Problem: Application of nitrogen (N) fertilizer in excess of crop requirements is a common practice in agriculture, to ensure maximum production. However, this usually incurs an additional cost to farmers and increases environmental risk due to N losses. Solution: Optical sensors can detect the nutritional status of crops before serious nutrient deficiencies become a growth limitation and be used to adjust N fertilizer rate and timing to meet crop requirements. This method has been developed for several crops (e.g., wheat, maize and potato) and under various environmental conditions. Optical sensors are available to buy or rent for carrying out measurements (Figure 1), although this is an additional cost for the farmer, in the long-term, they will benefit financially for saving N fertilizer costs. To mitigate varietal and environmental effects, it is advisable to have a reference band in the field with the recommended N rate. This will require an additional N fertilizer application to apply the recommended rate.