Biomass Accumulation and Its Relationship with the Demand and Concentration of Nitrogen, Phosphorus, and Potassium in Lettuce

Lettuce (Lactuca sativa L.) has a short cultivation cycle and a rare growth pattern. The nutritional management program of this species must be designed before its establishment in field or greenhouse. There are two scientifically based methods to design its fertilization management: the simplified rational model and the dynamics of nutritional accumulation. There is not much information to input into these methods. The present study reports about the biomass accumulation pattern of lettuce, cv. Coolward, and its relationship with the demand and concentration of nitrogen (N), phosphorus (P), and potassium (K). The study was conducted in a greenhouse with a slightly acidic sandy-loam substrate (pH 6.5) and a medium/high content of organic matter and nutrients. Twenty-one replicates were set in pots with increasing amounts of soil, depending on the time of harvest. Nine samplings were conducted from emergence up to commercial maturity (MC) (0-65 days after emergence, DDE). The plants collected were divided into two parts, aboveground (PA) and root (PR), and their dry weights and concentrations of N, P, and K were measured. With these data, biomass relative accumulation and nutrients were calculated. Total biomass association (BT) with the demand and concentrations of N, P, and K was estimated. The growth of lettuce during the first 35 DDE was slow and only represented 10% of the total accumulation of biomass, N, P, and K, mainly in PA. From the beginning of the rosette formation (IFRO) up to MC, the linear accumulation stage of MS, N, P, and K emerged. Over the last 22 days of its cycle, the crop acquired more than 60% of MS and nutrients measured at harvest. The PA accumulated nearly 90% of MS, N, P, and K, and the remaining 10% was found in PR. A close linear association was established between BT and the demand of N, P, and K in lettuce (R-2 = 0.99), which leads us to assume that the simplified rational model might be employed to estimate the dose of fertilization in this crop. The concentrations of N, P, and K in BT at MC were 4.0, 0.46, and 4.3%, respectively. These values are similar to those reported as internal requirements for this crop in other countries; on this account, they could be used tentatively to feed the models proposed.