Fertilizing

Discover Your Garden’s Fertilizer Needs

The amount of fertilizer to apply depends on the natural fertility of the soil, amount of organic matter, type of fertilizer, and the vegetables being grown. Get a soil test to determine your garden’s fertilizer needs.

In addition to soil testing, you also must measure your garden to determine the number of square feet it occupies. Garden fertilizer recommendations are based on 1,000 square feet, and an area of 1,000 square feet could measure 25 by 40, 20 by 50, 30 by 33, or other dimensions according to your plot layout.

If your area is smaller than 1,000 square feet, divide the actual area by 1,000; then multiply the decimal figure by the recommended lime and fertilizer rates. For example, if your plot measures 16 by 24, the area contains 384 square feet; 384 divided by 1,000 equals .384; multiply .384 by your recommended fertilizer rate to determine the amount of fertilizer to apply.

Vegetable plants require many different nutrient elements for good growth and production, but nitrogen (N), phosphorus (P), and potassium (K) are the three nutrients of concern to most gardeners. Calcium (Ca) and magnesium (Mg) are supplied by limestone. The other required elements are obtained from air, water, and soil.

Mixed fertilizers are normally sold by grade and contain two or three major plant nutrients. The numbers in the grade refer to the percent nitrogen (N), available phosphate (P2O5), and available potash (K2O).

Fertilizer sources of the major plant nutrients are ammonium sulfate (21 percent nitrogen, 21-0-0), a blend of ammonium sulfate and urea (34 percent nitrogen, 34-0-0), nitrate of soda (16 percent N, 16-0-0), calcium nitrate (15.5 percent N, 15.5-0-0, 19 percent calcium), urea (46 percent N), superphosphate (46 percent P2O5, 0-46-0), and muriate of potash (60 percent K2O, 0-0-60). Because many garden soils have been heavily fertilized for years, soil test results often indicate extremely high soil levels of phosphorus and potassium. In these cases, nitrogen is the only fertilizer recommended, since additional phosphorus and potassium are unnecessary.

Where nitrogen is the only fertilizer recommended, the usual recommendation is for 3 pounds of 34-0-0 (3 pints) per 1,000 square feet of garden space prior to planting. Fertilization with unnecessary nutrients can “salt out” the garden and damage plant growth.

Vegetables differ in their fertilizer requirements. Leafy greens like mustard, turnips, collards, cabbage, and spinach are heavy users of nitrogen. Broccoli and sweet corn also require more nitrogen than some other vegetables. While nitrogen is important to the plant growth of fruit and root vegetables, phosphorus and potash are important to the proper development of roots and seeds. Peanuts, southern peas, and beans get nitrogen from the air and do not require heavy nitrogen fertilization. Overfertilizing these vegetables with nitrogen causes excessive growth of leaves at the expense of the fruit.

Apply fertilizer before or at planting. Two methods of application are “in the row” and “broadcast.” For most gardeners, the broadcast method is more practical.

To broadcast, spread the recommended amount of fertilizer evenly over the soil surface and then thoroughly mix it into the soil during soil preparation. Heavy feeding vegetables need additional fertilizer (side-dressing) after the plants are well established.

For row application, apply the recommended fertilizer to the row. Mix it thoroughly with the soil so that it will not damage the seed and tender plants.

Fertilizer can be applied in a combination of broadcast and row applications. Broadcast two-thirds of the recommended fertilizer over the entire garden surface and mix it into the soil. Apply the remaining one-third of the fertilizer in furrows 3 inches to either side of the row and slightly below the level of the seeds.

Nitrogen fertilizer applied before or at planting time usually does not supply all the nitrogen needed during the growing season for heavy- and medium-feeding vegetables. Also, irrigation and rain can leach water-soluble nutrients, especially nitrogen, into deeper areas of the soil, out of the reach of the roots of shallow-rooted vegetables.

Apply (side-dress) additional nitrogen fertilizer along the row 4 to 6 inches from the base of the plants when plants are established, being careful to keep all fertilizer off plant leaves.

Side-Dress Applications of Nitrogen

(1 pint of 34-0-0 per 100 feet of row, 31⁄3 tablespoons per 10 feet of row)

• Beans at 3- to 4-leaf stage
• Beets, carrots 4 to 6 weeks after planting
• Bell peppers, eggplant, tomatoes after first fruit set and again at 4- to 6-week intervals
• Broccoli, cabbage, cauliflower, Brussels sprouts 3 weeks after transplanting or after danger of late freeze in spring; broccoli again when heads begin to show
• Cucumbers, muskmelons, watermelons, winter squash when vines begin to run
• English peas when plants are 4 to 6 inches tall
• Irish potatoes when sprouts break through soil surface
• Leafy greens (mustard, turnips, chard, collards) when plants are about one-third grown
• Lettuce, kohlrabi, Chinese cabbage 2 weeks after transplanting; 4 weeks after sowing seed
• Okra after first pods are harvested
• Onions (green and bulb)—from sets when tops are 6 inches high —from transplants when established and actively growing
• Peanuts none
• Radishes none
• Southern peas none
• Summer squash before bloom when plants are 8 to 10 inches tall and again in 4 weeks
• Sweet corn when 8 inches high and again when knee high
• Sweet potatoes none
• Turnips (roots), rutabagas 4 weeks after sowing seeds
• Tomatoes when first fruit are 1 inch in diameter; again at first harvest