4 22 Mineral Nutrition of Plants 



when an excess of moisture is applied and drainage is allowed to take 

 place is at a considerably higher level than that which would exist in 

 this same soil in the field. This higher moisture content may consider- 

 ably alter the aeration and the physiological status of the plants, especi- 

 ally in fine-textured soils. 



When the moisture content of the soil is at field capacity or above, 

 small changes in the hydraulic head can bring about corresponding 

 movement in the water in the soil. However, when the moisture con- 

 tent of the soil is below field capacity, which corresponds to soil mois- 

 ture tensions of the order of 50-150 cm. of water, then the unsaturated 

 permeability of the soil is small, and moisture movement, i.e., the 

 velocity of movement, is restricted even for large hydraulic gradients. 



When the moisture in soil has become reduced to near the wilting 

 percentage by plant root extraction, it is found that the process of 

 rewetting of the soil follows a fairly regular pattern. In order to get 

 water to move into and through the soil, enough water must be added 

 to the wetted zone to raise the moisture content of that zone above the 

 field capacity. It must be accepted as an experimental fact that there is 

 a comparatively steep moisture gradient in the wetting front. In order 

 for wetting to proceed, the moisture content of the wetting front must 

 be raised to field capacity or above, otherwise the permeability of the 

 soil will be too low to transmit moisture at a rate that will allow the 

 wetting process to proceed at an appreciable rate. When a limited 

 amount of water is added to dry soil, the moisture content of the 

 wetted portion is raised to field capacity and the rest of the soil will 

 remain dry. This phenomenon has been discussed by Shaw (/08), 

 Veihmeyer (120), Colman and Bodman (■?■?), Kirkham and Feng (68), 

 and others. As a consequence of this wetting phenomenon, it appears 

 that for practical purposes the only method for controlling soil moisture 

 during plant growth is to limit the degree of dehydration before the 

 whole root zone is brought to field capacity, or above, by irrigation. 

 Nevertheless, papers continue to appear in the literature reporting 

 experiments in which plants were grown at constant, controlled soil 

 moisture values. The average moisture content for the whole soil 

 volume may have been maintained nearly constant but there is con- 

 siderable doubt in such experiments as to whether the moisture was 

 evenly distributed after limited water applications. 



