426 Mineral Nutrition of Plants 



magnesium deficiency symptoms were quite prevalent on sweet pota- 

 toes and tomatoes growing in sandy soils in New Jersey during the 

 relatively wet season of 1946. They state that extreme magnesium defi- 

 ciency is not likely to occur on this soil except under conditions of high 

 rainfall or heavy potash fertilization. Boynton et al. (16, ly) have noted 

 that magnesium deficiency symptoms are more prevalent in the apple 

 orchards of New York during wet years than dry. 



The preponderance of evidence indicates that percolation of water 

 through a soil tends to effect the depletion of sodium, calcium, and 

 magnesium ions with relatively little removal of potassium. Jamison 

 (65) reports that potassium is readily leached out of certain sandy soils 

 in Florida in which the exchange capacity is largely provided by organic 

 matter because of the inability of organic adsorbents to fix potassium. 



It is probable that in most instances losses of nutrients by leaching 

 do not exceed contributions from soil decomposition and nitrogen fix- 

 ation over a long period of time.* 



High water tables and wet soils. 



It is evident that under certain conditions serious losses of plant 

 nutrients from the soil may arise from either surface runoff or down- 

 ward percolation. In addition, retention of excess soil moisture within 

 the root zone may have a seriously detrimental effect on the nutrition 

 and health of crop plants. Impervious layers in the subsoil that bring 

 about permanent or even temporary water tables affect soil aeration, 

 and, consequently, root growth, activity of microflora, nutrient avail- 

 ability, and nutrient entry. As mentioned previously, soils with low 

 permeability may also provide adverse conditions within the rhizo- 

 sphere following a heavy rain or after an irrigation. As the soil mois- 

 ture tension becomes lower, the soil pores become increasingly filled 

 with moisture and gaseous interchange is inhibited. Observations on 

 the composition of soil air illustrate the result of this relationship. 

 Furr and Aldrich (45) studied the composition of the soil atmosphere 

 at various depths under irrigated date palms. On irrigating, the soil 



* Since the completion of this manuscript, a pertinent study by Chapman et al. 

 on nitrogen gains and losses in lysimeters has come to the attention of the authors. 

 See H. D. Chapman, G. F. Liebig, and D. S. Rayner, Hilgardia, 19:57 (1949). 



