Potting Soil Calculator

Estimating container media is a geometry problem complicated by the fact that almost no flower pot is a true cylinder. A straight-sided round pot holds pi times the radius squared times the depth, but a tapered nursery pot is a frustum, and its volume is pi times height divided by 3, multiplied by the sum of the top radius squared, the bottom radius squared, and the product of the two. Because commercial pots typically have a base diameter of 65 to 75 percent of the rim, applying the plain cylinder formula to a tapered pot overstates the requirement by roughly 30 to 45 percent. A 30 cm round pot 25 cm deep works out to about 17.7 liters as a cylinder and closer to 12.9 liters as a frustum, which is a whole bag of difference across a dozen containers. Two physical properties govern how the numbers translate into practice. Bagged peat-based mixes as sold weigh roughly 0.4 to 0.7 kg per liter and around 0.8 to 1.2 kg per liter once saturated, so a 20 liter window box can gain 10 kg of water weight after irrigation, a real consideration for balconies, brackets and hanging baskets. Second, media are sold by loose volume and are compressed in the bag, and they settle further after the first few waterings, so a 10 percent purchase margin is prudent. A well-formulated mix retains 10 to 20 percent air-filled porosity after free drainage. Note that the perched water table at the base of a container is a property of the medium rather than the pot, which is why a layer of gravel in the bottom raises rather than lowers the saturated zone and does not improve drainage. The cost figure used here is a placeholder rate per cubic foot and should be replaced with local pricing.

Growing plants in containers of field soil was the norm until the twentieth century, and it failed reliably: garden soil compacts under repeated irrigation, drains poorly in a shallow profile, and carries weed seed and soilborne pathogens. The first systematic remedy came from the John Innes Horticultural Institution in England during the 1930s, where William Lawrence and John Newell developed the John Innes composts, standardized loam, peat and sand recipes with defined fertilizer and chalk additions that replaced the idiosyncratic mixes of individual nurseries. The move to genuinely soilless media followed in the United States. Kenneth Baker and colleagues at the University of California published the UC System for producing healthy container-grown plants, issued as Manual 23 in 1957, which paired sand and peat mixes with strict sanitation. At Cornell University in the 1960s James Boodley and Raymond Sheldrake formulated the Peat-Lite mixes, combining sphagnum peat with vermiculite or perlite plus lime and starter nutrients; these are the direct ancestors of nearly every bagged potting mix on retail shelves today. The mineral components had become available in bulk after the Second World War, perlite being volcanic glass expanded by heating and vermiculite an exfoliated mica. Since the 1990s coconut coir has been increasingly substituted for peat as concern over the carbon stored in peat bogs has grown, and the United Kingdom has pursued phased restrictions on peat in retail growing media.