Abstract
Cascade hydroponics, that is, the application of the circular economy concept in greenhouse hydroponic crops, may be considered as an alternative means to increase water and nutrient use efficiency in greenhouses. In such systems, the drained nutrient solution from a crop may be used as input in a second crop. However, the second (secondary) crop in the loop must be a crop that is less sensitive to salinity than the first (primary) crop. In the present study, the salinity tolerance of basil plants grown in rockwool and nutrient film technique (NFT) systems was investigated in order to study the potential of using a basil crop as a secondary crop in a cascade hydroponic system. In total, 4 electrical conductivity (EC) levels of the irrigation nutrient solution were tested (2, 4, 6, and 8 dS m−1), and salinity was imposed by NaCl or by macronutrients. Plant growth varied across the different substrates, with those grown in the NFT system being less affected as opposed to the rockwool-grown basil plants, which showed a significant growth decrease with EC values higher than 4 dS m−1. This relatively low growth pattern was associated with a decrease in water use efficiency (WUE) in the rockwool system. On the contrary, in the NFT system, the continuous flow of the nutrient solution in the root zone of the plants contributed to the alleviation of negative salinity effects, yielding up to 30 kg FM m−2 WUE even for the plants irrigated with the highest salinity treatment (8 dS m−1). The majority of macro- and micronutrients in the leaf tissue of basil were positively affected by the higher levels of conductivity in the nutrient solution. Therefore, basil cultivation could be efficiently incorporated as a secondary crop in a cascade NFT cropping system. This would contribute to drainage management in hydroponics, as the crop could be irrigated through the moderately saline drainage from a primary crop due to either NaCl or high nutrient accumulation in the leachates.
Highlights
The challenge to produce higher amounts of food with less resource use and low energy consumption presents a critical opportunity for the development of a circular economy (CE) in agriculture
Based on the calculations of water absorption, the total absorption of basil plants grown on rockwool cubes was 40 L m−2 for all salinity regimes and 38 L m−2 when the plants were cultivated in the nutrient film technique (NFT) system
From days after transplanting (DAT) 25 the fresh water needs increased with plant growth to the level of uptake recorded before the enrichment of nutrient solution (NS) with NaCl
Summary
The challenge to produce higher amounts of food with less resource use and low energy consumption presents a critical opportunity for the development of a circular economy (CE) in agriculture. The development of CE requires the adoption of closed-loop system layouts that work towards the goal of upgrading economic and environmental sustainability. The improvement of such systems is a departure from the traditional linear systems of production which act through the transformation of natural resources to products and to waste [1]. Greenhouse cultivation, especially of soilless crops, is a very resourceintensive cultivation method and may be as considered circular since plants can grow in closed systems where water and nutrients are recirculated [2].
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