Threshold value for chlorophyll meter as decision tool for nitrogen management of potato

We developed a practical system to improve crop N efficiency in potato (Solanum tuberosum L.). It is based on a strategy that includes split applications of N fertilizer and the use of the Hydro N Tester (HNT) leaf chlorophyll meter as a plant N status indicator. The method was tested in 12 potato field trials conducted from 1997 to 2000 on loam and sandy loam soils in Belgium. The fields were fertilized at planting with 70% of a field-specific N recommendation. Because of site-specific effects, raw HNT values measured during the growing season were not useful in assessing the crop N status and the need for supplemental N. In each field, there was a 200-m(2) subplot receiving no N fertilizer to obtain a control HNT value. After harvest, the 12 sites were separated into two groups according to the crop response to increasing N fertilizer rates. A parameter called the HAT slope was determined as the difference between HNT values measured in the field and those from the zero-N plot divided by the applied N rate. This parameter shows low values for sites presenting a weak crop response to N and high values for sites with a strong response. A threshold value to determine the need for supplemental N was found to correspond to a HNT slope value of 0.5. This value was successfully validated on a commercial scale at 10 sites in Belgium in 2001 and 2002 for nonirrigated crops of cultivar Bintje.