The Constant Gardener.epub
Iberio Ralda
Deep flow technique (DFT) systems involve the pumping of water up from a reservoir tank and across the roots of plants. The nutrient solution remains deep enough to cover the roots and typically constant circulation is used to ensure that oxygen content in the root zone remains adequate [39]. A benefit of DFT relative to non-circulating systems is an ability to constantly provide fresh nutrient solution, which allows for higher cropping density and production per area. There is a need for a simplified form of DFT which can be used to grow a variety of crops in tight areas. This simplified DFT system must be capable of growing high-value crops, require reasonable water inputs for a smallholder farmer and require only circulation that can be completed manually without electrical input.
The objective of this research was to develop a simplified DFT system that can be operated by a smallholder grower without access to arable land, electricity, running water or advanced training. In this study, we developed a simplified DFT system with primarily gravity-based circulation supported either by wooden or metal frames. We first tested the yields and water consumption of lettuce (Lactuca sativa L.) cv. Black Seeded Simpson (BSS) under constant circulation and with 94% reduction in circulation (four times daily for 20 min). Next, we validated that simplified DFT systems can produce a fruiting crop by growing bell peppers (Capsicum annuum L.) under constant circulation. Finally, we tested this approach in the target setting of rural Haiti, using only manual circulation to grow bell peppers.
A simplified DFT approach was designed for this study (Fig. 1). DFT systems are usually run under constant circulation. To test the importance of constant circulation in our DFT systems, we compared yields of BSS in constantly circulating systems to systems circulated only four times daily for 20 min. DFT systems on a wooden frame, named Victory Garden systems, were used for this experiment (Fig. 2). The average yield of BSS from all systems was 3631.75 g in replicate one, 5013.75 g in replicate two and 2836.25 g in replicate three. For replicates one and two, there were no significant differences in BSS yield per circulation regiment. For the third replicate completed during the summer, we found that there was a greater yield for the constantly circulating systems compared to intermittent circulation (Table 1). Therefore, BSS can be produced with DFT systems under intermittent circulation without substantial yield loss, though in some instances there may be a yield decrease compared to constant circulation (Table 1).
Constantly circulating systems is energetically costly. We hypothesized that intermittently circulating systems would produce greatly increased yields per energy usage. According to the U.S. Energy Information Administration, during June 2022, the average price per kWh of energy for commercial users was $0.1907 per kWh in Connecticut [40], and a constantly circulating system according to our experimental parameters uses 2.3814 kWh over 21 days while an intermittently circulating system uses 0.1386 kWh over the same time period. This translates to an estimated price of $0.45 per constantly circulating system versus $0.03 per intermittently circulating system. Yields per energy used were significantly higher in intermittently circulating systems than constantly circulating systems in all three replicates (Table 1).
Hydroponic systems require inputs of water and fertilizer and maintenance of pH in an optimal range. To test whether intermittent and constant circulation are different in their water requirements we measured the water consumption of each system after harvest of BSS. We did not find a significant difference in yield per water usage or in average daily water usage based on circulation treatment in any of the replicates (Table 1).
To test whether fertilizer and pH values fluctuated differently in the circulation treatments, electrical conductivity (E.C.) and pH values were measured. E.C. is a proxy for the nutrient content of a nutrient solution and as growth was similar between treatment groups, we hypothesized that E.C. would be similar between constant and intermittent groups at the beginning and end of the experiment. In support of this hypothesis, E.C. values were not significantly different between treatments at the beginning or end of the experiments in all three replicates (Table 2). In all experimental replicates, E.C. decreased between the first week and the last reading. Readings were similar in replicate one and two, but were lower in replicate three (Table 2). There were no significant differences in pH at week one between circulation groups (Table 2). In replicates one and two, there were no significant differences in pH values at the end of the experiment (Table 2). In our third replicate, we found that the pH was significantly higher in the constantly circulating systems and in all replicates, the average pH was greater at the final reading than at week 1 (Table 2).
To test whether bell peppers could be grown in simplified DFT systems, we grew them under constant circulation in Connecticut. Systems yielded an average of 3592.94 g of fruit with a standard error of 214.52 g. This is above the benchmark in-soil yield for peppers in Connecticut [41]. To test whether a minimal circulation approach could be applied to grow bell peppers without access to electricity, we grew Yolo Wonder bell peppers under manual twice-daily circulation in systems with a metal frame, called Babylon systems, in Pignon, Haiti (Fig. 3). Per system yields under manual circulation averaged 2574.13 g with a standard error of 140.84 g in the first replicate. In the second experimental replication, yields averaged 3308.35 g per system with a standard error of 303.63 g.
Research generally suggests that increased oxygenation increases yields [42]. Lower levels of circulation would likely decrease the oxygen content of the nutrient solution. For two of our trials, we did not see increased yields under constant circulation and therefore it is plausible that constant circulation does not significantly increase the oxygen levels of the nutrient solution relative to intermittent circulation in the context of our systems. Therefore, further research is warranted to see if systems can be adjusted to allow for increased oxygenation. Possible methods include adjusting the reservoir tank by making it smaller or wider. Another option is to increase fall distance between pipes to allow for greater water disturbance. Future research will focus on the dissolved oxygen concentration of the nutrient solution under different circulation regimes.
The interaction between temperature and circulation requirement was not examined. We found that yields were slightly decreased in intermittently circulated systems compared to constantly circulated systems in our third BSS trial, which took place during the summer when temperatures reached as high as 39 C. This trial had increased water usage for all system treatments compared to the two previous trials. E.C. values were lower for this trial as well, which may be due to a combination of increased transpiration rates and increased water additions to the systems which dilutes nutrient salts. Cooler water can hold more oxygen and it is possible that constant circulation increases oxygen levels of the nutrient solution in warmer temperatures in a way that is not relevant in cooler temperatures. Temperatures in Pignon, Haiti regularly exceeded 30 C during the described pepper trial but a comparative treatment with constant circulation was not possible due to electrical limitations.
In conclusion, simplified DFT systems with minimal gravity-based circulation can produce both lettuce and peppers on par with field production. In this system type, we did not see a benefit to constantly circulating the nutrient solution. This technology has broad potential applications for food insecure populations facing shortages in water and arable land. It is also possible to use this technology to sustain greater yields. The field of hydroponic technology is pushing towards increased mechanization and environmental control, but at the same time, we should explore its limits in the opposite direction to maximize its applicability.
Seeds were planted into 1 in.2 rockwool blocks. The rockwool sheet was watered frequently so the blocks were constantly moist. At the true leaf stage seedlings were thinned to one seedling per hole and quarter-strength nutrient solution was added in place of water. Once seedlings were 2 in. tall, they were transferred to 3-in. diameter plastic net pots. Once transferred, perlite was added to the pots to stabilize the seedlings. Seedlings were watered daily with quarter-strength fertilizer or water on an alternating basis. Seedlings were transferred into the Victory Gardens when they had three or four true leaves and roots were long enough to come through the holes of the net pots, at approximately 3 weeks for BSS and 4 weeks for bell peppers.
Hamden, CT, U.S.A. At the Connecticut Agricultural Experiment Station, Victory Garden systems were used to produce BSS. Systems were located inside a greenhouse where temperature was not regulated, but ranged between 15 and 22 C in replicate 1, 17 and 29 C in replicate 2, and 21 and 38 C in replicate 3. Four systems were constantly circulated, and four systems were intermittently circulated. The treatments were arranged in an alternating fashion to allow for electrical connection. All systems contained 20 plants. For the intermittent treatment, the pump ran for 20 min four times per 24 h and the continuous treatment ran the entirety of the experiment duration, apart from those same 20-min time periods when the intermittent system was running to avoid overloading the electrical system. E.C. and pH were measured with VIVOSUN E.C. and pH meters at 1 week after initiation of the experiment to allow complete circulation of the system and then the week of harvest (Additional file 2: Table S2). After 3 weeks in the systems, the aboveground mass of individual plants was weighed in grams (Additional file 2: Table S2). Further analysis was not completed so that the harvest could be donated to a local food pantry. The total decrease in water held by the system reservoir tank was measured in gallons and then converted to liters by multiplying by 3.7854 and then this was divided by the number of days in the experiment to calculate average daily water use (Additional file 2: Table S2). Three independent replicates of this experiment were completed (Additional file 2: Table S2).
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