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The soil in many home vegetable gardens is highly fertile due to the use of added fertilizer over the years. Submitting a soil sample to the WVU Soil Testing Laboratory tells us if there is sufficient phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg) in the soil. If needed, more of these nutrients can be provided by applying fertilizers or lime. When these nutrients are in the optimal or excess range, the addition of these amendments is not needed. Continuing to add amendments to the garden soil does not increase yields, can cause excess nutrients to be lost to the environment, and is a waste of money.
When the soil nutrients P, K, Ca and Mg are above optimal, vegetable crops only need nitrogen (N) to stimulate growth (Table 1). When legume cover crops, such as hairy vetch or crimson clover, are used in the garden to build soil organic matter and improve soil health, they also will provide a significant portion of this nitrogen requirement. For example, a thick, mature stand of crimson clover can provide over 100 pounds of N per acre when tilled into the soil. When a crop, such as white potatoes, that requires 175 pounds of N per acre follows this crimson clover stand, only 75 pounds of N per acre of additional N (175 pounds needed - 100 pounds provided by cover crop = 75 pounds) is needed to produce the crop (6 ounces of urea per 100 square feet). When additional N is needed the simplest and lowest cost source of N is urea fertilizer (46-0-0).
The N needed by vegetable crops is the total N for the growing season (Table 1). Since N is not stable in the soil, the plants respond best if no more than half is applied to the soil prior to planting the crop, with the other half of the recommended N applied when the crop is growing rapidly. Urea is most effective when it is tilled or watered into the soil to prevent loss into the air through volatilization. Urea is a natural product and nature provides abundant urease enzyme on and in the soil. This naturally occurring enzyme converts urea to ammonia, which, if not cultivated or watered into the soil, can volatilize into the air and be lost from the soil. Within the soil, the ammonia attaches to clay particles and is not lost into the air.
Fertilizer recommends in Table 1 are based on an area of 100 square feet. This area is fairly easy to see in the garden. When a bed is 4 feet wide, 25 feet of bed length equals 100 square feet in area. When rows are 30 inches (2.5 feet) apart, a 40-foot row is equal to 100 square feet.
High fertility garden soils commonly only need nitrogen to stimulate plant growth. This nitrogen is best provided by growing winter cover crops and by applying multiple small applications of a high nitrogen fertilizer like urea during the growing season.
Table 1. Nitrogen (N) needs of vegetable crops and the amount of urea fertilizer
(46-0-0) per 100 square feet of garden bed needed per year to meet that need.
Relative N need
N Class (pounds N per acre per year)
Crops
Ounces urea per 100 square feet per year
Low
50
Asparagus (growing crowns), kohlrabi, radishes, rutabagas, turnips, strawberries (maintenance)
4
75
Snap beans, carrots, head lettuce, parsnips, peas (shell, sugar snap, snow), sweet potatoes
6
Medium
100
Asparagus (maintenance), lima beans, beets, mustard and turnip greens, bulb onions, pumpkins, winter squash, summer squash, tomatoes (bare ground)
8
125
Garlic, leeks, leaf lettuce, endive, escarole, Brussels sprouts, cabbage, cauliflower, cucumbers, eggplant, muskmelons, cantaloupes, okra, strawberries (establishment), watermelons
10
High
175
Celery, parsley, peppers, white potatoes, sweet corn
14
200
Broccoli, collards, kale, mustard, horseradish, green onions, spinach, tomatoes (plastic mulched)
16
Authors: Ed Rayburn, Retired WVU Extension Specialist – Forage Agronomy; Tom Basden, WVU Extension Specialist – Nutrient Management; and Lewis Jett. WVU Extension Specialist – Commercial Horticulture
Last Reviewed: July 2022
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