Fast-K is a rapid test to determine the concentration of potassium (K) in soybean leaves under field conditions in an efficient and reliable way, speeding up decision making if potassium deficiency is identified. The Fast-K methodology makes it possible to evaluate potassium content in soybeans, separating them into three nutritional states: Deficiency, “Hidden Hunger1” and Suitable. Based on this diagnosis, one can define nutritional management strategies for the crop even in the same harvest season. The method is based on the use of a specific portable ion meter (K meter) Horiba Laquatwin® (Horiba, Ltd., Kyoto, Japan), or similar equipment to determine K concentration in liquid samples. Access the hot site http://www.embrapa.br/soja/fast-k to calculate K concentration from the information to be noted (Leaf samples, Final volume and Average Reading), in addition to obtaining more detailed information on sampling, necessary materials, procedures and data interpretation. It is worth mentioning that this determination must be complemented with standard soil and leaf analyzes to carry out a complete diagnosis of soil fertility and the nutritional status of the plants. [1] Hidden Hunger: A condition in which productivity is reduced due to lack of nutrients, without the plant showing visible symptoms of deficiency.