How to Fertilize Sweet Potatoes: Organic Feeding for Bigger Harvests

Sweet potatoes don’t need heavy feeding, but the right organic nutrients at the right times can dramatically improve vine vigor and tuber size. After more than three decades growing sweet potatoes in warm Sonoma Valley summers, I’ve learned that too much nitrogen leads to lush vines and small roots—while balanced organic feeding keeps tubers big, firm, and sweet.

Below is the exact fertilizing method I use in raised beds, mounded rows, and containers.

Table of Contents

Start With Fertile, Loose Soil

Sweet potatoes grow best in loose, sandy loam enriched with compost. Before planting slips, I prepare the bed with:

2–3 inches of compost worked into the top 8–10 inches
A light application of organic all-purpose fertilizer (4-4-4 or close)
A handful of bone meal per planting hole for phosphorus

This gives sweet potatoes the slow, steady nutrition they prefer.

Experience Tip:
I avoid high-nitrogen fertilizers—anything over a 5 in the N position—because they encourage vine growth at the expense of tubers.

The Three Key Nutrients for Sweet Potatoes

1. Phosphorus (P)

Essential for root development.
Bone meal, rock phosphate, and soft rock phosphate are my preferred organic sources.

2. Potassium (K)

Improves tuber size, flavor, and disease resistance.
Kelp meal, langbeinite, and greensand are ideal organic amendments.

3. Low Nitrogen (N)

Needed only in small amounts.
Nitrogen from compost is usually enough.

My Insight:
After years of trial and error, I’ve found that sweet potatoes grown in beds enriched with compost and a phosphorus-rich amendment consistently produce fatter tubers than those grown with traditional vegetable fertilizers.

Fertilizing Sweet Potatoes by Growth Stage

1. At Planting: Slow, Balanced Nutrition

Mix into the top soil:

1–2 cups organic 4-4-4 fertilizer per 10 square feet
½ cup bone meal per planting row
Optional: a light sprinkle of kelp meal

This sets the foundation for a strong start without overstimulating vine growth.

2. 4–6 Weeks After Planting: Potassium Boost

When vines begin to run, I use a light side-dressing of:

½ cup kelp meal OR
¼ cup langbeinite per 10 square feet

This improves tuber bulking and reduces stress during heat waves.

Experience Tip:
I apply potassium right before a deep watering so nutrients move into the root zone evenly.

3. Midseason (8–10 Weeks): Optional Feeding

If vines look pale or growth seems slow:

Add a small handful of compost around each plant.
Avoid nitrogen-rich fertilizers at this stage.

Most seasons, compost alone is enough.

4. Late Season: Stop Fertilizing

When vines begin to yellow naturally, I stop all fertilizing.

Fertilizing too late encourages soft tubers and delays maturity.

Fertilizing Sweet Potatoes in Containers

Containers dry out faster and nutrients leach more quickly, so I use:

A fertile potting mix with compost blended in
½ cup organic granular fertilizer mixed into the soil at planting
A midseason boost with liquid kelp or a light sprinkle of kelp meal

Experience Tip:
In containers, I watch the foliage closely—pale leaves often mean potassium is low.

Signs of Proper (and Improper) Fertilization

You’re fertilizing correctly if:

Vines are green and vigorous (not dark green and overly lush)
Roots are swelling by midseason
Soil stays loose and crumbly

You’re over-fertilizing if:

Vines grow 6–10 feet but tubers stay small
Leaves are deep green and oversized
Tubers form very late in the season

You’re under-fertilizing if:

Leaves pale or yellow prematurely
Overall vine growth is weak
Tubers are small and spindle-shaped

Final Thoughts

Sweet potatoes thrive with modest but well-timed organic feeding. Compost, bone meal, and potassium-rich amendments are the foundation of big, sweet, healthy tubers. With steady moisture, warm soil, and the right nutrients at planting and early growth, your sweet potato harvests can be abundant—even in small spaces or containers