Lettuce and Brassica winter production in NFT systems in Ohio
In recirculating systems producing leafy crops, one of the main factors in the control of the grower is the nutrient solution electrical conductivity (EC). In many systems, total EC, rather than single elements are controlled due to economics. In most commercial systems using electronic controllers and dosing pumps, concentrated fertilizer solution is added to the nutrient solution any time the solution goes below target EC. So, maintaining consistent EC levels is fairly straightforward, the main question becomes: What is the best EC? The answer to this question is based on two separate factors. The first relates to maintaining needed nutrients in solution. Essentially, the important question is how close to calculated nutrient levels does the solution remain over time. If there are large amounts of ions already in the source water (sodium, sulfate, or calcium for instance), this can cause the nutrient solution to become out of balance more rapidly meaning that ideal ratios of nutrients are not maintained. The second factor involves the movement of water through the plant. At lower EC, it is easier for plants to take up and transpire water. Therefore, under high light and temperature, and low humidity, lower solution EC levels makes it easier for the plant to move water. So, the EC that we use in our systems needs to address these two issues: 1) Maintain adequate levels of plant nutrients, and 2) not stress the plant too much in terms of taking up water needed for transpiration.
Seeding was done by hand into pre-moistened 1” x 1” x 1 ½” cubes. Seeds were germinated in 9” nursery channels that were receiving a continuous flow of nutrient solutions set at experimental levels. After 15 to 17 days, seedlings were transplanted to the production NFT channels at a spacing of 8” on centers. After transplanting, plants were grown in 4 ¾” channels until harvest. The nutrient solution was automatically and continually adjusted to maintain a target pH of 6.0. Electrical conductivity was maintained by hand additions of nutrient concentrate as
needed based on daily measurements of EC. These trials were carried out in a system designed to pull from four different 40 gallon nutrient tanks so that differing solution could be tested in a randomized block design. At harvest, shoot fresh weight was recorded individually for each head.
Timing and Conditions
* 400W metal halide lights were used to add approximately 30-40 µmol/m2/sec of supplemental light from 4 to 11 am during the lettuce experiment, but power usage constraints prevented lights from being used during the Brassica trial.
Biomass Yield by EC Treatment
Letters signify differences between EC treatments across all three cultivars. Treatments are only significantly different if followed by different letters.
EC x Cultivar Biomass Yield
Discussion on the two trials
These two runs of a fairly straightforward nutrient concentration test reveal some interesting results and, as useful tests should, provide some additional questions for future work.
1) Under the conditions of these trials, it is quite possible that nutrition was not always the most limiting factor. Yields were statistically similar in the lettuce trial for treatments where 1.3, 1.8, and 2.3 EC was maintained. This would suggest that all three of those EC treatments provided adequate nutrition and that the generally low yields in the trial, may have been due to low light conditions. Many times growers increase EC during the winter to push growth. Certainly under some conditions, that can be a valid technique, but it is also possible that the plants may not be able to parlay those available nutrients into increased yield. It should also be stated that our nutrient targets for recirculating systems are purposefully determined in excess of minimum nutrient levels to provide buffer in our systems and prevent yield reductions.
2) For some cultivars, quality impacts can be as large a determining factor in nutrient solution adjustment as yield. For any grower who has tip-burned romaine lettuce (and that includes a high percentage of those who have grown it), it comes as no surprise that quality issues are often more prevalent than in bibb lettuce. For some of our crops, then it may be maintaining crop quality rather than growth rate that is the determining management factor. So, if tipburn increases to a costly level, we may run lower EC even if slightly higher levels would lead to increased biomass accumulation. We also need to investigate other aspects of our management (air circulation, lighting, cultivar selection, etc.) to make sure that it really is a nutrient issue causing our decreased quality. This leads into many separate areas of research, but it is important to remember that only one of the lettuce cultivars and none of the Brassicas sustained tipburn in these trials.
3) For leafy crops other than lettuce, there still is likely room for improvement in our plant nutrition and crop management. Earlier in the discussion, I mentioned the fact that the clearer separation between EC treatments in the Brassica trial may have been due to less frequent tank changes and more opportunity for nutrient limitation in the 1.3 and 0.8 EC treatments. That is possible, but it is also quite possible that kale, arugula, and pac choi may have different optimum nutrient levels than lettuce (or each other for that matter). For the past several decades, bibb lettuce has been the focus of most hydroponic research and with an increase in the number of profitable crops that can be grown in greenhouses, it is quite possible that we still have a bit to learn about these other crops.
So, what are the questions for follow up trials??
1)How do seasonal conditions impact tests such as these?
As discussed earlier, both of these trials were carried out in low light, winter conditions. It is quite likely that when light is less limiting that more nutrition impacts will be present. Likewise, quality impacts (tipburn) may be more of a factor in summer trials.
2)What about the impact of the frequency of tank changes?
In most of our recirculating systems, our goal is to manage solutions to prevent nutrients from becoming limiting. Tank changes at specific intervals are often how we accomplish this goal (without purchasing specific ion probes). Only through nutrient testing and trials will we know for sure what the optimum EC and intervals between solution changes will be. While it stands to reason that tank changes may need to be less frequent at higher EC levels and/or higher light conditions, we need data to back up our practices and theories.