Tips for selecting a good potting soil
If you want to grow just a few really good plants in a smaller space and do not want to spend a lot of time in researching, shopping and mixing your own soils, then you may want to just buy some high end pre-mixed potting soil and get to work on the fun parts of planting, growing, and harvesting. But how do you choose a good soil when the bags and advertising seems so similar? Following are some tips for selecting the right soil for your small space garden projects.
Potting soil comes in many formulas, grades and price ranges. There is a difference in quality. We recommend buying from the smaller specialty organic companies. We can take a look at some of these factors and their function or contribution to the mix. We will discuss the soil itself but keep in mind that it also depends upon what you are growing and where! Primary factors to consider in evaluating which soil is best include the ingredients, the proportions (formula) and the processes.
1. INGREDIENTS: Obviously, starting with high quality ingredients is essential. Pumice, sphagnum moss, coco coir, and an organic compost are the foundation of a good All Purpose Potting Soil Mix. High quality ingredients and careful (simple straight-forward) processes are the basis for versatile high performance/quality soil mixes.
• Compost. Manures of one kind or another provide the high nutrient content and make
excellent compost. Some are too hot (contain too much Nitrogen) to use without composting first. The composting process serves a couple of important purposes. Following the National Organic Program (NOP) guidelines includes getting the compost temperature up to a specified range which kills the weed seeds but not the beneficial bacteria. It also involves turning and mixing the compost multiple times so that all of the compost material spends some time in the center of the pile where the heat is. Composting also breaks down the materials so that the nutrients are more readily available and do not take nitrogen from the plants in order to decompose properly. We use horse manure because we have a consistently high quality and reliable local source. They feed the horses pesticide free hay and grain. Wood shavings and/or hay usually come with the horse manure. It holds moisture, builds the soil, and if you have clay it will help prevent so much clumping.
• Peat Moss and Coco Coir. Sphagnum peat moss has been used in potting soils for many years to hold moisture. Coco Coir has many of the benefits and characteristics of peat moss. However, it also has the huge advantage of being a readily available and renewable by-product of coconut farming. We use both.
• Pumice (not Perlite). Pumice is used to aerate the soil and promote drainage. There is a dynamic balance required to retain moisture without getting soggy and having a well drained soil without it drying out too quickly. Pumice is similar in weight to other soil elements and therefore stays mixed and resists ‘floating’ to the surface. Perlite is lighter than most other soil components and when mixed with water it tends to float. That is why a layer of bright white granules is frequently seen on top of some generic potting soils. Pumice is not readily available in all parts of the US. Most pumice mined in the US is from Oregon. We are in Oregon. Pumice is better. We use pumice.
• Worm Castings. The term “mellow” comes to mind. Worm castings have a gentle slow nutrient release and do not ‘burn’ the plant with jolt of nitrogen. It is probably technically an amendment so we will talk about it more next time.
2. PROPORTION: The formula is critical…and also usually proprietary. You will want a good mix of quality ingredients without a lot of fillers (like dirt or bark). If you are planting a wide variety of plants under a variety of conditions, then you may want to use a general potting soil as a base and then customize your soil with a variety of amendments (next blog topic) to match your plants, containers and environment/climate. Another option is to use a premium soil with lots of amendments already mixed in for general planting. Our Premium Potting Soil Mix is awesome and includes the basic potting soil ingredients plus worm castings, bat guano, feather meal, and endomycorrhizae. Then select some special blends, such as a Cactus Mix or an Acidic Mix for specific applications.
3. PROCESS: Care and attention are required during each step in the preparation process. It begins with the sourcing of high quality ingredients. As you probably have seen by now, there are high, medium and low grade products in every category. Some companies just barely meet quality standards. Others voluntarily meet and exceed quality standards. They are continually looking for ways to “make it better” not just “make it cheaper”. Support and buy from people who insist on quality standards similar to your own.
As an example, the National Organic Program criteria for “organic compost” include:
“(iii) Maintained a temperature of between 131 °F and 170 °F for 15 days using a windrow composting system, during which period, the materials must be turned a minimum of five times.”
This temperature range is required to kill pathogens and weed seeds without killing of the beneficial bacterial. As you can imagine, even when the internal temperature is high enough, the outer layer is not. That is one of the reasons the compost needs to be turned and re-mixed multiple times. It is then screened to extract larger non-composted pieces before packaging. It is a time consuming process and it is tempting to skip steps and cut corners to save time and money. We don’t.
Buy the good stuff. Don’t risk your gardening experience on low grade products.
Next time we will discuss Soil Amendments