Equilibrium Content between Nitrogen and Phosphorus for Lettuce (Lactuca sativa L.) Grown in a Clay Soil

This study aims to investigate the effect of different nitrogen concentrations (60, 120 and 180 mg L-1) applied in the form of ammonium nitrate (NH4NO3) through fertigation and, also, phosphorus concentrations (15, 35 and 55 mg L-1) in the form of phosphoric acid (H3PO4), individually or in combination with each other, on the growth parameters and yield of lettuce plants (Lactuca sativa L.), as well as nitrate accumulation in plant tissues. The investigations were conducted under the field conditions during the two growing seasons of 2019/2020 and 2020/2021 at the experimental farm of Dokki site, Giza Governorate, Egypt, following a split plot design. Obtained results indicated that increasing N supplies up to 180 mg L-1 led to concurrent increase in the vegetative characters (i.e., plant height, stem diameter, number of leaves per plant and chlorophyll content) of lettuce plants. While the lowest growth and productivity of plants were obtained by applying 60 mg N L-1. Regarding the effects of P additions, increasing P supply up to 55 mg L-1 increased the growth and productivity of plants during both studied seasons. Interaction effect between N and P levels indicated that 180 mg N L-1 combined with 55 mg P L-1 gave the highest lettuce productivity followed by 180 mg N L-1 combined with 35 mg P L-1. The weight of lettuce head took the same trend of vegetative growth. The chemical analyses revealed that increase N dose led to increase the content of N, K, Ca and Mg as well as N-NO3- in plant tissues. Increase P level led to increase the P content. Finally, increase P level led to decrease N-NO3- and transformation rate of nitrate in plant tissues.