How mineral nutrition affects radishes grown indoors

Editor’s note: This is the second part of a four-part series on hydroponic radish production. Read the first part here.

While spring radishes are traditionally a field crop, their short production time, high planting density, and low air temperature performance make them an attractive candidate for controlled environment agriculture. In our first article, we discussed the importance of cultivar selection and highlighted some radish cultivars that perform well in the greenhouse. However, cultivar selection is only one piece of the production puzzle.

Mineral nutrition management is important for food crop production, whether in soilless, soilless, or hydroponic water culture mineral fields. Providing sufficient mineral nutrients helps plants realize their yield potential, in addition to ensuring that fruits and leaves do not suffer from deficiency or toxicity symptoms, and helps maximize their appearance and marketability. In trying to figure out how to fertilize radishes grown hydroponically, in trays with soilless substrate and sub-moisture, we could not successfully translate the recommendations for field-grown radishes in pounds of nitrogen per acre to parts per million (ppm) of nitrogen in a soilless greenhouse system. Therefore, we designed a study to determine what concentration of fertilizer would be suitable for growing radishes in a controlled environment.

Materials and methods

Based on previous research, ‘Crunchy King’ and ‘Red Castle’ were selected based on their performance in cultivar trials. Seeds were sown in 72-well trays filled with commercial soilless substrate and covered with a light coating of coarse vermiculite. After seeding, the dishes were moved to a glass greenhouse with a constant set air temperature of 68°F and a target daylight integral of 12 mol∙m–2∙d–1.

Seed trays were placed in one of six identical flood tables with internal dimensions of 36 in. W x 72 in. L x 7.4 in. H. The trays were initially hand-irrigated to saturation with clean water immediately after planting. The flood tables were flooded every morning for the first two weeks, morning and afternoon for the third and fourth weeks. Each of the different flood tanks had its own 40-gallon tank filled with a solution consisting of tap water supplemented with 15-5-15 Cal-Mag (Peters Excel; ICL Specialty Fertilizers, St. Louis, MO) to provide one of six fertilizer concentrations: 0 , 100, 200, 300, 400, or 500 ppm nitrogen (with corresponding ECs of 0.00, 0.76, 1.52, 2.28, 3.04, or 3.80 mS/cm, respectively).

Four weeks after sowing, data were collected. The diameter and fresh weight of trimmed and washed radishes were recorded, and other radishes were graded by hypocotyl diameter according to USDA grading standards. In addition, leaf number, length, and greenness were recorded prior to trimming to account for producers selling radishes with bunched leaves.

Effects of fertilizer concentrations

At the end of 28 days we could see clear effects of fertilizer concentration on the growth of ‘Crunchy King’ and ‘Red Castle’ radishes. In both cultivars, radish diameter increased and was greatest between 200 and 300 ppm N (1.52 to 2.28 mS/cm); when fertilizer concentration increased above 300 ppm N, radishes were smaller. In addition, variability in USDA grade of cut radishes increased as fertilizer concentration increased above 300 ppm N. We saw very similar results for fresh weight of cut radishes, and yield was highest at 200 to 300 ppm N. Although plants that were not provided with any fertilizer had fewer and smaller leaves than radishes receiving 100 to 500 ppm N, there were no differences in number, size, or greenness of radishes receiving fertilizer.

Putting science into practice

There are several considerations when determining how to fertilize radishes. First we used the classic 15-5-15 Cal-Mag fertilizer – and that was by design. Most commercial greenhouse fertilizers are a combination of ammonium forms of nitrogen including ammonium (NH4+) and urea (which is converted to ammonium and “counted” as ammonium) and nitrate (NO3-) nitrogen. We chose 15-5-15 because of its higher nitrate content than other fertilizers. Most of the ammonium is converted to nitrate for plant uptake, and nitrifying bacteria do this. However, at lower growth temperatures, the activity of nitrifying bacteria slows down, which can lead to excessive ammonium toxicity and – in some cases – ammonium toxicity.

The higher nitrate content of 15-5-15 Cal-Mag, also called a “dark weather feed,” makes it suitable for the cooler growing temperatures that radishes can grow in. We certainly encourage growers to try other fertilizers for soilless radish production, but we recommend evaluating your growing temperatures and matching them to the appropriate ammonium:nitrate ratio. Growers using warmer temperatures may use higher ammonium fertilizers, while growers growing at similar or cooler
temperatures should look at other products with a high nitrate content.

The electrical conductivity (EC) of the different nutrient solutions remained consistent throughout the experiment. Nutrient concentration management in ebb and flood systems is different from continuously circulating water cultivation systems such as the nutrient film technique (NFT) or the run-up or deep flow technique (DFT). However, in each repetition of the experiment, we used freshly prepared nutrient solutions. In commercial practice, we would use the fertilizer solution for a longer period of time. While the fluctuations in ebb and flood tanks may not be as great as in hydroponic water culture systems, don’t simply monitor the EC and pH of your nutrient solution. Instead, run regular lab tests to know the concentration of different nutrients and know when to start replacing the old solution with a new solution to avoid nutrient imbalances.

Key things

Based on our research, we believe that commercial growers interested in growing radishes in soilless culture with ebb and flood systems should try to use fertilizers at concentrations of 200 to 300 ppm N (plus other macro- and micronutrients). Unique combinations of cultivars, environmental conditions, and other cultural practices warrant that trials should be conducted to determine how these recommendations will work in your facility.