3 Useful Soil Mixes for Planters and Raised Beds
Once you’ve taken the time to create a container garden or raised beds, attention to soil mix leads to maximum yield.Posted Apr 23, 2014
For many dedicated gardeners, raised beds are a life-changer…
You may wonder why you waited so long to make the switch! Your back will thank you: just perch comfortably on the wooden edge to plant or weed. Your knees will thank you: no more painful kneeling on gravel or mud. Your plants will thank you: no more competition for essential nutrients with lawn grass or far-reaching tree roots. Your soil will thank you: the sturdy beams prevent erosion and leaching, and mulch stays in place. The earth never gets compacted because no one walks on it: no-till gardening becomes much easier. Your garden will look beautiful, organize easily, and produce bountifully.
For urban gardeners, container gardening is often the best (and sometimes the only) option. No yard? Pavement as far as the eye can see? No problem, if you have a balcony, fire escape, or sunny stoop — even an accessible rooftop. Landlord won’t let you dig up the lawn? Ask about adding tidy and removable planting containers. Containers can be small beds with slatted bottoms to hold soil in place, or large pots or window boxes for small groups of plants. Some small planters are so portable you can start them inside before the last frost, then move them out when the weather’s fine. The past decade has witnessed a revolution of gardening city-dwellers, making creative use of every corner of sun available!
But once I’ve set up my lovely new raised beds or planters, what do I fill them with? This is the number one question we get asked at Eartheasy. Can I just dig the soil out of my current garden or yard? If I’m buying soil, how do I choose? Keep in mind that your local conditions may have special considerations (gardeners in Phoenix will need more moisture retention, while in Seattle good drainage is crucial) but there are some guidelines which will help you get started.
Find out what’s beneath your feet
In many cases, using your own topsoil as a starting point makes good sense. For one thing, if you’re filling more than a few deck planters, buying bagged soil at your local garden center gets expensive. But before you start shoveling, now is a good time to test your soil. Knowing your soil’s pH is essential, so you can create optimal conditions for your crops — or choose plants likely to thrive. Rainy, woodsy areas tend towards acidic soil, while droughty regions are likely to be alkaline. If you live near midwestern prairies, your garden may be close to neutral pH. Sometimes you can find clues just by observing the local weed volunteers: plantain, oxeye daisy, sorrel and moss growth indicate acidity, while Queen Anne’s lace, chicory, and chickweed love alkaline conditions. Blueberry shrubs, potatoes and tomatoes like acidic soil, but many other vegetables will sulk if the pH is too low — or too high. Between 6.0 and 7.0 is ideal for vegetables.
Five pounds of lime per 100 square feet will raise your pH by 1
Luckily, adding compost to either acid or alkaline soil will buffer the pH, bringing it closer to neutral. For acid soil, you will also add some lime (dolomitic lime will help if you also have a magnesium deficiency). Five pounds of lime per 100 square feet will raise your pH by 1 point, although clay-heavy soils may need more. Applied sparingly, wood ash will also help alkalinize. If you want to increase your soil’s acidity, generous amounts of shredded leaves or peat moss will help lower the pH. Your soil test will also shine a light on any major nutrient deficiencies, pointing the way to the most relevant amendments.
Option 1: The Simple Blend:
Try this if you are filling many large beds, or if you have access to plenty of healthy loam. Screen your topsoil (or buy a truckload of screened topsoil) and mix it thoroughly with equal parts screened compost (kitchen, mushroom, manure, and/or fish). You can choose to discard the top layer of your soil to avoid weed and grass contamination. Put a tarp in the bed a pickup, get a shovel and ask around for good sources of clean local compost in bulk, as the cost of bags will mount quickly in larger beds. Fill your planters or beds with this basic mixture and add more compost every year in early spring.
Option 2: The Luxury Blend:
For small container gardens, or if you want to go all-out and ensure your beds start out absolutely weed-free, you can choose to start from scratch with 100% purchased materials. Your plants will thrive in this mixture that resembles potting soil, containing no garden-variety “dirt” at all: 1/3 vermiculite, 1/3 peat moss, and 1/3 varied composts. For larger planters, you can adjust this to one quarter each vermiculite and peat, with the remaining half comprised of compost. Perlite or coconut coir can substitute for vermiculite — these aerating and water-retaining mediums are particularly important for smaller containers, where the structure of the vessel can create compaction and moisture problems.
Option 3: The Lasagna Method:
Are you building extra-tall raised beds? Do you live near deciduous trees or have a good-sized lawn? Waist-high beds can be wonderful to garden, but filling the entire bed with good soil is very pricey. Leaves and grass clippings are great bulk organic materials which can be layered into the lower regions of tall raised beds, where they will slowly compost over time into rich soil. Aim for two parts shredded leaves to one part grass clippings. Add grass clippings in thin layers to prevent matting. Straw (not hay, which contains seeds), wood chips, or shredded bark could be included as well. Once the beds are full to within 6-12 inches of the top, add a compostable barrier such as untreated cardboard (it prevents your good soil from sifting down too quickly) and then fill the remainder with your chosen soil mix. Next year, you will find the soil level has sunk due to the decomposition and settling of the lower layers, so you will have plenty of room to add a fresh layer of compost on top! Eventually, the lower layers can be turned over and used as a soil amendment. If working with extra-deep containers instead of raised beds, you can add an inert “filler” to the bottom of the container, such as bricks, milk jugs, or stones. Cover the filler with landscape fabric, which will enable drainage but prevent soil loss, before adding your chosen soil mix.
Give your plants a balanced diet
Just like people, plants need a range of nutrients to thrive. If you make your own compost, the leftovers from your own balanced diet become food for your garden — but your kitchen compost is limited to the range of what your household commonly eats. Many gardening advisers recommend using at least five different sources of compost: for instance, your kitchen compost plus four different bags from a garden store. In addition, supplementing any known imbalances with targeted amendments leads to great rewards at harvest time.
An all-purpose organic fertilizer is the simplest way to go for the beginning gardener, but armed with the results of your soil test, you may want to fine-tune your beds. Essential nutrients for all plants include:
- Nitrogen: provided abundantly by leguminous cover crops, composted manure, fish fertilizer or alfalfa meal.
- Phosphorus: rock phosphate and bone meal are both good sources.
- Potassium: kelp meal or greensand are long-lasting potassium releasers.
- Calcium: commonly supplied by gypsum or lime. Glacial rock dust provides calcium and other minerals while raising the pH of acidic soil.
- Magnesium: epsom salts will raise magnesium without affecting pH, while dolomitic lime will raise both pH and magnesium.
- Sulfur: generally required only in alkaline soil. Adding sulfur will slowly lower pH through microbial action.
Gardening for years with a nutrient deficiency can be so discouraging that it’s hard to stay enthusiastic. Though in the excitement of spring, it may be tempting to rush into planting with whatever dirt you have on hand, take your time. All your labor will be in vain without the appropriate soil foundation. Take steps now to ensure some positive reinforcement for your efforts!