Soil Water Change as Related to Position of Wheat Straw Mulch on the Soil Surface

A field study was conducted in the Central Great Plains to determine the effects of the position of wheat straw mulch on soil water change of a silt loam soil (a montmorillonitic, mesic Aridic Paleustoll) during three 14-month fallow periods. Treatments of 4600 kg/ha of wheat straw mulch flat on the soil surface, ¾ flat and ¼ standing, or ½ flat and ½ standing (normal position following standard combine harvesting) were compared with bare soil. Standing stubble was 0.46 m tall. Soil water content of one 1-m³ hydraulic lysimeter within each treatment was measured daily and changes summed for each week, and at a distance of 2 m from each side of the lysimeter a neutron depth probe was used to determine soil water content weekly except when the soil was frozen. The relationship of soil water change for each weekly period with weekly values of total precipitation, total open-pan (U.S. Weather Bureau Class A) evaporation, average daylight-hour vapor pressure deficit, average maximum air temperature, average daily air temperature, average daylight-hour air temperature, total solar radiation, and average daily wind movement, was determined by single and multiple-correlation techniques. Soil water increases were related only to precipitation events with the highest correlation resulting from the ½ flat ½ standing wheat straw position treatment. Soil water losses were best correlated with wind movement with r² values of 0.55, 0.41, 0.41, and 0.32 for bare soil, flat ¾ flat-¼ standing, and ½ flat-½ standing treatments, respectively. Soil water loss occurred from both bare soil and where the mulch was flat with winds of 0.08 MS⁻¹. When ¼ or ½ of the mulch was standing, a wind of at least 0.55 MS⁻¹ was needed before soil water loss occurred. Standing wheat straw does not function as a wick for loss of water from the soil.