MULCH//Pros & Cons of the Deep Compost Mulch System

MULCH//Pros & Cons of the Deep Compost Mulch System

Covering your soil with a thick layer of compost is perhaps not as well-known of a no-till system as, say, straw mulch or cover cropping, but it could be argued that deep compost mulching holds the most broad potential for market gardeners for its simplicity and ease of implementation. That said, it ain’t a silver bullet, it has some downsides.

Briefly, though there are certainly some nuances to this system (many of which we tried to cover in this step-by-step from last week), the deep compost system is essentially the application of several inches of compost overtop of the soil—enough to suppress most weeds—forming raised beds. Seeds or plants are then placed directly into the compost beds. In a nutshell (a proverbial nutshell, though literal nutshells could make a good path mulch), that’s the system.

With any system, there are pros and cons. It is worth noting that a grower does not have to adhere strictly to any one system. If you only want to use the deep mulch system for direct seeded crops, for instance, and a straw mulch for everything else, or no mulch at all, that’s fine, too. Some amount of hybridization may be necessary on many farms. As always, we do not have all the perspective and experience, so please add your two cents here or at the forum—I like the forum, but I’ll do me and you do you.

Con: Finding Good Compost Can be a Hassle & Expensive (AF)

Because compost is going to become the main medium for growing your food in this system (though roots will still extend deep into the soil beneath), it is a good idea to find decent compost. Finding enough compost at all, however, can be a challenge, especially in more rural areas, and decent compost usually isn’t cheap unless you make it yourself (check out Ahavah Farm and Four Winds Farm podcast episodes for some interesting compost making ideas for small-scale growers). However, we consider the initial expense of creating the bed using 3” of compost an investment. One that, if the bed is managed properly, will pay dividends for years to come.

The best places to look are with landscaper suppliers. Landscaping is common everywhere, and they do often use or know where to get compost. Unfortunately, this may not be the best compost, but doesn’t have to be a deal breaker. If you’re close to a university, especially one with an ag department, check there as many larger institutions with dining halls are composting food waste to reduce disposal costs. Look for the best stuff from the most knowledgeable person you can find. You can source your fertility from other sources and use the compost strictly as a mulch, use it as bulk material to add your own compost, etc. If nothing is available, you may need to look into making your own or finding a mulch alternative.

Pro: The Compost Doesn’t have to Be Perfect

When you’re injecting the compost directly into the root zone via a tiller the quality is perhaps a bit more important than when it is acting as a mulch. In that case you cannot have super chunky compost as it may temporarily lock up nitrogen. You definitely do not want to inject anaerobic microbes into the soil if the compost was not made with care. The deep mulch system is seemingly more forgiving for mediocre compost as it encourages an aerobic environment and promotes soil structure and microbial life. Over time, the compost will work down—by way of root growth, broadforking, and marcobiotic soil activity—into the soil profile and increase organic matter rapidly. Get your compost tested and supplement what it is lacking in fertility.

Con: Some Compost can be Contaminated

If you are only able to find municipal compost or generic compost, make sure to ask as many questions as you can about what is in it and if it has been tested. “Rotten hay” or “grass clippings” can often come with persistent broad-leaf herbicides which an harm plants, especially nightshades (and their broad leaves).

Pro: Weed Suppression

The weed suppression can be dramatically increased through the use of deep mulch. We have seen as much as an 80% decrease (roughly) in weeds in places with mulch more two inches. If you’re being conservative with your compost, focus on getting at least two inches down (closer to four is recommended) when establishing a bed for the first time and be ready to mulch or wheel hoe your paths.

Con: Weeds Can Still Come Through

If bindweed is an issue for you, or johnsongrass, deep mulch may not (i.e. probably not) immediately remedy it. Grasses, if not removed or terminated before composting, can also make an appearance. Definately get a handle on your perennial weeds before implementing a deep mulch system. Tarping for a full season beforehand is a common practice in this context and we highly recommend it. If you absolutely have to plant now, relisten to Frith’s episode on TNTMGP for a workaround.

Pro: Addition of Organic Matter

Few substances add more carbon to the soil than compost. Not only will various micro and macro biologies happily bury the carbon into your soil even if the compost is laid on top, but also the organic matter on top of the soil will help to retain the carbon below the surface placed there by plant photosynthesis. If you want to (or need to based on a soil test) inject a bunch of carbon into the soil quickly, working in some compost than covering it up in the deep mulch fashion can be highly effective. If you’re doing an initial tillage, we recommend mineralizing your soil based on a soil test while you’re at it.

Con: Excessive Organic Matter, Salts and Phosphorous

There is such a thing as too much organic matter, and compost can potentially get you there. Same with salts. Same with phosphorus, which can hurt nitrogen uptake in plants and stunt growth. These are very complicated and nuanced issues, however—issues we intend to tackle in individual articles on the subjects. Some composts will be higher in salts than others. Some will have more phosphorous. Some will change drastically from batch to batch depending on the maker, materials, management, etc. For now, we will just say that if you start to see the effects of these issues in your crops, it is a smart idea to at least temporarily lay off compost applications. Again, more on all of this later.

Pro: Beneficials

Beneficial fungi love compost soil, beneficial bacteria love compost... compost can be a great way to inoculate your garden with the microbial life you want, and the deep-mulch compost method can provide an excellent habitat for those microbes to survive and thrive.

Con: Bad Compost Can Bring Bad Biology

Again, microbes love compost. But if it’s poorly made and not well-enough aerated, then that statement includes the bad ones—pathogens can love compost, too.

Pro: Water Retention

No matter where you live—even here in KY where we saw A FOOT OF RAIN IN FEBRUARY SO FAR, water retention is and will be increasingly important as some areas see longer periods between significant rain events and, when they do, it all comes at once. The deep mulch system helps water stay in the soil longer.

Con: Water Retention

Getting into your soil early is one advantage of this system—however, the compost may retain the moisture at levels in the spring that could potentially hurt some crops. That being said, increasing organic matter over time turns the soil from a bucket (in our clay soil, a drain if you’re in sandy soil) to a sponge. Combined with reduced/no tillage to support soil aggregation, it will be able to hold more water without the pronounced negative effects of waterlogged roots.

Pro: Helps with Erosion

We do have spots in our garden that have erosion, but nothing like when it was tilled. We are getting unprecedented rainfall, so it is nice to see that the mulch—in combination with over-wintered plants—are helping retain the vast majority of the soil, and where it is eroded tends to be primarily the compost itself, not the soil.

Con: Compost is Heavy

Moving compost is a big job. Establishing beds can take a lot of it, not to mention time and money. If you are working on anymore than an acre, a tractor and some sort of spreader will be necessary to do it profitably. Once beds are established, we’ve been adding less compost (approximately 1” between crops) and work more towards balancing the soil based on the needs of the crop, so it does get easier and less expensive!

Pro: Early Planting

Especially when tarps are strategically applied to keep major rainfall off of the compost, the deep mulch system can be superb for allowing the grower to get into their fields earlier in a wetter than normal Spring (ahem, 12” and counting). Again, some crops—particularly field tomatoes—may struggle with the excessive moisture.

Con: The Heat

Compost is dark, most of the time black, so in the middle of the Summer, although the soil beneath may be nice and cool, the actual surface of the compost may be excessively hot for planting. This may be an issue for Summer lettuces and greens.


Ultimately there is no perfect farming system. They all come with pros and cons. This system, in my mind, is best for people with moderate to low rainfall, have access to large amounts of good compost, and need to increase their organic matter sooner rather than later. Excessive rainfall areas may struggle with the loss of compost in heavy rain events. In this case, perhaps some growers may prefer a hybrid system—using deep compost for carrots and other roots, while using straw, peat, or no mulch at all for other crops. Many options there, and we are happy to explore them all.

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