Determining Mean pH for Standard Layers

Determining the mean pH for the 11 standard layers required three main steps:

1. For each component layer, computing the mean pH.
2. For each component, determining the contribution of each component layer to the 11 standard layers.
3. For each map unit, combining the contributions of all components to compute the mean pH for each standard layer.

The pH of a mixture of two soils having different pH values is dependent upon the specific chemical composition of the minerals and included water in the soils and the interactions between them. Factors such as the buffering capacity of constituent compounds and surface properties of minerals may cause the pH of the mixture to be significantly different from a simple average of the pH of soils which are mixed together. The STASGO Layer tables do include some additional variables, such as cation exchange capacity, carbonate concentration, and salinity, which might permit adjusting the mean pH for some of these effects. However, since all pH values in the Layer table are rounded to the nearest 0.1, and cation exchange capacity values are present for only about one-half of the layers, it appears that any such adjustments would result in, at best, a negligible inprovement in the final results.

For each layer of each map unit component, the STATSGO Layer table contains two values for the pH, PHH and PHL, defined as the maximum and minimum, respectively, for the range in pH for the soil layer or horizon. The mean pH for each component layer was computed as the arithmetic average of PHH and PHL.

For non-mineral soil layers, the STATSGO Layer table may specify that the pH for a layer is undefined by giving the values of PHH and PHL as 0.0. The computation of the average ph value for a standard layer ignores component layers for which the pH is undefined. If pH is undefined for all component layers which contribute to a standard output layer for the mapunit, then a pH value of 0.0 is used to indicate that the pH is undefined for that layer.

There were 507 component layers (approximately 0.4 % of all layers) whose TEXTURE1 (dominant texture) code corresponded to a mineral soil but a pH value was not entered or a value of 0.0 was used to indicate data not available. Roughly half of these layers had codes indicating the presense of coarse rock fragments (CB, GR, SH, ST); the remainer did not. The number of layers without valid pH for each texture code are tabulated below:

       TEXTURE1	    Count		TEXTURE1    Count

	C               6               GRV-S          39
	CBV-L           1               GRX-COS        12
	CBV-SIL		1	        GRX-S          96
	CBX-S		1	        L              16
	COS	       17	        S             153
	FL-L		7	        SH-C            4
	FS	       13	        SICL            2
	FSL		7	        SIL             8
	GR-COS	       22	        SL             39
	GR-S	       42	        ST-SIL          1
	GRV-COS	       16	        STV-FSL         2
        GRV-L           2                           

The contributions of each component layer to the standard layers for a given map unit were determined using the component layer depths specified by Layer table variables LAYDEPL and LAYDEPH, the mean depth to bedrock for each component calculated by averaging Comp table variables ROCKDEPL and ROCKDEPH, and the percent of the area of the map unit covered by each component as specified by COMPPCT.

For each component, the layers defined in the Layer table were compared with each standard layer in turn. If the standard layer was entirely included within one of the component layers, the pH value for the layer was multiplied by the COMPPCT value to determine the weighted contribution of the component to the standard layer. If the standard layer overlapped two or more component layers, the pH for each component layer were first weighted in proportion to the amount of overlap before multiplication by the COMPPCT value. The region from the bottom of the last component layer to the bottom of the last standard layer, if any, was assumed to be the same as the lowest component layer down to the mean bedrock depth. Below this depth, the pH was set to 0.

The weighted contributions of all components to each standard layer were then summed to obtain the mean pH values for the map unit. The result was rounded to the nearest 0.1; this matches the precision of the pH values in the STATSGO Layer table. If none of the component layers contributing to the standard layer were mineral soil or if the entire map unit was specified to be water, the pH was set to zero.

NOTE that for many STATSGO components, a depth-to-bedrock value of 60 inches (152 cm) was used to indicate that the soil was not examined below this depth, and bedrock was not actually encountered. In all cases, however, the pH was computed as if bedrock was encountered at the depth specified by the mean of ROCKDEPL and ROCKDEPH. Accordingly, the pH values for the two lowest standard layers (1.5 to 2.5 m) may in some cases be set to 0.0, indicating data not available, even though soil actually extends down to these depths.

8/27/2001