What Fertilizer Should I Use for My Cactus?

Fertilizer Components

• The three main components that occur in fertilizer are nitrogen, phosphorus and potassium. Each component has its own effect on plant physiology. Nitrogen promotes growth of photosynthesizing structures like stems and leaves, while phosphorus aids in development of fruits, flowers, seeds and roots. Potassium helps the plant fight off pathogens and contributes to overall health. In the United States, commercial fertilizer containers displays an NPK analysis, which indicates how much of each of these components the product contains. For example, a fertilizer with an NPK analysis of 10-20-10 contains 10 percent nitrogen, 20 percent phosphorus and 10 percent potassium.

Low Nitrogen for Most Cacti

• Cactus fertilizers should contain no more than 8 percent nitrogren, according to "The Encyclopedia of Cacti." High-nitrogen fertilizers have a deleterious effect on cacti precisely because they promote rapid stem growth. Slow-growing cacti are unable to support such increases, and the new growth becomes lanky, disease-prone and weakly spined. Fertilizers appropriate for cacti include special cactus formulations with low nitrogen as well as general fertilizers intended to promote flower growth.

Fast-Growing Cacti

• Certain species of cacti are fast-growing and can tolerate high-nitrogen fertilizers. These include fast-growing members of the genus Opuntia, such as prickly pears and chollas. Other examples include hedgehog cacti (genus Echinopsis) and Christmas cacti (genus Schlumbergera). As with any plant grown both for foliage and flowers or fruit, switch from high-nitrogen or all-purpose fertilizer to a low-nitrogen, flower-promoting formulation prior to the blooming period. High-nitrogen fertilizers can cause cacti to have few or no flowers.

Trace Minerals and Other Elements

• In addition to the three major nutrients, a cactus fertilizer should contain a variety of trace minerals. According to the book "The Cactus Family," these should include magnesium, iron, manganese, copper, zinc, boron and molybdenum. Lack of trace minerals can lead to poor growth and flower production, especially for cacti grown in soil-free potting mixes or specimens growing in the same soil for extended periods. Other elements useful for cacti include calcium, which leads to healthy spine growth, and sulfur, which supports overall plant vigor.