10 20 30 



DISTANCE FROM GULF (km) 



in free soil 

 chenier plain 

 from the gul f 



Figure 33. The decrease 

 water salinity (mg/g) of 

 marshes with distance (km) 

 (Rainey 1979). 



Soil Nutrients 



The nutrient content of delta marshes 

 is quite well known from a comprehensive 

 set of surface sediment samples taken 

 across the whole coast by R. H. Chabreck, 

 LSU, in 1968 and analyzed by Brupbacker et 

 al. (1973). Rainey (1979) used the same 

 data set to draw a number of conclusions 

 about the factors controlling sediment 

 nutrient concentrations. Because the 

 density of marsh soils varied from 0.05 to 

 0.97 in Chabreck's data set, a 20-fold 

 range, Rainey converted all nutrient con- 

 centrations to a volumetric basis as 

 recommended by Boelter and Blake (1954), 

 Clarke and Hannon (1967), and Mehl ich 

 (1972, 1973). 



When analyzed on a volumetric basis 

 (dry mass/volume wet soil), the distribu- 

 tion of nutrients across the marshes falls 

 into a predictable pattern. As one would 

 expect, the soluble ions associated with 

 sea water [sodium {Ha), chloride (CI), 

 potassium (K), magnesium (Mg), and total 

 soluble salts] are closely controlled by 

 the surface water salinity (Table 10). 

 This is also shown in Table 11, which 

 compares the ratio of soluble nutrients to 

 chloride in seawater and in the different 

 marsh zones. Sodium, K, and Mg ratios in 

 the marsh are never more than twice the 

 seawater ratio. 



Compared to the soluble ions, some of 

 the total available ions (the soluble plus 

 the exchangeable fractions) behave some- 

 what differently. Total available Na is 



closely related to surface water salinity 

 since it is a major component of sea 

 water. However most available K and Mg 

 are held in the soil exchange complex. 

 Therefore, available K and Mg are strongly 

 influenced by the adsorptive capacity of 

 the soil mineral component as indicated by 

 their high regression coefficients with 

 bulk density in Table 10. Phosphorus 

 distribution is also strongly related to 

 the mineral component of the soil. The 

 major source of phosphorus to the marsh is 

 probably from mineral sediment deposits. 



Neither total nitrogen (N) nor cal- 

 cium (Ca) (either soluble or exchangeable) 

 are closely related to salinity or to bulk 

 density. Unlike the other soluble cations, 

 Ca is abundant in freshwater, and runoff 

 from the surrounding upland areas into the 

 fresh marsh contains high quantities of 

 Ca. This explains the high Ca/Cl ratios 



Table 10. Multiple linear regression 



models of soil ions showing what factors 

 control their distribution in Louisiana 

 marshes (Rainey 1979). For each nutrient 

 the first soil factor entering the model 

 is shown with its R value. The total 

 proportion of the variability accounted 

 for when salinity, bulk density and or- 

 ganic matter are all entered in the model 

 is also shown. In general, one factor 

 accounts for most of the variability. 



*Independent variable that explains 

 greatest part of the variability, and 



the 

 the 



R value associated with it. 

 **Total proportion of the variability in 

 the dependent variability explained by var- 

 iations in the soil factors. 



36 



