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How Much Compost Do I Need?

Maynard, MA

The soil in my veggie garden is somewhat sandy. I need to add compost/organic matter. There's no way that I have enough of my own compost, even though it isn't a large garden. How can I figure out how much to order?

Anderson, IN(Zone 6a)

It is about 24 sq in to a plant here, that is 4 inches around the plant and 8 inches deep . That is enough to grow most , not all , I don't think there is a rule to go by , Where being a gardener with some intuition comes to this ,
I t also depends if the compost is finished or not , I guess that should give you a place to test or start from .

Everett, WA(Zone 8a)

I think about how deep I want to turn it under, and what % of the soil I hope to be compost at the start of the season. It will be "digested" pretty fast in sandy soil, especially if it is finished compost to start with.

Say you have a 3' x 15' bed - 45 square feet.

If you put down a 2" layer and turned it under for the top 6", the soil would start out as 30% compost. That would be 45 sq ft x 1/6th ft = 7.5 cubic feet. So 8 bags, less than $10. Not a full trunk-full.

It wouldn't hurt to fill your trunk (12 bags?). 12 cu ft would cover 45 sq ft 3.2" deep. If you turned that into the top 8" of soil, you would have 40% compost: very rich and water-retaining.

(My assumption is that rain will wash a fair amount of compost out of the top layers and into the sand deeper down. That's fine for up to 12", but many flowers and garden crops are shallow-rooted, and you could lose some of the advantage of the compost, depending on the crop.

MAYBE, and this is a pure guess on my part, but maybe a layer of corrugated cardboard or 5-sheet-thick layer of newspaper, 12 or 18 inches deep, would help prevent some of the compost from washing away.

(Or look up "lasagna gardening", put the cardboard on top of the sand, then 8" of compost on top of the compost, and grow plants in 100% compost! They say it works fine. (If you can afford that much compost: 6 cubic feet per square yard of garden.)

I tend to buy as much as will fit in my trunk, then go back for more while my budget and back permit.

If your "not large" garden have two hundred square feet, ask your budget how much compost you can afford. Say, $20. Around 15 bags. 15 cu ft / 200 sq ft = 0.075 ft = 0.9" of compost. You might want to grow some cover crop on half of the bed this year, and save the compost for the other half.

To some degree, the LESS compost you mix into soil, the less you'll lose to immediate microbial action. You probably can't keep more than 5-10% compost in well-aerated soil - it just gets digested. You might want to make some of the soil amendment something that only breaks down slowly, like bark fines. Even peat moss might break down a little slower than compost.

Some commercial "compost" like Cedar Grove loads the bag down with lots "soil conditioners" like wood shavings or even sawdust. Un-composted sawdust is bad for gardens: microbes eat the carbon in sawdust and multiply really fast, sucking all the nitrogen out of your soil before plant roots can get any.

When I want a bag of "compost", i usually buy "manure".

But one pack of crooks sold "manure" that was at least 15% pebbles! Too big to pass through 1/4" mesh, and I don't THINK that cows pass that much gravel! I don't buy that brand now, and I buy and screen one test bag of ANY new soil amendment before I buy several bags. Even "bark mulch" can range from filthy logyard trash with wood, stones and mus, to clean dry orchid-bark quality.

Try Starbucks and 7-11 for coffee grounds.
Fruit stands and restaurants for kitchen scraps.
Lawn mowing services for grass clippings.
Tree services and town departments for sawdust (for COMPOSTING) and wood chips for mulch.

P.S. I wish I could trade a few cubic yards or your sandy soil for my heavy clay!

Maynard, MA

Truly helpful, Rick. And yes, if we didn't live so far apart, a sandy-clay swap would be an ideal solution. Meanwhile, many thanks for your clear information.

Monte Vista, CO(Zone 4a)

Shayna, many horse-, goat-, and cattle farmers will let you get a pickup load of manure for free if you shovel it yourself. You still have time to turn it in, especially if it's aged manure. When I was a kid, it was my job to prepare the garden for my mom. I'd go out to the barn and get a couple of wheelbarrows full of goat manure, turn the garden with a shovel, and then she'd plant. It would take me a whole day and then some to get it all tilled and raked smooth. She always had bountiful gardens.

Everett, WA(Zone 8a)


You're very welcome. I hope you can afford lots and lots of organic matter for your sandy soil. A few inches, at least. Opinion is divided on how much it helps to mix it into the soil: I always do, in clay, but others never till once their soil is pretty good.

I don't know how many farms are near you, inside I-495, but they might giv e away spoiled hay if there are any. Horse-riding academies?

BUda, TX(Zone 8b)


I don't know if you have a Lowe's near you, but here in Texas they have a composted cow manure called "Black Kow" that is absolutely tremendous. I think it's only available at Lowe's, but I had done a little research and found out a few interesting facts. The main company is in Florida, and a local organic company bags the product in the Austin area. This company also custom blends and bags mixes for many of the organic retailers in our area.

SE Houston (Hobby), TX(Zone 9a)

Hey, Kevin!
How's the move going?

Cleveland,GA/Atlanta, GA(Zone 7b)

I am in Miami now and passed by mountains of Black Kow in a Black Kow paddy patch coming down the Turnpike in north central FL.. Black Kow has been around since dinosaurs roamed the earth which was when I started gardening. :) My kids used to love to send away for their yellow t-shirts; "Black Kow The Mature Manure". It's usually available at Walmart and the seed 'n feeds. Fresh animal manures should be composted for at least six months before putting in your garden for health safety. Black Kow is cheap enough for small gardens and good to go.

Maynard, MA

Hi Maypop,
We do have Black Kow here, at Lowe's, but it's $5.87 for a 50-lb bag. Someone on the Lowe's website said this is about 10 gallons. (Do you agree?) If it is 10 gallons, then it's about 1.5 cubic ft., and close to $3/cubic foot--making it a lot more expensive than the manure that Rick is buying. So unless the 10 gallon/1.5 cu.ft. figure is wrong, I'll keep exploring other local resources. I just sent emails out to a few nearby farms.

BUda, TX(Zone 8b)

The Black Kow is bagged here locally in Austin for Lowe's in Texas I guess, and I really like the price. We're paying $3.20 for the 50 pound bag of the manure, so for us, it's economical, and that's why it's all I'll use. Some of the local organic retailers have composted manure, but it's nowhere near the quality of the Black Kow....

Cleveland,GA/Atlanta, GA(Zone 7b)

I put in a supply in the shed a few years ago and am working on that. It sems like ten gallons in a bag is about right. I'd have to check when I get to my garden. I'm away right now. It takes just a sprinkle of Black Kow broadcast across the beds. We prepped the Spring garden a few weeks ago with two year old bags. Our prices are more in line with Kevcarr's. I was actually thinking around $2.99 a bag. Farm manure is definitely your best bet for the price. Just make sure you compost it for the right time. We used to use our own horse manure but the sprouting grass seed was a pain. Ditto my friends' free fertilizer who offered all the cow manure I can haul. Don't know what thrives best in pasture manure the fescue, clover or Bermuda seed. There was a time we had the gardens sprayed in liquid chicken litter each year. The aroma put a halt to gardening for days.

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