The three numbers that indicate fertilizer types stand for the percentages of nitrogen, phosphorous and potassium in a fertilizer. A 10-10-10 fertilizer has a balanced ratio of these three essential nutrients.
Mississippi State University Office of Agricultural Communications recommends balanced fertilizers, like 10-10-10 fertilizers, for garden beds with varied plants in them. Gardeners can have a lab or university test the soil for nutrients and then use a balanced fertilizer if the plants need more nitrogen, phosphorous and potassium. Gardeners should also use them if plants show signs of a nutrient deficiency, like very slow growth.
Gardeners should apply more fertilizer during the growing season and less fertilizer during seasons of slow growth or dormancy. For most plants, this means applying more balanced fertilizer during the early spring through mid-summer and then applying less during the cool seasons. However, plants that bloom during winter will appreciate winter fertilizer applications.
Washington State University warns that gardeners often overuse balanced fertilizers. Most plants use up the nitrogen in balanced fertilizers more quickly than the phosphorous or potassium. These leftover nutrients often end up in runoff and cause harmful algae blooms in nearby bodies of water. Gardeners should substitute high-nitrogen fertilizers for 10-10-10 fertilizers for very leggy plants, like corn and grass, during times of rapid growth during the spring.