Wadleigh and Richards 423 



INFLUENCE OF EXCESS MOISTURE ON NUTRIENT SUPPLY AND AVAILABILITY 



The influence of an excess of water upon nutrient supply and availa- 

 bility in soils may be segregated into three categories, viz: (a) surface 

 erosion, {b) leaching, and (c) presence of a high water table or other 

 conditions maintaining soil in a relatively wet state. Virtually no soil 

 is devoid of the occasional presence of one of these effects, and fre- 

 quently two or all three of them may be operative. 



Surface erosion 



Agriculturists in this country have become increasingly aware of 

 the vast losses in soil fertility incurred by surface runoff. This has been 

 so well publicized that further mention seems superfluous. Yet the 

 enormity of this loss mitigates any danger of overemphasis. Fippin (42) 

 presents data to the effect that the Mississippi River carries 475 million 

 tons of silt into the Gulf of Mexico during the average year, and that 

 this silt load contains 4.5 million tons of the exchangeable bases — 

 calcium oxide, magnesium oxide, and potassium oxide — and 1.5 million 

 tons of phosphoric anhydride and nitrogen. This is indeed an enor- 

 mous quantity of plant nutrients. Fippin also calculated that for the 

 year 1939, the average loss of plant nutrients per acre of row crops in 

 the Tennessee River system was "84.6 pounds of calcium, 97.9 pounds 

 of magnesium, 212.2 pounds of potassium, 13.0 pounds of phosphorus, 

 all expressed as oxides, and 23.8 pounds of nitrogen." These figures 

 amply indicate that surface runoff may bring about an alarming drain 

 on plant nutrients present in the relatively more fertile top soil. How- 

 ever, these values are much higher than those reported by Bryant and 

 Slater (23) for two New York soils subjected to seven different kinds 

 of cover. The small losses of nutrients which they found in surface 

 runoff would probably be compensated by contributions from soil 

 formation processes. 



Leaching 



Soil management practices inducing infiltration and curbing nu- 



.trient loss from surface runoff do not necessarily prevent nutrient loss 



by percolation. The objection might be raised that prevention of surface 



runoff merely alters the manner in which plant nutrients are removed 



