Gadenin日本語Get the appA rich bed gives you vine, not roots
Sweet potato is the one vegetable in the garden that punishes generosity. Given plenty of nitrogen it grows a magnificent sheet of heart-shaped leaves, runs fifteen feet in every direction, and when you dig it in autumn you find a handful of pencil-thin strings. The plant only starts bulking storage roots when top growth slows down, and heavy feeding never lets that happen.
The practical rule is to plant sweet potatoes on ground that fed a hungry crop the year before, and add nothing but a little potassium. If you have just spread manure or a nitrogen-rich compost on that bed, grow something else there this year and move the sweet potatoes to poorer ground. Gardeners who get twenty pounds from a short row are almost always growing on soil they consider disappointing.
If you only remember one number from this page, make it this: sweet potato wants roughly half the nitrogen you would give a tomato and about twice the potassium.
What the plant actually needs from the soil
Storage roots swell by pushing soil aside. That is a mechanical job, so the single most important property of the bed is looseness to a depth of about twelve inches (30 cm). In compacted or stony ground the roots meet resistance, fork around the obstacle and come out twisted and split. Sand, sandy loam and any soil you can push a hand into are ideal.
Nutrient-wise the plant is undemanding except for potassium, which it uses to move sugars down into the roots. Phosphorus matters at the start, for rooting. Nitrogen is the one to hold back on. Aim for a pH between 5.5 and 6.5, which is slightly more acid than most vegetables want, and do not lime the bed the way you would for brassicas.
| Factor | Target | What goes wrong outside it |
|---|---|---|
| Loose depth | 12 inches (30 cm) minimum | Forked, cracked, stunted roots |
| Nitrogen | Low. Nothing fresh added | Enormous vines, string-like roots |
| Potassium | Generous. Wood ash or sulfate of potash | Small roots, poor flavor and sweetness |
| pH | 5.5 to 6.5 | Above 7.0 encourages scurf and rough skin |
| Drainage | Water gone within an hour of rain | Rotting roots, especially late in the season |
Build a ridge, even on good ground
The ridge is not decoration. It raises the root zone above the water table, it warms faster in spring because more surface is exposed to the sun, and it gives the roots loose soil to expand into instead of the compacted layer under your paths. On heavy clay a ridge is the difference between a crop and a rotten mess; on light sand it still buys you a week of extra warmth at each end of the season.
Make ridges about ten inches (25 cm) high and two feet (60 cm) across the base, running them along the length of the bed. Build them two to three weeks before you plant so they settle. A ridge built the same morning you plant will sink four inches in the first heavy rain and leave the slips sitting in a trough.
- Loosen the whole bed first Fork the ground over to a full spade depth and break up any pan under the topsoil. Do this while the soil is dry enough to crumble, never when it smears.
- Pull soil up into a ridge Draw soil from either side into a rounded ridge ten inches (25 cm) high. Round, not flat-topped: a flat top holds puddles.
- Firm the sides, leave the top loose Pat the flanks with the back of a rake so they hold their shape in rain, but leave the top four inches loose for the roots.
- Cover with black plastic if your season is short Lay black plastic over the finished ridge and bury the edges. It lifts soil temperature by five to nine degrees F (three to five C) and suppresses the weeds you cannot reach once the vines run.
Feeding: what to add and what to leave out
Work a potassium source into the top six inches of the ridge as you build it. Wood ash at a couple of handfuls per yard of ridge, or sulfate of potash at the rate on the bag, is plenty. If your soil is genuinely poor and has grown nothing for years, add a light dressing of a low-nitrogen general fertilizer, something with a middle and last number higher than the first.
Leave out fresh manure, poultry pellets, blood meal and any high-nitrogen compost. Half-rotted manure causes a second problem beyond the vine growth: it attracts wireworm and encourages the soft rots that show up in storage. If you want to improve the structure of heavy ground, use fully finished leaf mold or spent mushroom compost, which add bulk without much nitrogen.
| Amendment | Use it? | Reason |
|---|---|---|
| Wood ash or sulfate of potash | Yes, worked into the ridge | Potassium drives sugar into the roots |
| Finished leaf mold | Yes, for structure on clay | Loosens soil with almost no nitrogen |
| Balanced granular fertilizer | Only on very poor ground, half rate | Easy to overdo and get vine |
| Fresh or half-rotted manure | No | Vine growth, wireworm, storage rots |
| Lime | No | Raises pH into the range that favors scurf |
Rotation and what grew there last year
Sweet potato is not a nightshade, so it does not share diseases with tomatoes or potatoes and can safely follow them. What it should not follow is itself. The two problems that build up in soil, black rot and scurf, both persist in the ground for three or four years, and a bed that produced scabby roots one year will do it again the next.
The ideal previous crop is a heavy feeder that stripped the bed: sweetcorn, brassicas, or a long-standing crop of squash. Ground where a cover crop of grass or cereal rye was recently turned in is worth avoiding for one season, because that residue is where wireworm larvae live and they will tunnel straight into your roots.
- Wait three years before repeating Do not grow sweet potato in the same bed more often than one year in four. Mark the bed on a plan so you are not relying on memory next spring.
- Follow a hungry crop Sweetcorn, cabbage or squash leave the ground exactly as depleted as sweet potato likes it. No extra preparation is needed beyond loosening and a potassium dressing.
- Avoid recently turned grass Ground that was lawn or grassy cover crop within the last year carries wireworm. Give it a season under a different crop first.