The water availability for plants is determined by the water potential of the soil. More precisely, it is the force that a plant must overcome to obtain water from the soil and the force that determines the distribution of soil moisture and the transport of solutions through the soil. The value of soil water potential is most often expressed in units of pressure and negative values. For example, if it is 0 MPa, it is full water capacity, all pores are filled with water and the plant has problems getting enough oxygen. If the values are in the range -0.005 to -0.015 MPa, it is a field water capacity, water is in the capillary pores and the plant has enough water and air. The wilting point occurs at -1.5 MPa, when transpiration exceeds water uptake by the wilting plant.