Mixes made with compost can be high in salts, which can inhibit germination. To test your mix, send it to a reputable lab for greenhouse testing. Remember, this is different from a soil test. For example, Penn State's Ag Analytical Services Lab has a "Greenhouse Soilless Media" test that will analyze your media's pH, soluble salt electrical conductivity , and nutrient content. Media sent in as a soil sample is tested differently and results will not make sense.
Premium potting mixes tested with the saturated paste method recommended for greenhouse media will have a pH between 5. Consider pretesting your potting mix by doing your own greenhouse bioassay.
- Test Your Mix Before You Use It!
- Make Your Own Potting Soil Mix.
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To do a bioassay, grow cress, oats, beans, lettuce, or another fast-growing crop with a high germination rate in your soil mix. If there is a problem with the mix, you will see it in reduced germination or poor seedling growth see sidebar. You may also compare your new mix to a mix that you are satisfied with.
Recently, handheld EC electrical conductivity meters have become more popular and available at reasonable prices. Lettuce seeded in potting mix with high salts right exhibited slowed and reduced germination rates. Even when making your own potting media, it is still important to ensure that the individual components of the media are specifically approved for certified organic production see sidebar, next page. If you are purchasing compost to add to your homemade potting mix, most certifiers will require this compost to be reviewed e. Fertility amendments, peat, coir, and other components must also be approved.
Check for the OMRI label and talk with your certifier.
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When you first start making your own potting mix, it's a good idea to try several different recipes that have worked for other growers and compare how they do on your own farm. A list of common potting mix recipes is provided at the end of this fact sheet.
Many organic potting mixes contain compost, which can provide many benefits. Compost adds organic matter to the mix and supports diverse microbial populations that can suppress soilborne-disease causing organisms Klein and Hammer Microbes break down organic material, releasing plant-available nutrients that are slowly available for your seedlings.
However, growers have increasingly reported problems with compost-based mixes.
Materials and Methods
This may be because they rely on microbial release of nutrients, which may occur too slowly to meet plant needs. A recent study compared 20 organic potting mixes Leonard and Rangarajan They found that transplants grown in potting mixes that contained blood meal or alfalfa meal in addition to compost were significantly larger. This was probably in response to ammonium nitrogen N levels two to three times higher than that of mixes without either compost or blood meal amendments.
It may be a good idea to use a mix with a more readily available N form, like blood meal or feather meal, in addition to compost. Blood meal seems to stimulate microbes and increase nutrient availability from compost. If you use compost, make sure you are using high-quality compost at the right stage of maturity.
Unfinished compost may release volatile organic acids that can negatively affect seedling growth and development Grubinger One classic method of evaluating compost readiness is by smell. Finished compost has a sweet smell.
Anaerobic, sour, or putrid smells are suspicious. If your nose detects an off smell, turn the pile and let it heat again before you consider using it in a mix Klein and Hammer Problems with compost-based mixes often occur during early season transplant production.
This may be because the mix is too cold, especially overnight when greenhouse temperatures drop. Compost supports an active biological system. Growers can also obtain commercially available mixes with different types of biological fungicides added to the mix.
Those containing mycorrhizae, though increasingly common, may have been prepared many months in advance and may contain insignificant concentrations of living inoculum. Compost-based mixes are also available commercially as a substitute for traditional soilless media, especially for organic production. See section below on organic vegetable bedding plant fertility.
Samples from soilless mixes are tested differently than samples from field soil.
Unlike field soil tests that extract nutrients with weak acid solutions, soilless media is mixed with distilled water at a standard dilution and then analyzed. There are three commonly used methods of testing soilless media using water as an extracting solution: saturated media extract SME , dilution method, and leachate Pour Thru. The values that represent each method of testing are different from each other.
Likewise, values for specific nutrients are likely to differ with testing methods.
Always use the interpretative data for the specific soil testing method used to avoid incorrect interpretation of the results. Most soil testing laboratories use the SME method.
The and Pour Thru are methods that can be used by growers on-site using portable soil testing meters. Since different soil testing labs may use different dilutions, it is not advisable to compare soil test results from one lab to those obtained from another. Use one laboratory for consistent results. In addition to carrying out a complete soil test, growers should routinely check the electrical conductivity or soluble salts EC and pH of their growing media. These tests can be done on-site using portable testing meters, or samples can be sent to a University soil testing laboratory.
Taking a Sample: Take several samples at root depth from several containers and mix together in a clean container. Sampling several containers is important because a sample from one pot or flat could be an anomaly values too high or too low misrepresenting the crop as a whole. Sample about 2 hours after fertilizing or at least on the same day. If slow-release fertilizer pellets are present, carefully pick them out of the sample. If the pellets are left in, they can break during testing and this may result in an overestimation of fertility. Be consistent in sampling procedures each time you sample.
A lot of variability can be introduced to tests due to inconsistent sampling and this diminishes the value of testing especially if you are trying to track fertility levels. Take about one cup of the medium and dry at room temperature. Identify each sample on the outside of the bag for your use.
Potting Mixes for Certified Organic Production
The pH can range from 0 very acidic to 14 very basic. Medium pH drives the chemical reactions that determine whether nutrients are either available for root uptake soluble or unavailable for root uptake insoluble. Also remember that smaller cells and plugs are subject to very rapid media pH change. Electrical Conductivity EC or Soluble Salts: Soluble salts are the total dissolved salts in the root substrate medium and are measured by electrical conductivity EC.
Most fertilizers except urea are salts and when placed in solution they conduct electricity. Measuring EC or soluble salts provides a general indication of nutrient deficiency or excess. Sometimes high EC levels occur when root function is impaired by disease or physical damage.
Best Potting Soil for Any Plant – Buying Guide and Recommendation
Always check the condition of the root system when sampling soil for testing. Water Quality and Alkalinity : The quality of water used for irrigation and mixing fertilizers should be tested each year for pH, alkalinity and electrical conductivity. Water containing a large concentration of dissolved salts can cause high soluble salts damage. Water alkalinity is a measure of the water's capacity to neutralize acids. An alkalinity test measures the level of bicarbonates, carbonates and hydroxides in water.
Test results are generally expressed as ppm of calcium carbonate.