The soil used for plants in containers has different attributes than the soil found in the garden. It must be lighter in weight, have larger pore spaces, provide drainage and add nutrients. Potting soil purchased in bags generally contains all of the elements needed to fulfill the potted plant's soil and nutrient requirements. There are specialized potting soils formulated to serve particular needs, such as plant-starter potting soil and soil formulated to accommodate specific plant types, like cactus and other succulents.
Soil Requirements of Plants
Plants require a soil that allows the plant to anchor its roots into the soil to provide support. The roots of the plant are where most of the moisture is taken in and where nutrients are absorbed, so plants need a soil that has enough density to support the plant. At the same time, the soil needs to have adequate air spaces to allow the tender young roots to grow and spread. Air spaces also allow water to penetrate and move easily throughout the soil, so the roots can in turn absorb it and the nutrients that it carries.
Potting Soil Components
Potted soils are formulated to fulfill the soil requirements of plants. Peat moss is used for density and support. Peat moss, harvested from peat bogs, is acidic in nature and can hold up to twice its weight in water. Used alone in a container, it could create a water-logged situation, which does not allow for air movement and would cause the plant roots to rot. When combined with a light, airy material such as perlite, pumice or vermiculite (also natural materials), air spaces are created. This union creates a potting soil that can absorb water, but also allows for water movement throughout the soil so the plant roots can use it. The excess water then moves out of the soil, adding good drainage to the potting soil attributes.
Potting Soil Nutrients
Plants growing in the ground can draw from nutrients that naturally exist in the soil, but plants planted in potting soil must be supplemented with nutrients. Naturally composted materials are sometimes added to potting soils to provide nutrients. Due to the bulkiness and weight of composted materials, time-release, pelleted fertilizers that contain complete nutrients, including nitrogen, phosphorous, potassium and sometimes iron, are commonly added to potting soil mixes. The fertilizer will break down slowly as the plants are watered, making the addition of fertilizer unnecessary for as long as three months after planting.
Special-Purpose Potting Soils
Special-purpose potting soils are designed to fulfill varied uses. Potting soils to be used as an amendment to the native soil in the ground will have a higher content of organic materials so that the components blend and work well with each other. A soil that is specially formulated for starting seeds will have fewer or no nutrients added, as seedlings are in more need of air space and moisture than nutrients to generate new, tender roots.
Crop-Specific Potting Soils
Cactus and succulent potting soils are very light, with ample quantities of pumice added to allow for quick-draining soils. Houseplant potting soil mixes are well-draining and have time-release fertilizers added to provide constant nutrients to hungry plants. Tree and shrub potting soils have additional composted materials to create a heavier mix to support the maturing root systems and to release nutrients over a longer period of time.