Ash Aftermath

Spring is a time of new beginnings, especially in the garden.

Most years, that fresh start includes clearing out the tattered remains of previous crops and renewing fertility by applying nutrients, tilling in cover crops or working in earthy-scented, life-affirming compost to jump-start the biological activity beneath the surface and improve the soil texture throughout.

But this spring, there’s an additional consideration: the question of whether the prodigious ash that the Thomas Fire deposited in every nook and cranny outdoors will have a deleterious effect on our gardens, be that to our soil; our permanent fruit trees, grapes and berries; or the new edibles that we plant in the months ahead.

Take a deep breath. Thankfully, there’s little worry. Truly, all that ash may even be a plus, delivering a nice dose of potassium, which is an essential element for healthy plant growth and the third of the three letters—N-P-K—in fertilizer.

Your Odds Are Good

Unlike the Santa Rosa Fire last October, which raged through both wildland and city neighborhoods, the Thomas Fire primarily burned wildland. Nearly all of our ash came from trees, shrubs and other chaparral. There was little in the way of manmade materials that could contain potential toxins. Our ash was relatively clean, plant-based material.

Also keep in mind that fire and ash have played prominent roles in cultivating land for centuries.

Ancient Mayans practiced milpa, slash-and-burn techniques to create new, fertile planting grounds by chopping down sections of forest, then burning the downed vegetation to generate potassium-rich ash that boosted the growth and productivity of their crops.

As early as the 1400s, farmers applied a condensed version of wood ash—potash—to their fields. Back then, potash was made by reducing wood ash to a fine powder in pots of boiling water.

Today, most potash is mined in the form of potassium-containing salts (potassium chloride or muriate of potash, also known as MOP) from underground seabeds. It’s typically applied to commercial carbohydrate crops, such as wheat, oats and barley, that can tolerate added salt.

A second form of manufactured potash, sulfate of potassium (SOP) does not contain chloride or salt and is best for the salt-sensitive edibles that we home gardeners often grow, including many vegetables, fruits and nuts.

Fireplace ash and bits of charcoal—provided they’re the untreated remnants of hardwood, not fake logs—are another source of potassium. Many of us add the remains, which also include phosphorus and micronutrients, to our compost piles.

How Potassium Works

First, regarding those N-P-K fertilizer labels—N stands for nitrogen, which promotes green, leafy growth above ground. P represents phosphorous, which helps with root development and flowering.

K is potassium or potash (the two words are often used interchangeably) and is responsible for overall plant vigor and health. To wit, potassium aids photosynthesis, helps roots extract nutrients from the soil, regulates proteins and starches in a way that contributes to a plant’s ability to stand erect, and enhances flowering and fruiting.

While potassium is generally present in most soils, its utility in clay and in dry soils can be limited, as it tends to bind to soil minerals rather being available to plants.

This can be problematic for permanent edibles, such as fruit and nut trees, berry bushes and grapes, all of which may be planted in native soil that’s reluctant to release potassium. For these plants, Thomas Fire ash may provide a welcome bonus.

Likewise, while you’re presumably cultivating your edibles in fertile, well-drained beds rather than in native soil, the new ash can still be a valuable supplement.

Indeed, many organic gardeners apply potash to their beds in late winter or early spring every year. Vineyard managers commonly do so as well, as potassium strengthens the vines’ ability to resist disease and boosts fruit development, size and flavor.

If you’re still not convinced, think back to how plants have behaved after other wildfires. Roses are one of the most demonstrable beneficiaries of ash, reliably producing extra-spectacular blooms the first year after a big fire.

On the Flip Side

But there are steps to take if you’re still concerned that your garden accumulated too much of a good thing.

Avoid fertilizing with a product that contains a high K number. Double-dig or rototill at least the first foot of soil to disperse the ash. Over the course of a few months, push the ash deeper into the soil by soaking the area each time the top inch dries out.

Better yet, use compost instead of commercial fertilizer. While compost does contain the big three—nitrogen, phosphorous and potassium—it breaks down slowly, so is not likely to overwhelm your edibles. Compost contains important trace elements and beneficial micro-organisms that will improve the texture and drainage capabilities of your soil as well.

Potassium and Particular Plants

Because potassium boosts flowers and fruits, your flowering and fruiting plants will benefit the most from replenishing it on a regular basis. The list starts with fruit trees, including avocado, citrus, pome fruit such as apples and pears; stone fruit such as apricots, nectarines, peaches and plums; and nut trees, including almonds, macadamias, pistachios and walnuts. Also think bushes and vines that bear edible fruit, such as blackberries and raspberries, along with grapes, kiwi and passionfruit. Vegetables, including tomatoes, melons, potatoes, pumpkins and squash, round out the list.

Note that edibles grown for their leaves, such as salad greens, kale and the like, don’t have as high a demand. Their greatest need is for nitrogen to fuel and sustain their foliage. But all edibles require at least some potassium, whether the source is fertilizer, compost or ash from wildland fire.