Using mass-balance principles to optimize nutrition of lettuce

The future of agriculture depends on ever more efficient uses of fertilizer inputs. Nutrient management approaches in closed-system hydroponic culture commonly emphasize the need to continuously monitor and control ion concentrations. Here we describe and validate a mass-balance approach that uses the ratio of growth to transpiration (water use efficiency) to calculate nutrient requirements and achieve optimal nutrition without the need for individual ion monitoring. We define optimal nutrition as the minimum fertilizer input to achieve maximum growth. The hydroponic solution volume was maintained by refilling with a nutrient solution in which ion concentrations were calculated based on previously measured leaf nutrient concentrations from healthy plants. Mass-balance recovery of most elements in this closed system was greater than 90%. The yield and nutrient uptake of three diverse lettuce cultivars was consistent over twenty-five consecutive crop cycles (707 days) without replacing any solution. Active absorption of nitrogen (N), phosphorus (P), and potassium (K) led to low and steady solution concentrations of 5 ppm N, 1 ppm P, and 10 ppm K. In the first of three studies, the concentrations of calcium, magnesium, sulfur, boron, and copper increased over time, which indicated that their concentrations were excessive in the refill solution. A high solution concentration did not result in "force feeding" these elements into the plants. Their concentrations were reduced in the second long-term study (seven crop cycles across 196 days) and further reduced in the third long-term study (nine crop cycles across 252 days). These nutrient reductions resulted in constant concentrations and an electrical conductivity of 0.3 mS cm^-1, which reduced the potential for gaseous N emissions. The third study included a treatment that replaced the nutrient solution after each harvest to maintain an EC of 1.0 mS cm-1. Despite the higher ion concentrations, solution replacement did not increase nutrient uptake or yield, and the increased calcium in solution did not reduce tipburn severity. We conclude that a mass-balance approach can be used to achieve optimal nutrition, maximum yield, and eliminate discharge across multiple crop cycles without continuous monitoring.