Essential crop nutrients (Table 1) are needed primarily to enable leaf photosynthesis, respiration and transpiration, so are initially concentrated in leaves. The leaves doing most photosynthesis are at the top of the canopy; hence the topmost and youngest leaves contain most nutrients. After leaves are formed, they continue to photosynthesise for several weeks, accumulating photosynthates. This dilutes some leaf nutrients such as N, P & K, whilst others remain stable, or even accumulate like Mg, Ca & B. Leaf analysis is commonly used to assess crop nutrient status. To minimize uncertainties of dilution or accumulation through development, leaves are best sampled from a standardized leaf position and age (like the youngest fully expanded leaf) and at a standardized growth stage (like GS31). Leaves are best analysed using the Dumas method for N and Inductively Coupled Plasma (ICP) spectroscopy for other nutrients; results are usually expressed as concentrations in dry matter (DM) and are then best compared with standard concentrations published for the same species, growth stage and region, or benchmarked against crops from a network of similar farms.