Soil condition has a tremendous impact on plant productivity and health when planting veggies and flowers. If you have raised beds or containers for these types of plants, it is recommended that you examine the condition of the soil each year and determine whether or not to add the right amount of necessary amendments, such as: bark, coir, compost, peat moss, perlite or vermiculite, sand or sulfur. The water in the desert has high levels of pH, which requires sulfur instead of lime.
Typically, containers are filled with potting soil, and this composite product is often comprised of peat moss, fine bark and/or compost, coir, perlite or vermiculite and small amounts of sand. Dr Q’s Filthy Rich Potting Soil Plus and Vegetable & Herb Mix are both top quality potting soils.
Coir is made from coconut husks, while perlite and vermiculite are mined or extracted then processed from various minerals, which provide the soil with the capacity to retain water. Perlite and vermiculite also add air spaces (or porosity) to the soil. A good quality potting soil should provide adequate water retention while also helping the roots to spread out in order to obtain oxygen. Perlite does not break down as much, which means if this amendment is chosen for use, not as much will need to be added later on. Peat moss also provides porosity but not water retention, and it breaks down to a significant degree through natural composting processes, which means that is it recommended to add a small amount of peat moss each year. If too much is added plants can yellow excessively due to the high carbon-nitrogen ratio.
Adding the amendments
First – thoroughly mix the soil in a wheelbarrow or large container, keep an eye out for any insects and moisten adequately. Feel around in the soil to determine if it lacks porosity or the ability to give the roots air space – the purpose of wetting the soil is to see how easily it will form a squeezable soil-roll. If the soil roll extends for ¾ inch and then breaks, this means that the pore spaces and water retention are adequate. If the roll can be forced into an inch and a half or longer, it is too fine and will require an aeration amendment (like perlite, vermiculite or more peat moss). Avoid adding more than 10%-15% peat in any year to keep the c/n ratio in check. If the roll breaks before it reaches ½ inch the soil will likely not retain water well, and coir will need to be added.
Next, check drainage – after adding the necessary porosity amendment put the soil back in the pot, saturate it with water and pay close attention to how it drains. The water should not form a puddle, and any drainage that comes out of the bottom should cease completely within approximately five minutes. Water retention should also be double-checked using a moisture meter roughly 24 hours after saturating the soil. Adding sand will increase drainage, while adding coir will increase water retention.
Soil for Raised Beds
Since raised beds are generally much larger, the cost of filling them with potting soil is usually prohibitive. A good quality bulk Top Soil Mix like what Star Nursery carries is usually the most economical option for soil requirements for a raised bed. A combination of washed sand or silt and a quality organic compost will provide a good balance to ensure proper plant health and growth. As the compost in the bed ages, it will break down into finer particles causing the porosity and drainage will diminish. Peat moss is an excellent additive for raised beds to keep the soil friable or provide easy oxygen access to the roots. Perlite or vermiculite can also be used for this purpose as well, but it is important to remember that they each also add water retention. If the soil in a raised bed drains too fast, adding about 5% coir will make a substantial difference in water retention.