There’s a saying among gardeners that “a poor gardener grows weeds, a good gardener grows plants but a great gardener grows soil.”
While weeds may be up for debate, experienced gardeners agree that it is the soil — and the microbes within it — that gives plants life. In a forest or meadow, animals and plant debris replenish the soil naturally. In a garden, we need to nurture our soil as it gets depleted by our tomatoes, roses and hydrangeas.
Fall is the ideal time to jump-start your spring garden by building a nutritious, supportive soil structure for your plants, allowing time for the microbes and worms to do their magic while most plants are dormant in winter.
To find out how best to prep our soil for next year’s garden, we spoke to Reingard Rieger of Tilth Alliance, a Seattle-based organization that advocates for growing healthy, sustainable food. Rieger manages the group’s Master Composter/Sustainability Stewards volunteer training program, which emphasizes the importance of soil science.
Know your soil
“Fall is definitely a good time to pay attention to your soil,” Rieger says. The first step should be identifying the kind of soil you are working with. Many issues can arise from over-amending soil or using the wrong ingredients. The best way, Rieger says, is to do an official analysis, which provides you with a full report on your soil’s makeup and nutrient content. (She’s not a fan of store-bought tests.)
In King County and many surrounding cities, residents are entitled to five free tests through the King Conservation District. Three-pound samples can be mailed or dropped off for testing at KCD’s Renton office.
No matter the type and size garden you are tending, soil tests and proper soil management can:
• Tell you the right amount and type of fertilizer to use for the plants you are growing
• Prevent contaminating the watershed with excessive fertilizers
• Increase your plants’ growth, yields and quality
If your main goal is growing vegetables, nutrient content is especially important. Reingard recommends soil testing in late winter/early spring for the most accurate reading, because winter rains can leach out significant nutrients, notably calcium.
If you’re unsure if you need a test, Rieger suggests contacting the Garden Hotline at gardenhotline.org or 206-633-0224.
For the basics about your soil, Rieger says, you can do a simple DIY test at home to learn about your soil’s texture and moisture-retention.
“It’s a fun project to do with kids, and it tells you what [kind of soil] you have,” she says.
A home test indicates the percentages of sand, silt and clay contained in your soil, which impacts how the soil holds and disperses nutrients and water. Start by sifting out a sample of your soil using a sieve or colander to remove rocks and twigs. Then fill a third of a Mason jar with the soil, add one tablespoon of dish detergent and fill the rest with water, leaving some space at the top. Cap the jar and shake to fully mix.
Set the jar on a level table for one minute and mark the top of the coarse sand layer settled at the bottom. Wait two hours and mark the next level: the silt layer. Wait 48 hours and mark the final layer: clay. Measure each layer’s height and the total height and calculate the percentage of each. You can find a worksheet to track your measurements at bit.ly/soil-jar-test.
Plant roots need a balance of water and oxygen to thrive. Sandy soil drains well, which also means it loses both water and nutrients faster than other types of soil. Clay soil retains water but compacts easily, making it hard for roots to “breathe.” It sounds complicated, but the good news, says Rieger, is there is one remedy to both situations: compost.
The magic of compost
Compost seems to act like an adaptogen for soil. It can help clay become more aerated or sand hold moisture better. Fall is a great time to add a percolating layer of compost as a nutritive mulch on new or existing beds, or even on a lawn.
“It’s so good to mix in organic material. It protects soil from pounding rains, retains water and nutrients and then slowly releases them. It’s almost like a miracle,” Rieger says.
You don’t want to smother plants with compost or other mulch, however. For new gardens, Reingard suggests a 3-inch layer in the fall and an inch or two each year after that. For clay soil, use slightly less; for lawns, about 1 inch.
For another mulch resource, resist raking up your deciduous leaves into the yard waste bin this fall. If you “leave the leaves,” they can become an excellent slow-release fertilizer as they break down. It’s best to chop up large leaves like oak with a lawn mower first. These can be used as a mulch, added to the compost bin or collected to use as needed later.
By mulching, “You’re mimicking what nature does. Leaves drop in the fall, making a protective layer,” Rieger says.
As for amendments, Rieger says, “Oftentimes, adding compost is enough, depending on what it’s made from. Essentially it brings most of the nutrients you need.”
If you do use an amendment, do so sparingly and follow the application instructions strictly, she says. Generally, you want to get the amendment in the top inch of soil by raking it in. Left on top, it can attract rodents or blow away.
And be gentle: A light touch when digging in your soil will preserve the delicate soil biome and network of mycorrhizal fungi that supports your plants.
Crops that feed your soil
Cover crops such as rye, vetch, buckwheat, legumes and clover act like living mulch aboveground while restoring the soil below. Legumes retain nitrogen in the soil, for instance. Fava beans can be sown into late November, offer elegant purple and white flowers before the beans, and can be lightly dug in after harvesting.
Now that you’re on your way to building some beautiful soil, Rieger also recommends fall as the best time to plant perennials, trees, shrubs and bulbs, and for looking into creating a rain garden.