If you’ve been growing tomatoes, greens, and herbs in 100% Deep Roots microbe-rich compost and loving the results — this post is not asking you to change a thing. Your compost method is exactly right for those crops. This post is about one important exception: root vegetables.

Why root crops need different soil

Root veggie rules: Carrots, beets, radishes, parsnips, turnips, potatoes and rutabagas play by different rules than everything else in your garden. The very richness that makes your compost great for tomatoes works against root crops. The good news: the fix is simple, and Deep Roots has made it even simpler by doing most of the prep work for you.

Roots store energy. Root crops aren’t trying to grow fast — they’re trying to store energy underground. Carrots, beets and parsnips only do that when they receive a specific signal that says “nutrients are present, but limited.” When nitrogen is too high — as it often is in manure-based compost — that signal never comes. Instead, the plant keeps putting energy into leaves. This is why gardeners sometimes see beautiful, bushy tops and disappointingly small roots. The plant is doing exactly what the soil chemistry is telling it to do.

Loamy, gritty, well-drained soil. There is also a physical reason. Pure compost lacks the mineral structure that root crops evolved in. It holds too much water, dries unevenly, and encourages shallow or forked rooting. Root vegetables evolved in mineral soils — loamy, gritty, well-drained — with only modest organic matter. That’s what allows them to grow straight, large, and dense.

The solution: one dedicated root veggie bed

You don’t need to change your existing beds at all. Simply set up one dedicated root vegetable bed or large container with a different soil. Deep Roots will sell you the perfect soil to fill it – pulverized mineral topsoil — a clean, low-salt, loamy base that gives roots exactly the growing environment they need:

Low in nitrogen – so roots get the signal to store energy underground
Mineral-based – so roots can grow straight and deep without resistance
Low soluble salts – so roots absorb water freely and expand easily
Well-draining – so roots don’t fork, rot, or grow shallow

PhoSul: your new root veggie fertilizer

PhoSul is an OMRI-certified organic fertilizer made from rock phosphate and elemental sulfur. It contains no nitrogen — only phosphorus, which is the nutrient most responsible for root development in carrots, beets, parsnips, and turnips. It also releases calcium and silica as it breaks down, which gradually improves soil structure season after season.

Build-and-maintain amendment: The best part is that PhoSul doesn’t leach. Unlike nitrogen, which washes below the root zone every time you water, phosphorus bonds to soil particles and stays put. This means it builds a slow-releasing reserve your root crops can draw on all season long — without constant reapplication. Think of PhoSul as a build-and-maintain amendment, not a seasonal fertilizer you must continually renew. Instead, top it up lightly once a year, and over two to three seasons the bed will develop a stable phosphorus bank that works quietly beneath your root crops all season long.

To apply PhoSul: Put a pinch of PhoSul in your planting hole for seedlings and seeds. It is very gentle and a little extra will not matter. When you buy Deep Roots crop soil, we will give you a small ziplock bag of PhoSul to add in at planting.

Note - Root topsoil and PhoSul coming to our store in April 2026

Garlic and Onions are NOT root crops

Garlic, onions and leeks are alliums. It’s ideal soil and fertilizer has similarities to BOTH root veggies and above ground (fruiting and leafy) veggies. Garlic and other alliums are a heavy feeders that require nutrient-rich soil, generous organic matter, and good nitrogen levels in early spring to grow large bulbs. The Deep Roots compost-rich beds your tomatoes thrive in are too nutrient-rich for garlic and cause tiny under-developed bulbs. Read our blog post: Planting Garlic in Fall.

Alliums need a soil blend: The solution is growing garlic and other alliums in their own bed or container using a soil blend of 30% high nutrient compost and 70% low nutrient mineral topsoil. Plant in fall (mid-October to early November), mulch with 2–4 inches of straw, and apply a pinch of phosphorus fertilizer next to the clove or seed. In spring when seedlings reach 4–6 inches tall, top-dress (spread on all soil growing alliums) about a half inch of compost and worm castings. Harvest garlic when one-third to half of the leaves have browned..‍ ‍

Soil blends for regular and sweet potatoes

Sweet potatoes prefer well-drained low-nutrient sandy or loamy soil, do not require large quantities of organic matter, and over-fertilization tends to produce foliage instead of roots. That makes a leaner mix, around 20% rich compost, better than a rich one.

Regular Potato Soil Blend
Mix 2 parts low nutrient sandy loam with 1 part nutrient-richcompost. Blend thoroughly, then fill beds or containers with loose, well-drained soil. This gives regular potatoes enough fertility without making the mix overly heavy.

Sweet Potato Soil Blend
Mix 5 parts low nutrient sandy loam with 1 part nutrient-rich compost. Keep the blend light and only moderately fertile, since sweet potatoes produce better roots in leaner soil and too much richness can lead to excess vine growth.

Starting and Transplanting Root Vegetable Seedlings

Root vegetables that can be started indoors and transplanted include leeks, onions, and specifically "Clancy" variety potatoes, as they are less sensitive to root disturbance than others. Beets can sometimes be started indoors if transplanted very early, but most root crops like carrots, radishes, and parsnips are best direct-sown.

Root Vegetables for Indoor Starting

Leeks: Start 8-10 weeks before the last frost, as they handle transplanting well.
Onions: Can be started indoors 8-10 weeks early, with tops trimmed to 3 inches for stronger, transplant-ready plants.
Potatoes ('Clancy' variety): These can be started from seed indoors.
Beets: Generally better direct-sown, but can be started indoors and carefully separated if moved while small.

Why Most Root Crops Dislike Transplanting

Root vegetables develop a single taproot that is very sensitive to disturbance. Transplanting them often causes stunted, misshapen, or split roots.

Tips for Success

Use biodegradable pots (like peat pots) to minimize root disturbance during the move to the garden.
Ensure all plants are properly hardened off before moving them outside.
For root crops that must be direct-sown (carrots, radishes, turnips), it is best to do so as soon as the soil can be worked.

What to expect

Learning to grow root crops is not a complicated change. Use dedicated beds or containers, pre-filled with Deep Roots mineral soil and amended with PhoSul. Everything else about your gardening stays the same. Your compost beds keep doing what they do best for tomatoes and greens. Your root veggie bed gets the lean, mineral, low-salt environment that finally lets carrots be carrots and beets be beets.

Plant something. Feed something. Protect something.

Deep Roots Project is here for every step of the journey.

“Grow Your Own Food” blog posts

See the full list of our Grow Your Own Food blog posts. Each post is assigned ”tags” which are under the post title. If you need a quick answer to a gardening question give us a call or send a text to our customer support team – support[at]deep-roots-project.org AND 708-655-5299.

Deep Roots online store

See our online store for details about prices, ordering and delivery of raised beds, planter boxes, microbe-rich compost, worm castings, leaf mulch and more. We don’t sell traditional soil, since we use 100% compost as our growing medium. Our online store has 2 sections – (1) raised beds and planter boxes and (2) compost, worm castings, fertilizer.

Please contact our customer support team before placing an order online so we can assist you with the details and answer your gardening questions. You can pay by credit card in the store or by check.

(708) 655-5299 and support[at]deep-roots-project.org

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Growing Root Vegetables - A New Method