Unlike fine wines and cheeses, potting mixes do not improve with age. Long-term storage of growing media in bulk or in bags can lead to production problems because physical and chemical properties change over time.
Storage conditions definitely come into play here. Mixes exposed to the elements can become dry and hard to wet or they may become too saturated in rainy, humid weather. Wet media is easily compacted resulting in poor aeration and drainage around the root system. Sunlight may break down plastic packaging, exposing the contents to contamination by weed seeds or diseases as with uncovered bulk mixes.
Most blends these days include some type of fertilizer. Starter charges of fertilizer can be consumed by naturally occurring micro-organisms as the mix ages. It may be necessary to apply additional fertilizer sooner than expected to avoid nutrient deficiencies. Any slow-release fertilizers are affected by temperature and moisture. Since all mixtures contain some moisture, releases begin soon after blending. High temperature and moisture cause quicker release and potentially high levels of soluble salts. In addition, pH and nitrogen levels may change over time, depending on mix components and moisture levels.
What can be done to avoid these problems? Use growing media as soon as possible. Don’t purchase more growing media than you can use in six months and store it in a cool dry place. In outdoor storage situations, use water impermeable barriers over top to prevent leaching, moisture retention, and contamination by weeds and diseases. This is a principle of Best Management Practices for Florida Container Nurseries. (http://bit.ly/geIa7n)
Check pH and soluble salts regularly, especially on older mixes. The Manatee Extension office offers this type of soil testing for a small fee.
Nutritional analyses can also be done by UF’s Extension Soil Testing Laboratory (click on Container Media Test) or through private labs. It could save you money in the long run, leaving more for those fine things that do improve with age!