Classified as legumes, beans (Phaseolus vulgaris) manufacture their own nitrogen directly from soil and, therefore, usually don't need more nitrogen than the amount they produce during the growing season. The soil may be lacking in other nutrients, however, and the only way to assess it is by performing a soil test. Otherwise, adding a low-nitrogen fertilizer at bean planting time and when the plants flower and produce bean pods generally meets the plants' nutritional needs.
Beans are a warm-season crop that can be planted in U.S. Department of Agriculture plant hardiness zones 3 through 10 when the soil warms to at least 50 degrees Fahrenheit. The National Gardening Association describes beans as light feeders that normally require very little additional fertilizing. If you choose not to have the soil's nutrient levels tested, then use a blend of 5-10-10 fertilizer, which provides nitrogen, phosphorus and potassium, the three most important nutrients that beans need. Fertilizer blends are gauged according to the percentage value of nitrogen, phosphorus and potassium they contain, and the percentage often appears on the packaging, with nitrogen represented by the letter N, phosphorus represented by the letter P and potassium indicated by the letter K. The rest of a fertilizer is composed of trace nutrients, substances such as copper and magnesium, which beans need less of than the three primary nutrients.
Apply 5-10-10 fertilizer either on the day of planting or the day before you plant, using it at a rate of 3 to 4 pounds for every 100 square feet of soil surface, such as every 100 feet of plant row or an area that measures 10 feet long by 10 feet wide. Work the fertilizer into the soil to a depth of 2 to 3 inches.
Cured compost and aged animal manure contain the same nutrients found in chemical fertilizer. Although they tend to work more slowly than chemical fertilizer, they deliver their benefits in a more regular and stable fashion and improve soil's texture. Cured compost or aged animal manure can be added to the soil at planting time or the day before planting. Use either of them at a rate of 3 to 4 pounds for every 100 square feet of soil, 100 feet of future plant row or an area that measures 10 feet long by 10 feet wide, and work it into the soil to a depth of about 6 inches.
If, however, you use fresh compost or fresh animal waste, then work it into the soil in fall or several months before planting time, which gives the material time to break down, resulting in less plant root burn from the heat generated by the material's decomposition. Use fresh compost or fresh animal waste at the same rate you would cured compost or aged animal manure to add the material to garden soil.
Second Application of Fertilizer
The beans' soil can be fertilized again after the blossoms open and the bean pods appear if you want to increase the plants' yield. Work into the soil the same amount of 5-10-10 fertilizer, cured compost or aged animal manure that you used during the first application of fertilizer. Place it along the bean rows a few inches from the plants' bases, and work it into the soil to a depth of about 1 inch.
Don't use fresh compost or fresh animal waste for this fertilizer application.
The process by which legumes such as beans manufacture their own nitrogen is called "fixing." Soil-borne bacteria attach themselves to the plants' roots, where the bacteria extract the nitrogen that is present in the soil as ammonia. The bean plants are then able to absorb the nitrogen in the amounts they need to produce flowers and pods. It's a good idea to keep this process in mind when using chemical fertilizers because excessive nitrogen produces lush foliage growth on bean plants but fewer beans.