To produce their own food, plants need minerals. They need six macronutrients (nitrogen, phosphorous, potassium, calcium, magnesium and sulfur) in substantial quantities and seven micronutrients (iron, copper, manganese, zinc, boron, molybdenum and chlorine) in small amounts. Nitrogen, phosphorous and potassium are often lacking in the soil. Using a fertilizer can improve growth and flowering.
A popular fertilizer for decades, fish emulsion is high in nitrogen, an element that promotes quick, green growth. It's a useful spring tonic for perennials just leafing out or a first feeding for annuals, but not as a regular feeding. Too much nitrogen causes lush growth with few flowers and needs to be balanced by phosphorous and potassium.
Composted steer manure is produced by mixing fresh manure with sawdust or other woody material. A good granular bagged steer manure has just enough nitrogen to get seedlings and perennials started in spring without overwhelming them. Use as a mulch or dig it into the soil to provide extra organic matter, useful for holding moisture and nutrients.
Granular Organic Fertilizers
Look for low-nitrogen formulas such as 5-10-10, often found in rose and flower mixtures or vegetables fertilizers. Either will work. The numbers refer to the percentage of nitrogen, phosphorous and potassium. Organics will have small quantities of micronutrients and should be used whenever possible. Be sure to water the plants before applying fertilizer, mix into the top 1 inch of soil and then water again.
Liquid Organic Fertilizers
Look for organic formulas with a low percentage of nitrogen. Liquid fertilizers are especially useful for hanging baskets and potted annuals. The nutrients are taken up by the roots quickly and produce quick results. Follow the directions on the package to avoid burning the plants with excess fertilizer. The nutrient will leach out of the soil quickly, so you do need to keep applying it on a regular basis.
These are especially useful for absent-minded gardeners with plants that need regular feeding: dahlias, roses, annuals or hanging baskets. You can spread the pelleted fertilizer on the garden once, in late spring or early summer, and they will release small amounts of minerals throughout the growing season. There is also less danger of burning the roots with a slow-release fertilizer, though be sure to follow the directions on the label.