Determining nitrogen fertility status using optical sensors in geranium with controlled release fertilizer

Abstract: Greenhouse production of geraniums is popular for sales in the spring, and monitoring plant nutrition is important for high quality plants. The objective of this study was to evaluate if nondestructive handheld sensors could be used to quantify nitrogen (N) status in Pelargonium × hortorum ‘Maverick Red’ using controlled release fertilizer (CRF). Fertilizer treatments of 0, 4, 8, 10, or 12 g of 16N-9P-12K were topdressed on greenhouse grown plants. Individual plants were scanned from 10 pots per treatment for Normalized Difference Vegetative Index (NDVI) and Soil-Plant Analyses Development (SPAD) over eight different sampling dates starting 7 days after fertilizer treatment application (DAT). Height, width, number of flowers, number of umbels and leaf N concentration were also recorded. Linear and quadratic trends were seen for both NDVI and SPAD. Plant height and width was highest in the 12 g treatment, but was not different than the 8 g or 10 g treatments. Number of flowers was highest in the 10 g treatment, but was not different from the 8 g and 12 g treatments. Number of umbels was not significantly different among fertilizer treatments, but all were greater than the control. For all measurement dates, a correlation was seen for fertilizer rate and leaf N concentration. Neither sensor showed correlations with leaf N concentration at 7 DAT or 14 DAT; however, both were correlated with each other and leaf N concentration starting 28 DAT. Results from this study indicated that 8 g CRF produced the best quality plants. Both NDVI and SPAD can be used to predict N status in potted geraniums grown with CRF, but consistency in sample collection and sampling time may be necessary to correlate the values with N status.