There are several types of fertilizers commercially available to growers: water soluble fertilizers (WSFs) and controlled release fertilizers (CRFs). Historically, WSFs have been used mostly in greenhouse production while CRFs have been used mostly in nursery crops grown outdoors. Both types of fertilizers offer advantages and disadvantages depending on the application.
WSFs are dissolved in water and drenched into the growing media. They provide an immediately available dose of nutrients to the crop and offer great flexibility. A grower can vary the timing, concentration or type of fertilizer formulation applied at any fertigation. Since any single application does not persist for very long in the root zone, WSFs must be continuously applied to provide the crop with adequate nutrients.
This can be labor intensive and crop problems can arise unless careful monitoring and application adjustments are made. Also, WSFs can only be applied when crops need to be irrigated and once a crop leaves the care of the greenhouse into the garden center or to a consumer, the WSF program can be disrupted or totally abandoned leading to declining crop quality.
CRFs are coated fertilizers. The entire fertilizer dose for a crop is applied only once at the beginning of crop production and this provides continuous feeding of crops over an extended period of time. The biggest negative with using CRFs on herbaceous crops is if CRF application rates are excessive for a particular crop or environment, excessive soluble salts and nutrients can accumulate in the growing media, causing crop damage. Also, once a full rate of CRF has been applied, the grower loses the potential benefits of “toning” a crop with the WSF choice.
So what is the best fertilizer method for producing flowering pot crops? The post-production aspect of combination fertilizer programs were studied in 2004 and 2005 in several trials at The Ohio State University conducted by Drs. Claudio Pasian and Peter Konjoian.
The 2004 study showed that, in general, plants that received both types of fertilizer were heavier than plants that received only one type.
In 2004, liners of petunias ‘Easy Wave Pink’ and ‘Wave Blue’ and verbena ‘Wildfire Rose’ were planted in May in 12-inch pots and grown outside on a concrete pad. Plants were subjected to 16 fertilizer treatments consisting of varying combinations of topdressed Osmocote Plus 16-9-12, 8-9 month Hi-Start and Peters Excel 15-5-15 Cal-Mag (Table 1). CRF treatments used were none, low rate (1.2 oz./pot), medium rate (1.8 oz./pot) and high rate (3.5 oz./pot) combined with WSF applied never, once per week at 300 ppm N, twice per week at 300 ppm N or once per month at 600 ppm N. Dry weights were determined at the end of the summer.
Plants with no fertilizer (either CRF or water soluble) performed very poorly (Table 1). The limited growth produced by these plants was likely achieved by using the starter nutrient charge in the growing mix. In general, as fertilizer concentration increased, plant growth (indicated as dry weight) also increased. The maximum dry weight measured was for plants grown at the highest concentration of CRF and 300 ppm N twice a week: 253.8 g, 220.8 g and 253.6 g for petunia ‘Easy Wave Pink,’ petunia ‘Wave Blue’ and verbena ‘Wildfire Rose,’ respectively.
In general, plants that received both types of fertilizers were heavier than plants that received only one type.
Plants that were fertilized once per month at 600 ppm N with no CRF were lighter (smaller) than any of the plants that received CRF. Even the lowest CRF rate produced more biomass than the plants in the treatment imitating how an average homeowner might fertilize (600 ppm N once per month).
In a follow-up trial in 2005, liners of geranium ‘Rocky Mountain Salmon Rose’ and petunia ‘Tiny Tunia Rose’ were grown in a similar manner to the 2004 trial. Plants were subjected to nine fertilizer treatments consisting of varying combinations of topdressed Osmocote Plus 16-9-12, 8-9 month Hi-Start and Peters Excel 15-5-15 Cal-Mag (Figure 1). CRF treatments were none, low rate (1.2 oz./pot), medium rate (2.9 oz./pot) and high rate (3.5 oz./pot) combined with WSF applied never, once per week at 300 ppm N or once per month at 600 ppm N. Dry weights were determined at the end of the summer.
As expected, plants with no fertilizer (either CRF or WSF) performed very poorly (Figures 1 and 2). In general, as fertilizer concentration increased, plant growth (indicated as dry weight) also increased. Although some dry weights for the 3.8 oz. per pot treatments were smaller than the ones for the 2.9 oz. per pot treatments, these differences were not statistically significant. Plants that received WSF at 600 ppm N once month with no CRF were lighter (smaller) than any of the plants that received CRF (Figure 2). Even the lowest CRF rate (2.9 oz. per pot) produced more biomass than the plants in the treatment imitating how an average homeowner might fertilize (600 ppm N once per month). Measurements for heights and diameters followed the same general trend as dry weights.
Consumer preferences were rated for petunias by eleven Master Gardener volunteers who indicated that the medium CRF rate is sufficient enough to achieve high consumer satisfaction (Figure 3).
General 2005 Conclusions
Like the experiment conducted in 2004, this study underscores the importance of using CRF in the production and postproduction of large container plants. Plants grown at even the lowest rate of CRF application performed better than plants fertigated at a regime similar to what an average consumer would use. It is imperative that growers preparing and selling mixed containers or monocultures intended to remain in the container where they have been grown apply CRF before plants reach the consumer environment. Dr. Pasian has decided to use CRF in the Mixed Container Trial at The Ohio State University in the future and continue to apply soluble fertilizers as a complementary fertilization treatment.
Some growers have reported that using combination fertilizer programs (low-to-medium rate of CRF, along with supplemental applications of lower concentrate WSF as needed) can round out a potentially inefficient fertilizer program and provide benefits:
- Growers who raise many crop types at once can simplify their WSF programs. Use one concentration of WSF on all plants. CRF can meet additional needs of heavy feeders or plants with special fertilizer requirements.
- The WSF program allows growers to react to specific situations that arise, e.g. pH issues, micronutrient deficiencies, N draft from green bark.
- CRF provides a base feed when you can’t use water soluble fertilizers–during cool, cloudy weather (no need to irrigate) or during busy shipping times (no time to irrigate or to mix up fertilizer stock tank solutions). CRF maintains better foliage color due to constant, uniform feeding throughout the crop cycle.
- CRF continues after the plants leave the greenhouse, fertilizing for the retailer and homeowner, providing your customers with a value-added difference.
- Potential to minimize nutrient run-off by utilizing controlled release fertilizers.