Adding compost to your garden soil is a step that you really shouldn't be skipping. A well-made compost is the key to soil health because it adds essential nutrients, improves soil structure, and promotes healthy plant growth. The perfect soil recipe for your garden bed is going to use organic matter to work with your native soil porosity to create the ideal water retention and balance of soil nutrients.
Some folks do backyard composting from kitchen scraps, grass clippings, and shredded leaves. Others prefer to leave it to the professionals and buy bulk compost from a landscaping company or garden center. Understanding your ideal soil health is a longterm project that depends on your choice of plants, your soil composition, and the types of beneficial microbes found in your climate, but there are some general ratios that you can follow for gardening success year after year.
Ratio for Vegetable Gardens
The goal is for your outdoor plants to produce vegetables and not just leaves. This requires that different nutrients are available over the course of the gardening season.
- General Ratio: 1 part compost to 2 parts soil.
- Transform heavy clay soil by aerating as you add compost and then mixing the clay with sand to help with drainage and promote beneficial soil life.
- Raised Beds: 1 part compost to 1 part soil. These beds typically require better drainage and richer soil.
Ratio for Flower Beds
We want flower beds with beautiful plants and vibrant blooms to attract and feed all of those pollinators. The best way to do this is with strong root systems to grab nutrients from a good mix of nutrient-rich compost.
- General Ratio: 1 part compost to 3 parts soil.
- Sandy soil will need more organic material as it doesn't hold onto the nutrients very well.
- Annual Flowers: 1 part compost to 2 parts soil for more nutrient-demanding plants.
Ratio for Trees and Shrubbery
Successfully growing trees and shrubs requires a long-game approach to develop and maintain healthy soil. It's a good idea to take soil samples annually so that you can adjust soil nutrients before your plants show signs of problems.
- General Ratio: 1 part compost to 4 parts soil.
- Some species prefer alkaline soil while others are acid-loving plants. Make sure your shrubs and trees have the same preferences.
- Planting New Trees/Shrubs: Mix compost with soil from the planting hole in a 1:3 ratio to encourage healthy root growth and to give young plants a strong start.
How Much Compost Should You Add to the Soil Every Year?
A good rule of thumb to maintain soil fertility and a consistent soil structure, is to add a 1-2 inch layer of compost to your garden beds each year. This can be done by spreading the compost on the surface and lightly working it into the top few inches of soil. You can do this in the fall with soil amendments and fresher organic matter. If you are building up your soil bed with more dirt, you will want to mix organic matter (worm castings, mature manure, aged leaves, yard waste) 6-8 inches deep so that it can break down over the winter and have time to attract beneficial microorganisms for the garden season. For your spring gardening, you can buy bags of garden soil or organic potting soil mixes that already have a balanced soil-compost ratio to give your seedlings a strong start.
What Happens When You Add Too Much Compost to a Vegetable Garden?
Adding too much compost can lead to nutrient imbalances, excessive salt buildup, and poor plant growth. Overly rich soil may cause vegetables to produce more foliage than fruit, and limit the movement of water which can cause plant roots to rot. Learning how your climate uses different soil ingredients is part of the gardening journey. It's essential to follow recommended ratios to ensure a balanced and healthy garden ecosystem.
