Perhaps one of the few things that every gardener around the world agrees with is the benefit of using compost in the garden. Compost is loaded with nutrients and is often the go-to solution to improve poor soils. But is it necessary to grow a bountiful vegetable garden?
Below is everything you need to know to get started using compost in your vegetable garden, including different types of compost (such as mushroom compost and composted manure).
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Do You Need Compost to Grow Vegetables?
If you’ve read any gardening blogs or binge-watched gardening videos, you’ve likely noticed a lot of gardeners use compost in their vegetable gardens. But is it necessary?
While compost is considered the holy grail of building a productive, organic vegetable garden, it’s not necessary if you already have relatively fertile soil or soil that is not too sandy or consisting of heavy clay.
The main benefits of compost are three-fold: it adds nutrients, benefits soil life, and builds structure to your soil, all of which are immensely beneficial for growing vegetables.
Compost Adds Nutrients
The reason why most of us add compost to our garden beds is because compost is a gentle, organic fertilizer packed with nutrients. And compost slowly releases those nutrients over several years, meaning less plant nutrients are wasted compared to chemical fertilizers.
The NPK values of compost depends on what organic matter was used to make the compost, but in general, a well-balanced compost will have an NPK of 1-1-1, most of which will not be available immediately. This is why some people choose to grow in pure compost with good results.
As compost continues to break down, nutrients are absorbed by your vegetables. But eventually there will be leftover organic matter that cannot break down any further, which is called humus. One major component of humus are humic acids, which are not a nutrient but actually help plant roots absorb nutrients more easily, promoting plant growth.
Compost Builds Up Soil Life
Compost is created by a combination of bacteria, fungi, and animals (mainly worms and sometimes insects) eating organic matter. Compost is teeming with life, so when you add this to your soil, you’re inoculating it with beneficial microbes and critters which will invariably improve the quality of your soil.
It’s for this reason that the number one way to improve poor or “dead” soil is to add compost.
Why Is Soil Life Important?
Unless you solely grow vegetables with chemical fertilizers, it’s soil life that releases the nutrients in compost and organic fertilizers. If there is little to no soil life, nothing you add can break down.
Compost Improves the Structure of Soil
The third benefit of compost which most people tend to gloss over is that compost itself improves the structure and physical characteristics of your soil.
Good compost is generally slightly spongy, holds on to water well, but is also fluffy enough that it can break up easily in your hand after squeezing.
For this reason, compost can greatly improve soils that are too sandy or too heavy with clay. In the case of sandy soil, adding compost will help it hold onto nutrients and water more easily. For heavy clay soils, compost lightens up the soil structure.
What Compost Do I Need for Vegetables?
Not all composts are created equal, and this makes it very difficult to choose the right one for your garden when you’re inundated with a whole host of choices at your local garden center or nursery.
For most home gardeners, regular compost is ideal for all vegetables either as a soil amendment or as the main growing medium, although well-composted manure is usually just as good as a soil amendment.
Below are some of the main types of compost you may have heard about. All are good but not equally good for vegetables.
|Type of Compost
|Readily available to buy, easily made at home, reduces waste, gentle fertilizer, safe to use for all vegetables in small or large amounts
|Nutrient profile and quality varies depending on what went into the compost
|Rich in nitrogen and can be easily found; produced for free by livestock; safer to use on plants than fresh manure
|Varies by type of manure; is often more dense than regular compost; is sometimes more expensive than regular compost; some risk of containing harmful herbicides or bacteria
|Vermicompost (Worm Castings)
|Very nutrient dense, improves water retention, reduces kitchen waste
|Expensive to buy; requires several vermicomposting bins to make large amounts at home
|Composting Toilet Waste
|Composting toilets generate composted waste easily and is free
|Contamination risk when used for vegetables; better for fruit trees and ornamental plants
|High in nutrients and greatly increases water retention of soil
|Too concentrated to use in large amounts or as the main growing medium; much better mixed with soil before applying or used as a shallow mulch
1. Regular Compost
Most generic composts, either homemade or store-bought, are gentle fertilizers that add nutrients, beneficial microbes, and build up the structure of your soil.
However, the quality of compost depends on what was put into it. Homemade compost made from a good mix of green and brown organic matter is usually well-balanced for any garden.
Many store-bought composts are of similar quality, but sometimes you’ll notice some cheaper composts are mostly just filled with punky wood chips and sticks. This is not bad if you’re adding bulk to your garden beds, but not good for pots or starting seeds.
2. Composted Manure
You’ve likely have seen bags of “composted manure” or “manure compost” in the nursery and wondered if that’s safe for your vegetable garden or if it’s even any good.
Composted manure is just manure that has been aged and composted over months, and can come from any animals, but in the stores they’re mostly from horses, cows, or chickens.
Composted manure is not the same as fresh/raw manure. Fresh manure is often (but not always) to concentrated or “hot” and can burn the roots of your plants. And some which are okay to add fresh can be a disease risk if used for vegetables grown close to the ground (like lettuce) or root vegetables. Also, fresh horse manure tends to have a lot of viable weed seeds which can quickly cause a headache in the garden.
Pro-Tip: When buying composted manure, make sure it’s explicitly labeled as composted.
Composting manure makes it much safer to use for vegetables. The NPK of composted manure is low, usually below 1-1-1 but is higher in nitrogen than phosphorus and potassium. For example, composted cow manure is around 0.8-0.5-0.5, while aged/composted chicken manure is approximately 1-0.8-0.4. Composted manure is a gentle, slow-release fertilizer, much like regular compost.
The disadvantages of composted manure are the usually higher cost associated with them and the fact that they don’t build up the structure of soil as well as regular compost. Composted manure tends to be more dense and less fluffy than regular compost. Another possible risk is the presence of herbicides which can harm broad-leafed vegetables (e.g. most vegetables except corn, wheat, and other grasses), but if bought from a trusted source this is usually not an issue.
The recommended use of composted manure is as a soil amendment, either top-dressed or mixed into the soil.
3. Vermicompost (Worm Castings)
Earthworms are always a good sign that your soil is healthy and rich in organic matter. In cold composting methods, the resulting compost is often filled with worms. The worms themselves eat decaying matter and their poop (worm castings) are like little nuggets of garden gold.
Vermicomposting is basically a form of composting where worms do all or most of the composting, so the resulting compost is all worm castings.
Worm castings are very nutrient-dense, with NPK ratios getting up to 3-1-1, although this also varies and some worm castings can have an NPK similar to regular compost. Worm castings are rich in beneficial microbes and often have some earthworm eggs mixed in with them, meaning you’re also inoculating your vegetable beds with more beneficial worms.
The disadvantage of vermicompost is mainly the higher cost and lack of scale. If you want to make a lot of vermicompost, you need a lot of worm bins, which take up space, time, and effort. And if you want to buy worm castings, they tend to be very expensive compared to regular compost.
The recommended use of vermicompost is as a soil amendment or organic slow-release fertilizer, sprinkled into planting holes before transplanting or top dressed around established plants so none of those hard-earned nutrients are wasted.
4. Composting Toilet Waste (Human Compost)
You won’t find this in the store, but if you’re wondering if you could fertilize your garden with compost made from human waste, you technically can but should avoid it for vegetables.
Another name for human waste used as fertilizer is night soil which comes from the historical practice of collecting it from cesspools and latrines at night. It’s still used in some countries as a sustainable source of fertilizer in built-up areas, but modernized to reduce the risk of disease.
If you live out in a very rural area or have a cabin, you might have heard about people using composting toilets. This is the same principle. Most people just bury the “human compost” when the composting toilet pit gets full, but you could in theory collect the waste and use it in the garden… but don’t use it for vegetables.
The disadvantage of using composting toilet waste is that there is a much higher risk of spreading harmful bacteria to your vegetables. Even aged, well-composted human poop could harbor E. coli and other harmful coliform bacteria.
The recommended use of composting toilet waste is around non-edible plants like ornamental plants, or fruit trees where the fruits are high above the ground to avoid crop contamination. Not recommended for vegetable gardens.
5. Mushroom Compost
Mushroom compost is a by-product of the mushroom industry and is a newer type of compost that’s becoming all the rage.
Most mushrooms you get at the grocery store are grown on either manure/compost (white, cremini, and portobello) or woody/fibrous plant matter like sawdust, grains, or straw (e.g. shiitake, oyster, king oyster, enoki, etc.). After the mushrooms have been harvested, the spent growing medium with the mushrooms’ “roots” (mycelium) can still be broken down, and that is what is used as mushroom compost.
Mushroom compost has excellent water-retention properties and can add lots of nutrients including trace minerals to your soil.
The main disadvantages of using mushroom compost are the higher cost associated with it and the fact that it is very concentrated, so using it to start seeds or as the main growing medium can overwhelm young plants.
The recommended use of mushroom compost is as a soil amendment, either as a mulch or mixed with soil before applying.
What Can I Use Instead of Compost?
What if you don’t have compost or can’t get enough of it to feed your plants?
Aside from compost, there are several easy ways to add nutrients to your soil: fresh manure, mulch, plant-based fertilizer tea, and trench composting.
1. Using Fresh Manure
Fresh manure can be used directly in your garden, however, it’s not as safe as using composted manure, due to the risk of contaminating your vegetables with harmful bacteria.
The USDA has guidelines on how to safely use manure in the garden [PDF]. In brief, if there is any chance that the harvested vegetable will come into contact with the soil (such as root vegetables, leafy greens, or low-hanging vegetables), you should apply fresh manure at least 120 days before harvest.
Some manures are also too concentrated or “hot” to use directly, such as chicken manure (which is fantastic after it’s aged for several months). Fresh horse manure tends to have a lot of weed seeds which will sprout as soon as you add them to the garden. Rabbit manure is probably the best fresh manure to use since it can be applied directly to plants and doesn’t have any weed seeds. Some people bury a few pellets of rabbit manure into planting holes before transplanting vegetables.
2. Using Mulch
Mulch added around the base of your vegetables can be used in a pinch, and will break down gradually, releasing nutrients down to the soil below.
Wood chips are a common mulch but break down very slowly, however, straw or grass clippings will break down faster. Fresh grass clippings will immediately start releasing small amounts of nitrogen (grass clippings have an NPK of 4-0.5-2) and then break down gradually. Add a thin layer of grass clippings around your vegetables to get the most benefit.
However, for adding nutrients to your vegetable garden, you can save your grass clippings for fertilizer tea.
3. Fertilizer Tea
Fertilizer tea is water that is infused with fresh plant matter (grass clippings, leaves, weeds, etc.) that is allowed to steep for days or sometimes weeks. Not only will the leaves and green stems release some nutrients immediately, but bacterial will begin to break them down to release even more.
Making fertilizer tea is easy: simply fill up a bucket with fresh green plant material, like grass clippings and pulled weeds, then add just enough water keep your garden waste submerged or floating. Leave uncovered or cover with a mesh screen to keep out mosquitoes, and stir daily. After a few days (or weeks), you can strain the liquid gold and toss the solids into your compost bin or bury them in your garden.
Fertilizer tea can be used “full strength” or diluted, either as a foliar spray on your vegetable leaves or poured around the base of your plants.
4. Trench Composting
While this is technically compost, you’re actually using kitchen and garden waste to make compost without a compost pile.
You can bury kitchen waste like stems, peels, coffee grounds, even fish scraps, and it will gradually break down, releasing those nutrients for your plants. This is called trench composting. You will want to bury your scraps at least a few inches into the soil. And you can use garden waste as well, such as grass clippings.
Although you can bury meat and fish, make sure to bury it deep and don’t bury too much, to avoid attracting animals. Also avoid burying big bones which will take a long time to break down.
Which Is Better: Manure or Compost?
Both manure and compost are extremely common and it can be difficult to decide which one to use in your garden.
For a vegetable garden, both fresh manure and compost add nutrients to the soil, and both can improve soil structure. However, compost is safer to use and tends to improve soil water retention and increases the fluffiness of the soil more.
As mentioned earlier, fresh manure should be applied at least 120 days before harvest if there is any risk of the part you’re eating being in contact with the soil.
If manure is your only option, try to find composted manure, which is less risky (I would still follow the 120-day USDA guideline [PDF]) and also will have fewer viable weed seeds.
Can I Mix Manure with Compost?
Manure can be mixed with compost as both provide similar nutrient profiles, and the slightly higher nitrogen content of manure and better structure of compost can work synergistically to produce an excellent soil amendment. As a fertilizer, use roughly a 1:1 ratio of cow/horse manure to compost, but to fill beds or containers, use mostly compost with smaller amounts of manure mixed in.
There are a few things you should consider before you mix your manure with compost. First, some manures, like chicken manure, can burn roots if used when fresh. Secondly, fresh horse manure and to a lesser extent cow manure will have some weed seeds that remain viable and will sprout in your garden. You can avoid both of these problems by using composted manure.
Can You Grow Vegetables in Compost Only?
If using a well-balanced compost, and it is broken down well, you can grow vegetables entirely in compost.
The main downside of using only compost to grow your vegetables is the cost.
When choosing (or making) the right compost, make sure there is not too much undecomposed plant matter like wood chips and twigs which will pull significant amounts of nitrogen out of the soil as they continue break down. But compost that is already very well-decomposed will work fine.
Can You Fill Raised Beds with Compost?
While you can fill raised beds with compost, it can quickly get expensive when filling up several raised beds.
To save costs, you can fill the bottom layers with cheap compost, topsoil, or other garden soil, then use better quality compost to fill the top foot or so where the majority of plant roots will grow. This approach can also be used when filling your raised beds with other expensive growing media like potting mix.
Can You Just Put Compost on Top of Soil?
Using compost as a mulch on top of your soil is not only a good idea, it’s one of the core principles of no-dig or no-till gardening. Compost protects the bare soil underneath from drying out while also releasing nutrients right to the feeder roots of your vegetables.
If you don’t have enough compost to fill up your raised beds or you want to improve the quality of your native soil, the simplest thing to do is place a layer of compost on top, at least 1 or 2 inches thick.
Not only will compost on the surface of your soil feed plant roots below, it will also act as a mulch, protecting the soil from drying out.
Some no-till or no-dig gardeners use this to gradually build up their soil with successive layers of compost over the years, even if the original soil below was very poor, too heavy, or too sandy.
Should I Add Compost to My Garden Every Year?
Compost has a low NPK, but it also breaks down slowly. Is it worth placing in your garden every year or is it overkill?
If you are only adding a layer of compost or spreading it around your garden, it’s a good idea to add a new layer every year to your garden. However, the compost you add will continue to break down and provide a slow, steady stream of nutrients for several years.
If your garden beds are mainly filled with compost to begin with, it’s not necessary to add more compost every year, although a top dressing of compost never hurts.
When Should I Spread Compost in My Garden?
There are two trains of thought about when to spread compost: spreading compost in early spring, several weeks before planting, or in late fall, after your harvests.
The idea behind both approaches is that compost is actually not completely broken down, so it takes time for soil microbes and worms to work their magic and release nutrients into the soil. By spreading compost in your garden weeks (early spring) or months (late fall) before planting, you’re giving those soil organisms a head start.
However, you can top dress throughout the growing season by spreading lightly around the base of your plants.
What’s the Difference Between Topsoil and Compost?
Compost is organic matter that has been at least partially broken down, whereas topsoil just refers to the top layer of soil, which can include some organic matter, sand, silt, and clay, as well as debris.
Topsoil is usually very cheap because it’s often a waste product from construction as developers dig and level out the ground. Topsoil can vary in quality. Low-quality topsoil may be barely better than your native soil, and could include rocks, pieces of wood, and other large pieces of debris. Higher quality topsoil is usually sifted and may be mixed with compost.
Use pure topsoil as a cheap filler in your garden or mix it with compost to make a decent growing medium for garden beds.