Performance of wick irrigation system using self-compensating troughs with substrates for lettuce production

ABSTRACT

Subirrigation systems in which water and nutrients are supplied to the substrate through wick strips for upward nutrient solution (NS) movement can be a feasible alternative to improve lettuce quality with low environmental pollution, enabling production with reduced labor and electricity or in regions with high air temperature. The objective of this study was to compare the performance of two wick irrigation systems using self-compensating troughs filled with either pine bark (WPB) or coconut coir (WCC) with nutrient film technique (NFT) hydroponic system for greenhouse lettuce production. The daily monitoring of electrical conductivity (EC) and pH allowed the experiment management according to the recommended values for optimal lettuce growth. The EC showed variation among troughs and salt accumulation in substrates, with WPB exhibiting two times greater EC than WCC (ranging from 0.95 to 7.57 and from 0.68 to 3.67 dS·m⁻¹, respectively), while the pH values were stable over time. The WCC promoted greater root length and shoot diameter, while WPB produced shorter plants compared to the other two treatments. NFT resulted in an 83% lower leaf area and 44% lower root volume than WPB and WCC. The fresh and dry shoot masses with NFT were 58% and 24% lower than with WPB and WCC, respectively. The fresh root mass was also reduced in NFT plants, which was 67% smaller than WCC and 59% smaller than WPB. Root dry mass of NFT was 35% lower than the average of WPB and WCC. Nitrate (NO₃)-nitrogen (N), ammonium (NH₄)-N, phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), boron (B), copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn) concentrations in plant shoot and root at the end of the experiment as well as the same nutrients, chloride, sodium, and bicarbonate concentrations in substrate and NS determined weekly differed among the treatments (P < 0.01). The EC and nutrient concentration in the substrates increased over time. The wick irrigation system with both substrates resulted in higher productivity than NFT, with higher yield and plant quality in WCC, indicating its feasibility as an alternative system for lettuce production in greenhouses. However, due to the salinity buildup, water and nutrition management needs to be optimized for self-compensating troughs to avoid an increase in substrate EC over time.

KEYWORDS:

• subirrigation
• hydroponics
• nutrient concentration
• Lactuca sativa L. ‘Vanda’, greenhouse
• NFT

Acknowledgments

We thank Dr. Flávio Bussmeyer Arruda, Antonio Carlos Ferreira Filho, Maurício Madoglio Sultani, Renato Traldi Salgado and Vicente Dias Martarello for the technical collaboration, Dr. Marc W. van Iersel for reviewing the manuscript and the constructive criticism, Antonio Carlos Ferreira Filho for making the figures using Google Sketchup, the Board of Directors of the School of Agricultural Engineering for donating the material for the assembly of the experimental plots, and the Hidrogood Horticultura Moderna Company for donating the self-compensating troughs. The authors declare that the mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product and does not imply its approval to the exclusion of other products or vendors that might also be suitable.

Funding

The National Council of Technological and Scientific Development (Ministry of Science and Technology, Brazil) provided a PhD scholarship to the first author. Funding for this research was provided through the PRP/FAEPEX/UNICAMP (award no 261/10).