Study of conifer nitrogen nutrition using hydroponic cultures: Application to fertigation systems

Nitrogen (N) optimization of a nutrient solution for ornamental conifers was obtained with two different experiments in hydroponic cultures: First, by using three different NO₃ ^‐/NH₄ ⁺ ratios (60/40, 55/45, and 40/60 in percentages of the total N supplied); and, secondly by testing three total N levels (3.7; 4.7 and 5.5 mmol L^‐1). Best growth was obtained with a NO₃ ^‐/NH₄ ⁺ ratio of 55/ 45 and a total N level of 3.7 mmol L^‐1. With these experiments, reference levels of foliar concentrations of the macronutrients N, phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and the biochemical indices, such as chlorophyll and starch levels, were obtained with the treatments corresponding to the plants with the higher growth. In the course of a growth cycle, a substrate assay with two different pot mixes (moss peat plus perlite and black peat plus sepiolite 60/40 ‐% v/v‐) was carried out by using the best N ratios and doses in nutrient solutions as obtained in hydroponics. The results indicated that the same N fertilization in fertigation systems changes depending on the different physicochemical properties of the substrates used; in this case, depending on the different physical properties of the two substrates. By applying DRIS to perform nutritional diagnosis, it is possible to find nutritional limitations to plant growth, but not additional factors, such as water‐air relationships in the growth media.