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.