30 min 120 min Silt loam Sand 200 min Water Water movement in soil layers

sand silt

Water Movement through Stratified Soils Unsaturated flow is dominant in most soils Water always moves from high to low potential Unsaturated flow moves from large to small pores Water moves into a contrasting textural layer only after matric potential approaches 0 and gravitational potential increases (i.e. a positive head is built up at the interface)

Impact of stratified soils Abrupt change in pore size inhibits water flow and percolation –Upward or downward flow is impeded Capillary rise is cut off, percolation slows, perched water table may form –Texture differences, plant residues, gravel layer under a foundation, sand or gravel lenses, golf course construction, buried waste

Sponge demonstration The sponge can be likened to soil in that it has many pores of different sizes. Dip the sponge into a bucket and allow it to completely wet, filling all the pores. This is called "saturated." Hold the sponge and allow it to drain. Big pores will drain first, followed by the smaller pores. Once most of the water has drained out by itself, this is what is referred to as "field capacity" in a soil. By squeezing the sponge further, you mimic a root taking up water and reduce the amount of available water holding capacity. The available water holding capacity is the amount of water between field capacity and wilting point. Water can still be in the soil at wilting point, especially in clays, but the water is held very tightly by the small pores and is unavailable to plants. The plants cannot exert enough energy to extract the water.

L Gravitational water

Capillary water easymedium hard Plant available water

Soil organic matter by weight % Wilting point Field capacity