Most vegetables want 12–18 inches of soil depth, but root depth varies by crop — shallow-rooted lettuce can get by in 6 inches, while a deep-rooted tomato or squash wants real room to stretch.
| Depth | Examples |
|---|---|
| 12–18" | Lettuce, spinach, beans, cabbage, broccoli, onions, radishes, corn |
| 18–24" | Peppers, cucumbers, peas, carrots, potatoes, kale, chard |
| 24–36"+ | Tomatoes, asparagus, winter squash, pumpkins, sweet potatoes |
Fresh compost feeds plants for roughly 4–6 weeks. After that, side-dress with an all-purpose organic fertilizer at about 1 cup per cubic foot of soil per season, or follow your specific product's label rate. Worm castings are gentle enough to add generously at any time.
This calculator gives you loose-fill volume, but rain and watering compact soil over time, so the level in your bed will drop a bit after the first season. Filling to the top (or slightly over) at the start accounts for that settling, rather than ending up short by midsummer. Topping off with a bit more compost each spring is completely normal, not a sign anything went wrong.
The NPK Balancer checks your fertilizer against a plant's generally ideal ratio, and scales the application rate to a reasonable baseline nitrogen target. That's genuinely useful for a fast check at the garden center — but it doesn't know what's already in your soil.
Professional agricultural recommendation tools (the kind university extension services offer) start from an actual lab report of your soil's existing phosphorus and potassium levels, then calculate exactly how much more you need for your specific crop — because soil that's already rich in phosphorus needs a very different fertilizer than soil that's depleted, even for the same plant. If you're managing anything beyond a home garden, a real soil test will always beat a generic ratio.
Looking for mix ratios instead? See the Soil Mix Guide, or head back to the calculator.