Plotting Your Plat

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This year, use crop rotation for fertility and pest control

STORY AND PHOTO BY BILL THORNESS

Carica_papaya_seedling_macroOn a nice day soon, I’ll get down level with the soil and press my rake handle into it, creating a shallow furrow. Time for seeds to bring the garden back to life. But as I contemplate the future harvest, I focus on the past, too. A bit of advance planning on crop rotation helps keep the soil fertile and the pests and diseases at a minimum.

I consult a messy, fold-out sheet in my garden journal. This hand- drawn chart of my vegetable beds has brief notes of what I’ve planted in each bed each year (or season; there might be multiple crops in a year if I’m using the bed year round). With a note that says “2015: tomatoes,” I will know what not to plant there this year, and hence I can rotate in something else.

Here’s an example from one bed:

2010: Lettuce (L), Beets (R)
2011: Broccoli (Fl), OW cover crop (Fl)
2012: Peppers (Fr), OW cover crop (Fl)
2013: Lettuce (L); Carrots (R), OW Fava beans (Fl) 2014: Beans (Fr); OW Asian greens (L)
2015: Tomatoes (Fr)

If you’ve been doing this yourself, you might recognize my shorthand. OW stands for overwintering, which is a crop that is started in the fall and left to cover the soil for the winter. It will shoot up and mature in early to mid spring.

I use the overwintering technique to grow some edibles for harvest during the “hungry months” of February and March, when the raised-bed cupboard is relatively bare. But I also use it for cover crops, which is a generic term for something sown primarily to protect and feed the soil, rather than to produce food.

Fava beans make a good cover crop, whether the large-seed culinary varieties like broad Windsor or the small-seed type that you often see in a cover-crop blend.

plotting your plat

FERTILITY MANTRA

The parenthetical letters next to the vegetable type are less obvious. They refer to an old mantra I learned at Seattle Tilth many years ago. It is “leaf-root-flower-fruit” and refers to a crop rotation pattern that can help me keep soil fertility up — or at least keep it from plummeting fast. The rotation pattern is based on the part of the plant that you eat.

The idea is to plant crops that need different amounts of fertilizer and that take up that fertilizer in different soil zones. This season, plant a leafy crop. In the same space next season, plant a root vegetable, and so on. Lettuce is a leaf crop that feeds very lightly — mostly on nitrogen, the fertilizer component that boosts leafy, green growth. Tomatoes, on the other hand, are heavy feeders, and they require a full spectrum of fertilizer.

Those two plants also use fertilizers in different zones. If you’ve ever pulled up a mature tomato plant at the end of the season, you have probably gotten a Medusa’s head of tangled, stringy roots. Tomato roots can travel three feet into the ground and spread far beyond the plant’s canopy. They snake out to seek water and nutrients. By contrast, even the most robust, fully realized lettuce plant will have a modest Tribble of roots — a compact, round mass extending just a few inches into the soil.

What this means for plant rotation is that you could fertilize once in a location, and both your lettuce and your tomato, planted in succession, could eat from that same bowl at different times and different levels. If you have a bed with good fertility and are following lettuce with tomato, you might just have to add a small amount of fertilizer, maybe in the form of compost, to satisfy the big vine growth.

It’s important to consider fertilization in the crop rotation equation for a couple of reasons.

First, why spend more on your garden than is necessary? You don’t want to grow the dreaded $62 tomato. If you rotate other crops before you grow tomatoes in the same spot again, the soil fertility will recover somewhat on its own.

Second, why pollute? That’s right: Too much fertilizer could actually be considered pollution. Some fertilizer will “leach out” through the soil and down into our groundwater. Water washes the fertilizer downward, whether from our copious winter rains or our regular summer watering scheme.

FAMILY AFFAIR

But fertility rotation is not the only consideration. By rotating by plant family, especially for brassicas, nightshades, cucurbits, and alliums, I can keep the pest and disease population down naturally and cheaply, too.

Putting your tomato plants or others of the nightshade family (Solanaceae) in the same soil year after year will deplete soil fertility while causing an increase in a type of fungus known as late blight. The tomato family also includes peppers, eggplant, tomatillos, and potatoes, so include those in your rotation. By rotating these crops into different beds each year, you’ll also confuse the little chewers that show up annually, like aphids and flea beetles.

Similarly, if I plant my squashes and cucumbers (Cucurbitaceae family) in the same place every year, I’ll very likely increase the population of powdery mildew spores. This fungus lives pretty freely throughout the garden, but it will come on earlier and stronger if you serve it a nice, fresh squash host every year.

Ditto the brassica family (Brassicaceae). This large group of my favorite veggies includes broccoli, kale, and cabbage, but also radishes, mustards, arugula, turnips, and Asian greens. The list of potential problems is nearly as long: The fungal disease clubroot can really stunt the growth of broccoli and deform radishes. Root maggots and cutworms can also attack these plants below ground. But probably the most visible brassica problem is the peripatetic little white moth, known as the cabbage looper. You’ll see these all over the garden, and if they always find your brassicas in the same spot, they will become nearly a year-round problem.

With alliums (Amaryllidaceae, or onion, family), you might see basal-root rot, which pretty much turns your garlic or onions into a slimy mess below decks, and really, with onions, that’s where the money is. So pay attention to the rotation of onions, garlic, leeks, scallions, and shallots to avoid this disheartening problem.

Instructive crop-rotation charts for family and fertility are offered in Seattle Tilth’s Maritime Northwest Garden Guide. I was the editor for the newest, expanded edition, and my compatriot Lisa Taylor, also author of Your Farm in the City, was the principal author. We devoted particular energy and space to outlining crop-rotation needs, because in our verdant, cool climate (where we can grow some of these crops year round), these problems can become significant.

  • Copious winter rains can have a significant effect on soil fertility, which could result in a much larger fertilizer bill or less satisfying harvests.
  • If your garden is in a very mild microclimate, some of the pests will overwinter, and the populations can come on very strong before the counteracting population of beneficial insects can get to work en masse in the spring.
  • In a climate where the ground rarely freezes, the soil-dwelling fungal and bacterial organisms are ever present and can build up.Practicing crop rotation is a challenge. As you review the list of desired vegetables for this year’s garden, hold these questions in your mind: How much and in what zone are the plants eating, and as they grow, what might be eating them? Your garden is as much a dinner table for critters as the one in your dining room is for your family, but with a bit of advance planning, the seeds you sow now will bring you a larger, healthier harvest.
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