Good potting soil is essential for houseplants. Because potted plants live within a small, defined area, generalized potting soil mixes have a fertile base that usually contains peat moss or coconut husks, pine bark, perlite or vermiculite and sometimes compost and top soil. Potting soils should be light and fluffy. Most commercial potting soils contain added essential nutrients and micronutrients that plants need to grow. Specialized potting soils for plants with specific needs, such as African violets, are also available.
Plants need nitrogen in large quantities as it is a necessary component of chlorophyll needed for photosynthesis and other plant metabolic processes. Common nitrogen sources in potting soil include blood meal, fish meal, feather meal, soybean meal and bat guano. Some of these sources, such as blood meal, release nitrogen faster than their counterparts and may be found in specialized mixes.
Potassium and Phosphorus
Along with nitrogen, potassium and phosphorus form a trio of macronutrients essential for plant growth. Potassium is essential for virtually all plant metabolic processes, while phosphorus aids flower and fruit production along with root growth. The weathering of parent rocks produces most potassium in natural soils. Potting soil phosphorus sources include bat and seabird guano, bone meal and rock phosphate. Hardwood ash, kelp meal, greensand and langbeinite may be sources of phosphorus.
Calcium and Magnesium
The elements calcium and magnesium, along with sulfur to a lesser extent, are other macronutrients found in potting soil. Calcium is essential to building cell walls and helps retain other nutrients. Dolomitic lime, gypsum and superphosphate are common sources of calcium. Magnesium is essential for photosynthesis and helps activate plant growth enzymes. Organic additives and dolomitic lime are sources. Sulfur plays a variety of roles in production of protein and enzymes. It also improves root growth and seed production. Found in gypsum, sulfur may be added to potting soil through fertilizer.
Micronutrients are elements essential for proper plant growth, but only in small quantities. These are also called trace elements, although the American Society of Agronomy and the Soil Science Society of America prefer the term micronutrients. Micronutrients are boron, copper, iron, chloride, manganese, molybdenum and zinc. Specialized potting soils may include higher amounts of certain micronutrients. Potting soil for acid-loving plants, for example, includes higher amounts of manganese. Sources of trace elements include oyster shells, dolomite lime, glacier rock dust and alfalfa meal.