Fertilizers usually have 3 numbers on the label that tell you how much nitrogen (N), phosphate (P) and potassium (K) they contain by weight. For example, a fertilizer with an NPK of 10-15-5 means that 10% is nitrogen, 15% is phosphate and 5% is potassium. The rest (70%) are other ingredients.
Common high nitrogen fertilizers are:
- Ammonium sulfate (21-0-0). Contains 28% sulfur. pH lowering
- Calcium nitrate (15-0-0). Contains 17% calcium. pH neutral
- Urea (47-0-0). pH neutral.
- Blood meal (12-1-1). pH lowering. May burn young plants if too much.
- Feather meal (12-0-0). pH neutral.
- Fish emulsion (5-1-1). pH neutral.
Blended fertilizer products usually add nitrogen from one of these or other sources so check the label. It is more economical to buy a nitrogen fertilizer without additional ingredients that you probably don’t need.
For an average plot size of 600 sq. ft., this chart will help you calculate the amount you may need. Adding too much nitrogen may harm plants.
| NPK | Weight in pounds |
| 10-0-0 | 16 |
| 15-0-0 | 12 |
| 21-0-0 | 8 |
| 27-0-0 | 6 |
| 48-0-0 | 4 |
Small amounts of nitrogen may be added every 3 to 4 weeks during the growing season. If compost is not added to the soil, it may be necessary to add a small amount of phosphate at the rate of 4 lbs. of a 0-20-0 fertilizer once a year. Scale down these values if your growing area is smaller.