The hand-held soil plant analysis development(SPAD) chlorophyll meter has proved to be a promising tool in evaluating the nitrogen status of the potato and guiding fertilization recommendations. In the process of N evaluation of potato plants and N recommendation, it is critical to establish the threshold SPAD value(SPAD reading), below which nitrogen supplement is required. And taking convenient using into account, the threshold needs to be dynamic throughout the potato growing season so that the users can test their potato plants and make fertilization decision at any growing time of potato. To complete this goal, field experiments with different nitrogen supply levels were conducted in different sites in northern China from 2009 to 2011. The results showed that threshold SPAD values decrease as the growing season progresses for all cultivars and planting sites. By statistical analysis, the threshold regression models were established respectively as: y=-0.003x2-0.0507x+58.213(y, threshold SPAD value; x, days after emergence) for the potato cultivar Kexin 1, and y=-0.003x2+0.017x+52.489(y, threshold SPAD value; x, days after emergence) for the cultivar Shepody, from which, the threshold SPAD value at any day after emergence can be calculated.
