Ideal fertilizer use for culinary organic herb production
Unlike ornamental flowering crops, the edible nature of herbs creates more opportunities for adding value by using organic production practices. Suståne is a certified organic slow-release fertilizer that is well-suited for containerized herb production.
In a previous article, “Ways organic fertilizer improves culinary herb production,” we reported the results of a study evaluating the use of Suståne (fine particle size) on growing organic herb plugs. In this follow-up study, we wanted to determine what concentrations of medium-sized particle Suståne could be used to successfully finish culinary herbs in 4.5-inch containers.
Study of six organic herbs
We chose six of the most popular culinary herbs to determine what organic turkey litter fertilizer concentrations were most appropriate for producing finished containerized plants, including sweet basil (Ocimum basilicum ‘Nufar’), purple basil (Ocimum basilicum ‘Red Rubin’), common sage (Salvia officinalis), flat-leaf parsley (Petroselinum crispum ‘Giant of Italy’), cilantro (Coriandrum sativum ‘Leisure’) and dill (Anethum graveolens ‘Hera’).
Prior to filling containers, the substrate was amended with a medium-grade particle size of organic turkey litter fertilizer (8N–4P–4K; Suståne) comprised of composted turkey litter, feather meal and sulfate of potash at one of six different concentrations: 0, 4.2, 8.4, 12.6, 16.9 or 21.1 pounds of fertilizer per cubic yard of substrate. Throughout the experiment, containers were grown on expanded metal benches in a glass-glazed greenhouse with day and night air temperatures maintained at 72 degrees Fahrenheit and 65 F, respectively, and a 16-hour day with a target daily light integral (DLI) of 12 mol·m–2·d–1.
Data was collected three to four weeks after transplant, representing the standard production time for 4.5-inch containers planted with plugs. Data were collected to characterize the root zone [pH and electrical conductivity (EC)] and plant growth (shoot height, width, leaf area and shoot and root dry weight), and shoot tissue was submitted to a commercial laboratory to determine mineral nutrient concentrations.
Fertilizer affects macronutrients
Turkey litter fertilizer affected the EC and pH of all crops in a similar manner. As the concentration of fertilizer increased, so did the substrate EC and pH.
Containerized herb growth, including height, width, and shoot and root dry mass was affected differently by fertilizer concentration amongst the different species. For example, as fertilizer concentration increased from 0 to 8.4 lbs./yd³, sage and purple basil height decreased, cilantro and dill height increased, and sweet basil and parsley height were relatively unaffected. However, as fertilizer concentration increased to 12.6 lbs./yd³ or more, height was negatively affected for all species. Although shoot dry weight increased for all species as fertilizer increased from 0 to 8.4 lbs./yd³, it decreased as fertilizer increased above that; alternatively, root dry weight decreased with increasing fertilizer concentration.
Organic turkey litter fertilizer concentration affected macronutrient tissue concentrations differently. Increasing fertilizer concentration increased nitrogen, phosphorous, potassium and sulfur for all herb species. Alternatively, calcium decreased with increasing fertilizer concentration for all species. Magnesium concentrations of species were affected differently by fertilizer, with increasing fertilizer concentration decreasing magnesium in cilantro, dill and parsley, but increasing magnesium in sweet and purple basil and sage. Across all herb species, increasing fertilizer concentration increased most micronutrient concentrations.
Implications for commercial growers
One of the goals of any containerized crop producer is to produce plants that are proportional to the container they are grown in. For mineral nutrition, this means providing sufficient fertilizer that will produce healthy-looking plants without promoting excessive growth. This can be an even bigger challenge for containerized crops like culinary herbs since there are no chemical plant growth retardants to inhibit unwanted shoot growth. Although no conventional fertilizer was used in this study, increasing organic fertilizer concentration did not result in the same type of excess growth that we see when using conventional water-soluble fertilizer.
Rather, the most pronounced effect of fertilizer on plant size we saw was diminished growth at high concentrations, which we attribute to the high root-zone EC. Using a high fertilizer concentration to suppress growth and keep plants proportional to containers is an ill-advised strategy, however.
Increasing fertilizer concentration decreased root growth for all species. This is not an uncommon response, as root growth can increase as plants are trying to “find” nutrients for uptake. The diminished root growth observed at the moderate fertilizer concentrations that produced healthy foliage may not have a negative impact on postharvest performance. Many herbs grown in 4.5-inch containers are used as potted plants indoors, not transplanted into garden beds, where more root growth may speed up landscape establishment.
Fertilization practices during production directly affect the appearance and marketability of crops. In our research, we did see some lower-leaf chlorosis — a symptom of nitrogen deficiency — in cilantro, parsley, and sweet basil when no fertilizer was provided. The more pronounced effect on appearance was toxicity symptoms at the highest fertilizer concentrations that also resulted in insufficient shoot growth.
We suggest 8.4 or 12.6 lbs./yd³ of medium-grade 8–4–4 Suståne be used for finishing culinary herbs when starting from adequately fertilized plugs. Use 8.4 lbs./yd³ for cilantro, dill, purple basil, and 12.6 lbs./yd³ for sage, sweet basil and parsley. For species that were not included in our research, start with an intermediate concentration, around 10 or 11 lbs./yd³, and adjust as needed based on your results. Additionally, if less fertilizer is provided to plugs than what we used in our study, a higher fertilizer concentration may be needed during finishing.
This article was originally published in Greenhouse Grower Magazine.