Summer bibb lettuce yield and quality as impacted by nutrient tank management techniques
In recirculating hydroponic systems producing leafy crops, one of the main factors in the control of the grower is the frequency of tank changes (ie pumping out and turning over the recirculating solution). Since we generally manage the solution based on EC, we are assessing the total amount of solutes in the water. We don’t know the balance of each nutrient individually, so tank changes are carried out to try and maintain necessary levels of hydroponic lettuce nutrients. Essentially, our goal is to change the nutrient solution often enough that we don’t have detrimental buildups of unused ions or depletions of important nutrients. Also keep in mind that different water sources have different background ion levels that can slow or speed up imbalances in the nutrient solution.
This tank change practice is really based on cost efficiency. There are nutrient solution management systems that have the ability to manage based on individual ions, but these are much higher cost than the typical systems installed in small to mid scale greenhouses. This hydroponics system cost savings comes at the price of more frequent tank changes that maintain a safe margin of hydroponics nutrient solution.
Without installing a new nutrient management system, there are methods to try and extend time between reservoir changes. One common method is the addition of small amounts of nutrient concentrate that address the most commonly depleted nutrient ions. Typically, these additions focus on N, K, and P. In this trial, the daily additions were intended to provide 10 ppm N and P, and 15 ppm K. An ion that can increase over time in some solutions is Ca (from source water and calcium nitrate additions). So, potassium nitrate and monopotassium phosphate were the fertilizer materials added to avoid adding Ca. Additionally, some treatments were formulated with a slightly lower initial Ca target to see if this could limit Ca increases in solution over time. The goal of this nutrient tank change test was to investigate how additions of nutrients compared to tank changes in these normal and low calcium nutrient formulations. This trial was carried out in our experimental system where we have the ability to provide four different nutrient solutions. Four treatments were based on these solutions and goals. 1) Normal solution with one tank change at midway point in production (8/25), 2) Normal solution with no tank change but daily additions of KNO3 and KH2PO4 after tank change was carried out in other treatments, 3) Lower calcium solution with one tank change during production (8/25), 4) Lower calcium solution with no tank changes but daily additions of KNO3 and KH2PO4 after tank change was carried out in other treatments.
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 13 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 at 1.8 for all treatments through the entire experiment from seeding onward. These trials were carried out in a system designed to pull from four different 40 gallon nutrient tanks so that differing solution treatments discussed above could be tested in a randomized block design. At harvest, shoot fresh weight and a visual rating of tipburn was recorded individually for each head.
Timing and Conditions
- No supplemental lighting of any kind was used and there was no shade cloth on the greenhouse in the summer of 2014.
- Environmental data represent only the first 24 days of the experiment.
Biomass Yield by Treatment
Nutrient Solution Data
Final Nutrient Solution Data
Discussion on the trial
There are some interesting points to bring out looking at the data presented on both plant and solution variables.
1) In this trial, there were not significant differences in final fresh weight yield in the four treatments as is clear by overlapping confidence intervals. Likewise, there were no distinct differences in tipburn ratings (intervals not shown). There was quite a bit of variability in the tipburn rating among treatments, and more work will be needed to see if less frequent tank changes may increase the risk of tipburn. While covering these results, I want to make sure to point out that this is a preliminary study and additional work should follow. It is intended to lay the foundation rather than provide a prescription for current changes to nutrient solutions. This test was designed to replicate as closely as possible the starting point of solution conditions that would be present in the systems of many of the growers. More advanced questions and detailed work would follow to develop recommendations about practical steps that growers could take to prolong the distance between refreshing the solution in you nutrient tank. Keep in mind that individual conditions in source water will have a strong impact of whether trends observed in this work would or could be replicated elsewhere.
2) In the area of essential nutrients, we see that some of our original assumptions were not necessarily correct. Reviewing the solution data, we can see that calcium levels actually did not increase to undesirable levels over the course of the experiment. In fact, these solution values indicate that Ca in the normal recipe was probably more in line than in the low Ca solution. Mg was present in fairly consistent and generally adequate levels throughout the trial. By the end of the trial, N was present in all solutions, but certainly not in excessive amounts. K and P were present is quite sufficient amounts. With K and P, it is likely that adjustments could even be made to our daily addition recipe to optimize these levels. P could be lower in treatments 3 and 4. Probably the most detrimental attributes of the solution that are revealed by these samples was the increase in S and Na content by the end of the trial. One key way that several of these issues could be addressed is by the use of a nitric acid solution instead of a sulfuric acid solution to lower the pH. This would increase the level of N without needing to add K (as is necessary in the addition of KNO3) and also reduce the buildup of S in solution. The increasing levels of Na in solution were caused by source water conditions and are much harder to change. More testing will be necessary to determine of the increased Na levels over time were detrimental to lettuce quality. Finally, it is important to note that lower levels of some micronutrients, especially Mn were observed in this study. If tank change intervals are to be increased, then daily additions should likely be including the most highly used micronutrients, and higher initial targets likely needed to be started to prevent deficiencies even under regular tank change intervals.