structure and texture, boosts water holding capacity, and adds plenty of humus.
It’s also so easy to make your own leaf mold compost. You just need two ingredients: leaves and time. It’s best to start with shredded leaves, as they break down quicker. To shred, use a chipper/shredder or mow over the leaves a few times to chop them into smaller pieces. Place the leaves in a compost bin, a ring-shaped enclosure made with wire fencing, or gather them in a free-formed pile. I like to make a five to six foot diameter ring with wire fencing as it prevents the leaves from blowing away. Plus, it’s an inexpensive DIY compost bin. You can also buy a wire compost bin for an instant set-up. Fill the enclosure with the shredded leaves and wait. You can water the pile if the weather is dry or turn it with a garden fork to incorporate some oxygen and speed up the process. It takes one to three years for a leaf pile to turn into gorgeous leaf mold. Use finished leaf mold to enrich garden soils or mulch around plants.
Peat moss has been sold for many years as a ‘soil conditioner’. It’s light and fluffy and made from ground up dried sphagnum moss. It’s also a key ingredient in potting mixes. If you’ve ever tried to rewet dry peat moss you’ve likely noticed that it’s very difficult to do. Dry peat moss repels water and is therefore not a great amendment for mulching or top-dressing. It also contains very little, if any, nutrients or microorganisms and can acidify the soil.
Peat moss is also a controversial amendment as it’s harvested from peat bogs, a biodiverse habitat for animals, plants, birds, and insects. And while peat companies work to restore bogs after harvesting, it can take many decades or longer to truly renew a peat bog. I don’t add peat moss to my garden beds.
A couple of years ago one of my neighbors bought a truck full of ‘black earth’ bags from a building supply store. They were just $0.99 each and he thought he scored an amazing deal. After spending hours filling his new raised vegetable beds and using the black earth for shrub and perennial borders, his plants failed to thrive. I guess if a deal seems too good to be true, it really is. This inexpensive black earth was just black peat and with its dark brown color looked like a rich garden soil amendment but it isn’t. It’s the material from the bottom of a peat bog and is acidic, doesn’t contain or hold nutrients, and doesn’t offer many benefits to a garden. Buyer beware!
There is another produced also labelled as black earth called chernozem. This is truly a wonderful amendment and is rich in humus and nutrients. It’s less common than black peat but, if you can find it, I recommend using it in your vegetable and flower gardens.
Kelp is one of my favorite garden soil amendments, particularly as I live very near the ocean. Washed up seaweed can be gathered from above the high tide line, brought home, and added to a compost bin or chopped and dug into the soil in autumn. Seaweed is extremely rich in micronutrients and plant hormones that promote vigorous growth. Gardeners who live far from the sea can buy bags of kelp meal to give their gardens the same boost. Kelp meal can be added to vegetable or flower beds in spring. I like to include a handful in each planting hole when I transplant tomato seedlings.
Should you buy bagged or bulk garden soil amendments?
The decision of buying bagged or bulk comes down to a few considerations: 1) How much do you need? 2) Can you find it in bulk? 3) Is there an extra delivery fee if you need to get bulk amendments? Sometimes it’s cheaper to buy in bulk, sometimes it’s not. And if you’re buying bulk compost, ask what it’s made from? If you can, check it out before you buy, giving it a squeeze and looking at its texture.
If buying pre-bagged amendments read the labels carefully to see what exactly is in the bags. I’ve bought bagged composts that turned out to be little more than bark mulch and did nothing for my soil. Bagged amendments are convenient and often screened for rocks, sticks and other garden debris. They may also be sterilized to kill weed seeds.
If you can, start making your own soil amendments by gathering leaves, garden debris, and other organic materials to make compost and leaf mold. My homemade compost is, by far, my best soil amendment and I wish I had space for a dozen compost bins so I could make enough for all my raised beds.
When should you apply garden soil amendments
There’s no need to wait until spring to improve your soil. I often add soil amendments to my garden in late summer and autumn, a time when it’s easy to source organic materials like leaves. And adding in autumn gives the soil food web time to break these materials down so your plants can take advantage in spring.
There are three times I apply soil amendments to my raised bed vegetable garden:
- In spring before I plant. I use amendments like compost, aged manure, and kelp meal to feed the soil.
- Between successive crops. To maintain high soil fertility, I add a light application of compost or aged manure.
- In the fall. Once I’ve cleaned up the vegetable beds that aren’t full of crops for fall or winter harvesting, I dig in amendments like chopped leaves or seaweed. These slowly break down improving the soil structure, fertility, and feeding the soil food web. By mid-spring the beds are ready to plant.
I also add amendments to my container gardens in late spring. A mixture that is roughly two-thirds high quality potting mix and one-third compost keeps my potted veggies and herbs thriving all summer long.
How much should you add?
Garden soil amendments are mixed into the soil while mulches are applied to the soil surface. Application rates of garden soil amendments depend on the general health and structure of your soil as well as the selected amendment. Healthy garden soil typically contains 4 to 5% organic matter. In spring I apply a two to three inch layer of composted manure or compost to my raised vegetable beds. In between successive crops I add another inch of these materials. If I was applying kelp meal, I would follow the recommended application rate on the package.
For further reading be sure to check out these excellent articles:
- 12 creative ways to use leaves to feed your garden soil
- Fertilizer numbers: what they mean and how to use them to grow better plants
- How to build a worm bin
- Easy ideas for a DIY compost bin
What’s your go-to garden soil amendment to add to your vegetable and flower gardens?