Guttation is the exudation of drops of xylem sap on the tips or edges of leaves of some vascular plants, such as grasses. Guttation is not to be confused with dew, which condenses from the atmosphere onto the plant surface.
At night, transpiration usually does not occur because most plants have their stomata closed. When there is a high soil moisture level, water will enter plant roots, because the water potential of the roots is lower than in the soil solution. The water will accumulate in the plant, creating a slight root pressure. The root pressure forces some water to exude through special leaf tip or edge structures, hydathodes or water glands, forming drops. Root pressure provides the impetus for this flow, rather than transpirational pull.
Guttation fluid may contain a variety of organic and inorganic compounds, mainly sugars, and mineral nutrients, and potassium. On drying, a white crust remains on the leaf surface.
If high levels of nitrogen appear in the fluid, it is a sign of fertilizer burn. Excess nitrogen must be leached from the soil by addition of large quantities of water. This may result in water pollution, but is the best way to restore soil fertility.