Soil and Water Management & Crop Nutrition

For the first few days after heavy rain or irrigations, water drains from the soil profile until its water content approaches a relatively stable value called the drained upper limit or field capacity. When plants have extracted all of the water available to them, the root zone water content approaches a lower limit of available water, or permanent wilting water content. The water held by the soil between these two limits is called plant available water. These two limits are often associated with water content values at specific soil water potentials, (a measure of pressure at soil water is extracted). Field capacity is often taken as the water content of a soil at -33 kPa water potential. Permanent wilt is taken as the water content at -1500 kPa. The methods typically used to determine plant available water are slow and inaccurate. We present here (1) a method for measuring field capacity using a tensiometer and an extrapolation technique, and (2) a method for measuring permanent wilting water content with a dew point potentiameter and an extrapolation method which are much faster and more accurate than traditional methods. Prevention and rehabilitation of degraded land to achieve sustainable agriculture By Patcharin Jankong SWMCN Laboratory Seibersdorf, Joint FAO/IAEA Division Patcharin Jankong joined the Soil and Water Management & Crop Nutrition (SWMCN) Subprogramme in June 2009 as an intern from Thailand. She is currently receiving training and provides technical support in the use of Fallout Radionuclides (FRNs) to assess soil erosion and sedimentation rates in agricultural catchments.