Organic potting soil contains microbiolic organisms that add nutrients to the soil and make the soil reusable.
Non-organic soil contains ingredients such as manure and hummus, these are discarded after just one season.
Soil Weight
Lighter soil is ideal for plants that grow in shaded arrears, however this soil dries out quickly which could damage your plant if not maintained.
Medium soil is used for plants that are exposed to sunlight, this soil holds water more efficiently then lighter soil.
Heavy soil is to be avoided, this will cause problems with air getting to the roots and can contain to much sand. Sand does not hold nutrients which will cause problems.
Fertilizer
Purchasing soil with fertilizer included is beneficial to your plants as it provides added nutrients, this will help your plant develop. Fertilizers come in ratios of Nitrogen, phosphorus and potassium.
Leafy plants require more nitrogen, the ideal mix for this would be 20:10:10.
Flowering plants require more phosphorous, the ideal mix for this would be 10:20:10.
To increase your plants health and disease resistance use more potassium in the ratio of 10:10:20.
