Compost is the organic matter used to fortify soil and make it more nourishing for plants. It also helps the soil retain water, ensuring that plants’ roots always have plenty of water to draw on.
So, what ratio of compost to soil should you use? The amount of compost you use varies from plant to plant; while a full-grown tree may need less compost, a fledgling tree may need you to mix much more compost into the soil. And depending on the type of soil you’re planting in, the compost to soil ratio may change.
Using compost is not quite as simple as going to the store, buying a bag of compost and using it generously across your garden. Too much compost can give a plant an overdose of nutrients, which will make them grow too fast and prevent them from developing in ways they need to.
Read on to explore what you need to consider when deciding what compost to soil ratio to use.
How to Measure Compost?
Before you start to learn about ratios, you first need to understand how to measure compost and soil.
While compost is sold by the kilogram, most gardeners will measure compost by inches – for instance, they’ll look at how many inches of compost to pat down into the soil.
You can also measure compost by using a container or a garden trowel and simply measuring it in comparison with the soil, where one scoop of soil and one scoop of compost are equal.
Types of Soil
There are several types of soil, each composed of different amounts of sand, clay, silt, rocks, water, air, and organic matter.
Compost works differently in different types of soil – for instance, compost can help sandy soil retain water.
So, when deciding on the ratio of compost to soil to use, you need to consider the type of soil you’re dealing with.
Sandy soil is comprised of large solid particles that do not retain water well. As a result, plants in sandy soil are not able to access much water which they need to survive, which is why beaches typically have very few plants growing on them.
To help sandy soil retain water, mix two parts of compost with one part of the soil. The compost will also add nutrients to the soil making it more nourishing for growth.
Unlike sandy soil, clay soil has no problem holding onto water. However, clay soil can get too wet, drowning out all other nutrients, and tends to run cold.
If you’re gardening in clay soil, fortify it by adding one part of compost for every part of soil – a ratio of 1:1.
Compost will help attract worms to clay soil, which will burrow through it and break up its density. It can also help balance clay soil’s pH levels, making it more workable.
Chalky soil is another relatively hard soil to garden in. It has a high alkaline pH and has minimal nutrients.
Also, it’s free-draining, which means it retains little to no water.
Chalky soil needs a lot of help from compost to nourish plants. Mix three parts of compost for one part of chalky soil; the compost will inject the soil with much-needed nutrients and save plants from growing stunted.
Silty soil is considered one of the best gardening soils to work with; it holds on to just enough water and also retains nutrients very well. However, at times, it may hold on to too much water and prevent plants’ roots from absorbing nutrients.
To amend silty soil, mix one part of compost to every two parts of soil. Or, add an inch of compost over silty soil beds.
Loam soil is the holy grail of soils; it is the ideal combination of sandy, clay and silty soil, and has the advantages of each without their drawbacks.
Loam soil needs very little help to boost plants, but you can mix one part of compost for every five parts of soil.
Types of Plants
The other way to decide on what ratio of compost to use is to consider which plant types you’re trying to grow or strengthen. Each type of plant requires a different amount of water and nutrients, and you need to consider how compost will impact their needs.
When adding compost to vegetable beds, use one part of compost for every five parts of soil. Place one inch of compost over your vegetable bed and then till your soil until your shovel has reached five inches.
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Compost will improve microbe activity in the soil, and help the vegetable roots pick up water.
However, you should choose the type of compost you use based on what vegetables you’re growing; root vegetables like beetroots, carrots, and turnips need phosphorus-rich compost while leafy vegetables thrive on manure-based compost.
For annual flower beds, use one part of compost for every four parts of soil. Work one inch of compost into your flower bed and then till the soil until you reach four inches deep.
Perennial flowers, which bloom only in spring and are dormant through the rest of the year, need more compost than annual flowers; use one part of compost for every two parts of soil in perennial flower beds.
Once the compost settles in, it will nourish and condition the flowers’ roots and help produce colorful flowers!
Adding compost to your lawn will feed soil microbes that will create more nutrients in the soil. This will then help the grass stay green and prevent weeds from growing in your lawn.
When planting a new lawn, use one to two parts of compost for every six parts of soil; till one to two inches of compost into the soil, you want to plant your grass seeds on.
If you want to revitalize the soil in an already established lawn, top dress your lawn with ¼ to ⅛ inches of compost and then gently rake it into the grass for the soil to absorb.
Potted plant-soil tends to run dry, so it’s helpful to add a liberal amount of compost to your potted plant soil to help it keep moisture.
A ratio of 1:1 or 1:2 would work best; either mix equal parts of compost and soil together or mix one part of compost for two parts of soil.
Shrubs and saplings
If you’re planning new saplings or shrubs, mix one part of compost and nine parts of soil.
Or, gently rake the compost into the soil surrounding the sapling – be careful not to get it into the hole where you will be placing the sapling.
If compost does get into the area marked out for the sapling, its roots will not grow past that area, stunting its growth.
Established trees don’t need as much compost as shrubs and saplings and don’t need compost, which has been mixed with soil.
Sprinkle the top of the soil around the tree with ½ inch of compost until the tree’s drip line – the soil which is covered by the trees’ canopy.
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While these can act as a general guide for how much ratio of compost to soil to use, using compost correctly can get much more complicated.
What if, for instance, you want to add compost to a vegetable garden that has clay soil as well as to a vegetable garden that has silty soil? Will the ratios change?
Use some basic calculations and your intuition to help you answer that question and experiment with different soil to compost rations.
It’s also helpful to talk to experts at gardening stores, who will be more familiar with the type of terrain and soil in your area.
Over time, as your expertise in gardening grows, you’ll become a lot more knowledgeable about the compost to soil ratio you need in each scenario; just like a chef is intuitive about what ratios of ingredients he or she needs!
ReadyToDIY is the owner of this article. This post was published on 2019-11-22.
Being mindful and deliberate about how much compost you need will help your plants thrive, and you’ll be rewarded with a beautiful garden.