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Home » Follow these steps to replenish nutrients in raised garden beds: Ask an expert

Follow these steps to replenish nutrients in raised garden beds: Ask an expert

It’s been a mild and springlike March and that means it’s time for gardening. If you’ve got questions, turn to Ask an Expert, an online question-and-answer tool from Oregon State University’s Extension Service. OSU Extension faculty and Master Gardeners reply to queries within two business days, usually less. To ask a question, simply go to the OSU Extension website and type one in and include the county where you live. Here are some questions asked by other gardeners. What’s yours?

Q: We have two raised garden beds in our backyard and I was looking for advice on how to best prep the soil for this year’s garden. Both garden boxes are approximately 8 -by-2 feet long by 2-feet wide by 2.5- feet deep. One was built two summers ago and the other was built last summer. Both beds were filled with compost when they were built.

Last year we added a little bit of compost to the original garden box and mixed it in with the soil that was already there. Other than that, we have not added anything else to the soil. Is there something that we should add to these garden boxes to replace nutrients used up by plants over the last couple of years before planting in them this year?

In one garden box we grew potatoes, bell peppers, basil, carrots and garlic. The newer of the two had tomato plants, green onions and carrots, although the tomato plants took over so the other veggies didn’t survive. – Marion County

A: Vegetables will usually do well for a year or so in a new bed, but because they are annual plants, they quickly deplete the nutrients, which must be replenished regularly.

Compost is an excellent amendment to condition your soil and improve it over time. However, decomposition is a slow process so compost is not considered a fertilizer. Many necessary plant nutrients are minerals such as those found in rock. Nitrogen is rapidly used by plants and leached out of the soil by rain; that is why most fertilizers contain some combination of nitrogen (N), phosphorus (P), and potassium (K).

This is not to say that vegetables need more nitrogen (in fact, too much can result in lush foliage and little fruit); but the nitrogen must be replenished more often. Typically, you will see numbers on packaging for fertilizers for the three primary ingredients in the order mentioned above, but many fertilizers also include secondary and micro nutrients that are also needed by plants in small amounts.

In a raised bed, it is difficult to customize the nutrients and soil pH for various vegetables. Your best approach is to strive for a slightly acidic soil (6.0 – 6.5 pH). Rotate your vegetables each year so those in the same family are not grown in the exact same spot, which will also help reduce pathogen build-up that may cause diseases. Families include Solanaceae (tomatoes, peppers, potatoes, and eggplant), the allium group (onion, leeks, garlic) and the leafy greens.

Ideally, you should have a soil test to determine what your soil is now, and then amend or fertilize accordingly. Many people do not wish to test for a small garden, but understand it is a bit like working in the dark without a test.

A good soil is about 3{77ce8f5c13f5afba9075485d3cf7c497b0ab33a4437bbe9a3594fe3360863398}-5{77ce8f5c13f5afba9075485d3cf7c497b0ab33a4437bbe9a3594fe3360863398} organic materials in various states of decomposition. This will provide good soil health, tilth, and maintain healthy soil micro-organisms. In addition, you will also need fertilizer which can be either organic or non-organic (i.e., conventional or synthetic). Look for a quality balanced fertilizer that has proportionately higher phosphorus and potassium than nitrogen, such as 10-20-20.

For more information, please see this OSU publication.

Also, this publication provides an excellent guide for vegetable gardening in general.

One last point I’d like to make is that tomatoes are often better off planted in a very large pot where they will not crowd other vegetables. You can also stake and prune them so they grow up, not out. Read more in this publication. – Lynn Marie Sullivan, OSU Extension Master Gardener

Q: I want to implement no-till gardening and combine that with green manure/cover crops to improve my soil structure. However, I am confused about how to start. My garden has issues with massive amounts of weed seeds, clay soil, compaction where I have been walking, and a section where we are battling invasive blackberries and thistle. How can I implement no-till gardening with all of these problem areas? – Marion County

A: You may wish to deal with the obvious problem areas first to get the garden started and then implement the no-till/green manure techniques once you get started. Damp, unprotected clay soil is highly prone to compaction, so I would sketch out the garden space and decide what areas would be dedicated pathways and what is going to be growing space.

The existing compaction in the growing spaces is a short-term problem, so you might want to consider digging in organic matter when the soil is dry enough to break that up using a minimally invasive tool like a spading fork. You can maintain the structure of the soil after doing this with the use of mulch or green manures.

I would not let the weed seeds discourage you. Remove any perennial weeds and any annuals as they germinate and you will gradually reduce the seed bank and the number of weeds you have to deal with. The blackberries are an invasive weed so, of course, you want those gone as well, but they may have had the side benefit of protecting, or even improving, soil structure beneath the canopy and suppressing other weeds.

Removal of these, including root systems, may reveal a soil already ready for no-till gardening, with the caveat that there are undoubtedly blackberry seeds that will germinate, requiring ongoing weeding of those. You don’t mention which thistle is involved, hopefully it is an annual species and not Canada Thistle, which is perennial and very difficult to eradicate. Feel free to write back and elaborate on this.

This short article provides some additional suggestions:

This Extension publication provides a lot of additional guidance, though not specifically on no-till gardening. As I mentioned, feel free to write with questions. – Neil Bell, OSU Extension horticulturist

Red clover in Washington County. Oregonian file photo.

Q: We grew red clover as a cover crop this winter. We turned it over last Saturday. My Master Gardener material says to wait three weeks before planting. How critical is this period? What are the disadvantages of planting sooner? Does it matter whether we are putting in seeds or starts? – Multnomah County

A: In this Washington State University Extension publication, here, under the section about terminating a cover crop, is “Residues from freshly terminated cover crops can inhibit seed germination, including germination of desirable vegetable seed. Waiting to plant vegetable crops for three to four weeks after terminating cover crops often improves their establishment.”

The OSU Extension publication “Growing Your Own” gives timelines for planting. The soil is too cold and wet for most planting this early, so perhaps waiting won’t hurt your garden plans. – Jacki Dougan, OSU Extension Master Gardener