An alternative, indirect but quicker approach might be to measure leaf greenness because chlorophyll content is usually related to nitrogen content. The Soil-Plant Analyses Development (SPAD) chlorophyll meter estimates chlorophyll content, as does the Normalised Difference Vegetation Index (NDVI).
In the present experiments, seeds of two geranium cultivars ('Horizon Deep Red' and 'Horizon Tangerine') were grown in pots and watered with solutions containing 0mg, 50mg, 100mg and 200mg per litre of nitrogen. The SPAD meter was used to scan four mature leaves per plant and an NDVI pocket sensor was used to scan whole plants. Leaves were then removed from each plant and analysed chemically for nitrogen content.
Although SPAD values for the two cultivars differed, NDVI and SPAD values were related to leaf nitrogen content. The SPAD meter would evidently have to be calibrated for each cultivar but then either sensor could be used to monitor the nitrogen content of geranium leaves.
Dr Ken Cockshull, Associate Fellow, Warwick Crop Centre, University of Warwick
Use of An Active Canopy Sensor & SPAD Chlorophyll Meter to Quantify Geranium Nitrogen Status by Wang, Dunn, Arnall and Mao (2012). HortScience 47 (1): 45-50. The author's abstract can now be seen in full at http://hortsci.ashspublications.org but ISHS members can view HortScience online at www.ishs.org.