作物所需要的营养元素,主要来源于土壤,其中以氮、磷、钾三要素需要量最大,而土壤中这三种元素由于气候、地形、土壤、水肥条件以及产量水平的变动,而常常发生不同程度的缺乏或过多,严重地影响作物生长和产量水平。因此必须经常测定“三要素”作为指导当地生产的参考。 (一)土壤样本采集根据不同土壤类型,地形,历史情况,耕作制度,产量水平与不同土壤肥力状况,按地块采取混合土壤样品,即多点混合取土。一般采集耕作层0～20厘米,在高产地块可酌情深些,(0～20～50厘米取两层)。取土样的数量和密度 The nutrient elements needed for crops mainly come from the soil, of which nitrogen, phosphorus and potassium are the most important elements, while the three elements in soil often occur due to the changes of climate, topography, soil, water and fertilizer conditions and yield levels To a certain extent, lack of or excessive, seriously affect the crop growth and yield levels. Therefore, the “three elements” must always be measured as a guideline for local production. (A) soil sample collection According to different soil types, topography, history, farming system, yield levels and different soil fertility conditions, according to plots to take mixed soil samples, that is, multi-point mixed soil. Generally collected tillage layer 0 to 20 cm, in the high yield plots may be more appropriate, (0 ~ 20 ~ 50 cm take two). Take the number and density of soil samples