Analyses for other mineral nutrients included potassium, calcium, 

 magnesium, sulfur, iron, and sodium in both 1971 and 1972. An analysis 

 for manganese was included in 1972. Regression analysis was used to 

 determine if there were significant linear or quadratic relationships 

 between the nutrient concentrations in plant tissue and the nitrogen rate. 

 Of the nutrients checked in 1971, only potassium was found to be signifi- 

 cantly related to nitrogen rate (Tab. 35) and this was true only when 

 phosphorus was applied. In 1972, when no phosphorus was applied, phos- 

 phorus, calcium, and manganese concentrations were significantly affected 

 by nitrogen rate. Calcium and manganese concentrations increased as 

 nitrogen rate increased, while phosphorus concentration decreased as 

 nitrogen rate increased. The decrease in phosphorus concentration indi- 

 cates that there is a limited amount of phosphorus available and the 

 amount in the plant tissue is diluted as growth is increased due to nitro- 

 gen fertilization. Where phosphorus was applied, only iron and manganese 

 concentration was significantly affect by nitrogen rate. Manganese 

 increased and iron decreased as nitrogen rate was increased. 



Table 35. The Relationship of Nutrient Concentration in the Plant 

 Tissue to Nitrogen Rate* 



Nutrient 



Year 



P Rate 

 (kg/ha) 



R 2 





Regression Equation 



K 



1971 



74 



0.59f 



Y = 



0.987 + 0.0033(X) 



P 



1972 







0.68+ 



Y = 



0.09 - 0.00007(X) + 0. 00000007 (X) 2 



Ca 



1972 







0.61$ 



Y = 



0.33 + 0.00036(X) - .00000039 (X) 2 



Mn 



1972 







0.62+ 



Y = 



15.75 + 0.0092(X) 



Fe 



1972 



74 



0.59+ 



Y = 



320.0 - 0.219(X) 



Mn 



1972 



74 



0.57+ 



Y = 



18.8 + 0.117CX) 



*X = Rate of nitrogen fertilizer (kg/ha) 

 tSignificant at the 0.05 level 

 ^Significant at the 0.01 level 



Total uptake of mineral nutrients increased as nitrogen rate was 

 increased (Figs. 59 and 60). This was due mainly to an increase in yield 

 in response to nitrogen and phosphorus fertilization, producing more plant 

 material in which nutrients in adequate supply are incorporated. The 

 standing crop of these nutrients is controlled by yield which is in turn 

 limited by availability of nitrogen and phosphorus to the marsh plants. 

 The amount of nitrogen and phosphorus available is the limiting factor in 

 the total amount of nutrients exported from the marsh in the dead plant 

 material which contribute to the nutrient cycle of the adjacent estuary. 



An obvious feature of the data for uptake of mineral nutrients is the 

 large difference in sodium uptake between 1971 and 1972. This was probabl) 

 due to periods of high salt concentration in the substrate in 1972. The 

 level of sodium in the substrate varies with rainfall and frequency of 

 flooding. At Ocracoke, southwest winds during summer often prevent 



129 



