Vegetable Gardening – Nitrogen Recommendations

Introduction

Soil should contain an adequate supply (40 ppm) of nitrogen (N) when the garden is planted. A soil test conducted the previous fall or in early spring is the best way to determine if more nitrogen is needed. As the plants use up the soil’s supply of nitrogen, they become starved for more. While N starvation may not be obvious it may affect the quality and quantity of the vegetables you harvest. 

Note: If a soil test indicates a high level of N to start the season, additional N during that growing season may not be needed. 

If a soil test was not conducted, the application of eight ounces (one-half pound) of N per one-thousand square foot of the vegetable garden, prior to planting is recommended. In areas where beans, peas, or other legumes are to be planted the application of N should be avoided as this can reduce the yield of these vegetables. 

Calculating the amount of fertilizer needed

The recommendations in this fact sheet are based on the weight of N, not the weight of the fertilizer that needs to be applied. Synthetic fertilizers¹¹ and organic products used as fertilizers contain varying levels of N. To ensure you don’t over apply nitrogen, the percentage of the N in the product needs to be considered when applying it. The recommendation of four ounces of N per one-thousand square foot area of garden does not mean four ounces of fertilizer should be applied. 

To determine the amount of fertilizer needed to apply the recommended amount of fertilizer, divide the ounces of N recommended by the percentage of N in the fertilizer. 

Examples

Ammonium Sulfate with 21% N: If four ounces of N is required, divide 4 by .21 (% of N in the product). Thus 19 ounces (1 pound plus three ounces) of ammonium sulfate are needed to supply the 4 ounces of N needed. Note: One cup of ammonium sulfate weighs approximately 8 ounces. 

Corn gluten with 9% N: When four ounces of N is needed then slightly over 2 ¾ pounds of corn gluten needs to be applied (4 ÷ 0.09 = 44.4 ounces (2.78 pounds). Note: One pound=16 ounces. 

Recommendations 

The following information provides guidelines for additional amounts and timing of N needed by vegetables during the growing season, to ensure they produce the best crop possible. 

Asparagus 

Apply 1.6 to 2.4 ounces of N per 100 square foot area in early spring as the asparagus emerges and again after the last harvest in June. 

Beans 

Use an inoculum of nitrogen-fixing bacteria–do not side-dress²² with N. 

Broccoli 

Side-dress with 4 ounces of N per 250 feet of row when plants are half grown. 

Brussels sprouts 

Apply one side-dress application of 4 ounces of N per 250 feet of row when plants are 12 inches tall. Water appropriately to keep the crop growing vigorously during the heat of summer. 

Cabbage 

Side-dress with 8 ounces of N per 250 feet row when plants are half grown. 

Carrots and beets 

Side-dress with 4 ounces of N per 250 feet of row, 4 to 6 weeks after planting. Do not apply fresh manure; misshapen roots may result. 

Cauliflower 

Side-dress with 4 ounces N per 250 feet row when plants are half grown. 

Eggplant 

Side-dress with 4 ounces of N per 250 feet of row when plants are half grown and again immediately after harvesting the first fruits. 

Leafy Vegetables 

Side-dress with 12 ounces (¾ pound) of N per 250 feet of row, 4 to 6 weeks after planting. 

Onions 

Apply 8 ounces (one-half pound) of N per 100 feet of row 3 weeks after planting. Repeat every two to three weeks until the necks start to soften. 

Avoid applying N after bulbs begin to form as this can result in late maturity, large necks that are difficult to cure, soft onion bulbs, and overall poor storage quality. 

Peas 

Approximately 80% of the N requirement of peas comes from nitrogen fixation by Rhizobia. Apply one-half pound of N per 1000 square foot area at seeding if the soil test shows the N-NO3 level below 5 ppm. This will help ensure nitrogen deficiency does not occur before N fixation occurs. Rhizobia inoculation is strongly recommended. 

Peppers 

Apply 4 ounces of N per 250 feet of row after the first flush of peppers is set. 

Potatoes 

Apply 12 ounces of N per 250 feet row per season as follows: 

30% (3.6 oz) of N at planting; ~20% (2.8 oz N) at tuber set when tubers are approximate nickel size, ~20% (2.8 oz N) two weeks later followed by a final application of ~20% (2.8 oz N) two week after second application but no later than July 31st. 

Sweet Corn 

Apply 8 ounces of N per 250 feet row when plants have 8 to 10 leaves.  

Apply 3.2 ounces (1/5 pound) of N per 250 feet row when silks first appear. 

Tomatoes 

Work 4 ounces of N per 250 feet of row into the soil at planting; Side-dress with another 4 oz N when fruit are about 1/3 grown. After picking the first ripe fruit, side-dress with another 4 oz N. Urea or ammonium nitrate fertilizers are not recommended sources of N. 

Vine Crops – melons, pumpkins, squash, cucumbers, etc. 

When the plants begin to vine, in midseason, apply a side-dressing of 2.5 ounces of N for each 250 feet of row.