Organic Root Media and Fertilizer: Making the Switch From Conventional Work for You
April 10, 2020 
Impatiens grown with feather meal 13-1.4-0.2 (center) and blood meal 14.4-1.4-0.2 (right), both also with bone meal 6.3-33-1.4 and potassium magnesium sulfate, incorporated to provide the same amount of N, P, and K as Osmocote 14-14-14 (left, rate of 7.7 g per liter mix). All were leached twice with a 20% leaching fraction before transplant of the plugs.
Photo by Jason Nelson
With increasing interest from consumers in certified organic and sustainable production, an alternative market niche has developed. But switching from conventional to organic root media and fertilization practices comes with some challenges.
Organic Root Media: Pre-Mixed vs. Mix Your Own
Until about 15 years ago, most organic growers had to make root media from scratch, often using their own compost mixed with peat or soil. Incomplete pasteurization sometimes resulted in weed and disease problems with these home-made root media, leading to inconsistent results. In addition, compost can be expensive to transport because of its weight and odor. Commercially prepared mixes are consistent, ready to use right out of the bag or bale, are less expensive to transport, and do not smell bad. For all these reasons, using a commercially prepared mix holds appeal.
The peat, perlite, vermiculite, dolomitic limestone, and gypsum in most conventional media are acceptable by organic standards. However, synthetic wetting agents and chemical fertilizers used for the pre-plant nutrient charge are restricted items. One way that root media producers have dealt with the challenge of not using synthetic wetting agents is to use a less hydrophobic blend of peats, which are also used by the mushroom industry. Secondly, a higher moisture content — 55% to 60% moisture for organic media as opposed to 40% for conventional mixes — encourages rewetting. One common organic-certified wetting agent is a concentrate derived from Yucca schidigera plants.
Several organic fertilizers are appropriate for use as pre-plant nutrient amendments, with the goal being to provide three to six weeks of nutrients as a starter charge. Pelletized fertilizers work well for this purpose, and a poultry manure/litter formulation is often used in commercial organic mixes.
One Caution with Organic Certification
Organic certification of pre-mixed root media is difficult through the Organic Materials Review Institute (OMRI), because raw materials cannot be certified. The result is that custom mixes for growers cannot automatically meet certification requirements. Regardless, growers who use these media are readily certified as meeting the National Organic Program (NOP) guidelines. An evolving list of approved and restricted products can be found at omri.org.
Organic Fertilizers: Uniform Delivery and Quality Improving
Organic fertilizers include both liquid products, like fish emulsions and hydrolyzates, and solid and slow-release granular products that can be incorporated into the root medium. Organic fertilizer options have consisted of single sources of plant and animal by-products such as bone and blood meals, composted manures, and alfalfa meal that varied in overall quality, nutrient analysis, and nutrient release from batch to batch and source to source. Manufacturers are now providing an array of organic fertilizer products that are more uniform in nutrient analysis and nutrient-release rate.
Fertility Management Not Without a Few Hurdles
Fertility management with organic nutrient sources is significantly more challenging than with conventional fertilizers. Organic fertilizers release nutrients differently than conventional fertilizers, with nutrient availability associated with microbial activity. Factors that affect the activity of microbial populations will influence organic fertilizer breakdown into plant-available nutrients.
For example, cool temperatures will slow conversion of ammonium to nitrate because the bacteria that accomplish this process are functioning more slowly. This is in stark contrast to constant liquid fertilization with soluble ammonium-N and nitrate-N that are immediately available for absorption by plant roots. Indeed, some of the nutrients in organic fertilizers, such as feather meal, are mineralized so slowly they may never be available to container crops.
Another reason organic fertilizers are more challenging to use than conventional fertilizers is that their nutrient formulations are different. The percentages by weight of macronutrients are often much lower in organic fertilizers than in conventional fertilizers, and nutrients in organic formulations rarely are present in ratios similar to those in plant tissue.
For example, a typical N-P2O5-K2O analysis in a soluble, conventional fertilizer is 20-10-20 or 13-2-13 compared to analyses in organic fertilizers such as 2-5-0.2 in a liquid fish hydrolysate and 13-1.4-0.2 in a solid feather meal source. However, some organic nutrient sources will often produce plant growth comparable to conventional fertilizers even with nutrient ratios that would traditionally not be recommended. Why? One reason may relate to length of the production cycle: short-turn crops, like vegetable transplants, may not develop detectable nutrient deficiencies or toxicities during the four to six weeks they are grown.
Another reason organic fertilizers are more challenging to use is that some, such as fish emulsions, may contribute high salt levels that can damage root systems, especially early in crop production. Part of the problem is that nutrients are typically added based on parts per million nitrogen, but because organic fertilizers tend to contain lower percentages of nitrogen compared to conventional soluble fertilizers (e.g., 2% in organic compared to 20% N in conventional), higher unneeded salts from other fertilizer constituents are also applied.
Constant Monitoring for Salt Levels a Must
While it is important that salt levels (EC) be measured routinely under an organic fertilizer regime just as they should under a conventional model, the test results are less reliable. Why? Because while nearly all of the EC from conventional fertilizers are from plant essential nutrients, some organic nutrient sources contribute excessive nutrients, such as phosphorus, or unneeded elements, such as sodium. These issues dictate the need for growers to more intensively monitor crop nutrient status when organic fertilizers are used.
Best Practices for Handling Difficulties
Following are a few suggestions to help growers mitigate or navigate the challenges with organic nutrient sources. First, consider the length of the crop production cycle. Pre-plant incorporation of organic nutrient sources is more likely to meet all the crop’s needs for short-turn crops like vegetable transplants than longer cycles required of many flowering crops. Plan to supplement nutrients for longer-term crops with soluble feed or top-dressing.
It is important to monitor the amount of soluble (plant available) nutrients that will be quickly available from pre-plant nutrient sources. This includes knowing the EC that the amended root medium will generate after watering in. If the initial EC is too high, a grower can leach before transplant.
Consider these research results: a normal pre-plant EC range is 2.0 to 3.0 dS/m, but when feather meal or blood meal were incorporated into a peat-based root medium at rates to meet nutrient needs for the entire production cycle, EC levels rose to 4.0 to 4.5 dS/m during the first week of production. These high initial salt levels damaged young impatiens seedlings. But when the same root medium was prepared and leached with two irrigation events prior to transplanting, EC at transplant fell to within the 2.0 to 3.5 dS/m range and plant growth matched that of those grown with Osmocote 14-14-14 and Peter’s 20-10-20 constant liquid feed with no additional fertilizer additions (See photo at top of this article).
Trialing and testing solutions on a small scale before making a wholesale switch to a new root medium or fertilization program is essential. Additional monitoring helps growers navigate the transition by providing information about the differences between conventional and organic fertility.
Deep-Dive Learning
Kimberly Williams is co-editor of the book “Water, Root Media, and Nutrient Management for Greenhouse Crops” from which the information in this article is derived.
This comprehensive manual, available from anrcatalog.ucanr.edu and booksellers such as Amazon, has chapters written by more than 20 different greenhouse industry professionals and academics.
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