How Much Soil Amendment Do You Need?

– Posted in: Hardscaping and Projects, How-to
11 comments

Mud season is over and your soil is finally drying out, and you’re contemplating adding soil amendments to your vegetable bed or mulch to your ornamental beds. The question is: how much do you need? Probably more than you think. To get an even remotely accurate answer, you will need a tape measure. I like to use one similar to the one pictured on the left for this kind of task. Measure the length and the width of your bed. Write that down. Then you need to come up with a rough idea of how much amendment will achieve your goals. For example, Tracy DiSabato-Aust, in her book The Well-Tended Perennial Garden: Planting and Pruning Techniques, recommends adding four inches of organic matter to a new landscaping bed. You may have seen a different recommendation somewhere else. But you need to have a depth measurement in mind before you can proceed.

Do The Math

Now, you can either do the math the old-fashioned way, with pencil and paper, or the semi-old-fashioned way, with hand-held calculator. Visit North Coast Gardening for instructions on calculating how much mulch or compost you need. Genevieve also links to this page which calculates area for you (you’ll need those length and width measurements you wrote down) and this page which calculates the amount of amendment you’ll need to buy, in bags or even truck loads. Ha-ha. I made you nervous, didn’t I? You thought you were going to have to do math, but you actually only have to put some measurements in boxes and click a virtual button. If only spreading and digging in your amendment were so easy!

If you already know the price of the amendment you’re considering, sit down before you make this final calculation of total cost. It can be shocking if you’ve never done this before. Multiply the price of one bag by the number of bags the previous calculator came up with.

If you find this whole discussion rather abstract, take a look at this Garden Math video by Greg Draiss. He demonstrates quite graphically how little one bag will cover. You’ll realize it makes sense to figure out how much you need before you hop in the car to go get it. If you just guess, you’ll probably need to make a second trip.

About the Author

Kathy Purdy is a colchicum evangelist, converting unsuspecting gardeners into colchicophiles. She would be delighted to speak to your group about colchicums or other gardening topics. Kathy’s been writing since 4th grade, gardening since high school, and blogging since 2002.

In its own way, frost may be one of the most beautiful things to happen in your garden all year . . . Don’t miss it. Like all true beauty, it is fleeting. It will grace your garden for but a short while this morning. . . . For this moment, embrace frost as the beautiful gift that it is.

~Philip Harnden in A Gardener’s Guide to Frost: Outwit the Weather and Extend the Spring and Fall Seasons

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greg draiss May 20, 2011, 10:33 am

Thanks for the video link! I always find great advice on this blog and refers readers here quite often

Kima May 6, 2011, 11:14 am

So very true! Excellent way to break this down – bags labeled ” X cubic yards” always seems like so much in theory! Thanks for the great explanation – I will use it to explain to clients why I do almost everything by the truckload and not by bag, even when it’s on sale. The only reason I go by the bag is if I have a long haul over non-cart/barrow/heavy trash can- friendly terrain. (That situation is not too uncommon here in NW Montana.) And that leads to my additional tip to share – rolling trash cans (not the large city pickup size – too heavy) have proven to be my best hauling tool for amendments or bark mulches. If you’re careful not to let your loads get to heavy, trash cans can do stairs, go up terrace walls, thru narrow paths, navigate sloped ground, and can be dumped very specifically, missing those tender plants. Try it – Rubbermaid cans have held up the best for me – check for solid handle and wheels. Oh – and don’t lift the bottom by the wheels when you dump – lots of them will rip right off. Happy Planting!

Kathy Purdy May 6, 2011, 3:55 pm

Thanks, Kima! Excellent tip!

Ian Hall May 5, 2011, 11:35 am

Hi Kathy,

I will be forwarding this to all my skeptical design clients. With the heavy clay soils in our area I always encourage getting it by the truckload…just to make sure there is no second trip! Thanks so much for giving us this truly useful tool!
Best!~ Ian

Renee May 5, 2011, 8:44 am

Kathy, this is terrific! Thanks so much for sharing. Customers always look aghast when I answer honestly how much soil amendment they need for a new garden. Then they buy 25% of what I recommend, if that. This stuff gets expensive when you buy by the bag. Oh, and it’s a LOT of work. Isn’t gardening supposed to be easy/instant?

Ferdi Nel May 5, 2011, 6:53 am

Thanks for the great links. This will make my calculation work a lot easier.

I also heard about the recommended 4 inches of compost for a new bed.

Genevieve May 4, 2011, 10:48 pm

Kathy, thanks so much for including me in this round up of information on buying mulch and compost! I hadn’t seen Greg’s video before and I just adore it! Your point about planning ahead is exactly right. Plan ahead, or plan to make a second trip. That’s so true.

Kathy Purdy May 4, 2011, 10:52 pm

I’ve had that page on your site bookmarked for many months. I always thought calculators such as those you linked to would be mighty handy to have. Now we just need it as a smartphone app.