Welcome to the Michigan State University - US Forest Service Soil
Drainage Index and Fertility Index web page

This page is designed to help you calculate the Drainage Index (DI) and Fertility Index of any soil in the world, as long as it is classified within the US system of Soil Taxonomy. For help with this taxonomic system, please go to the official NRCS page or to the Pedosphere page. This work was performed under the supervision of Dr. Randall Schaetzl, Department of Geography, Michigan State University, under contract with (and supported by) the US Forest Service. Special thanks go to Frank Sapio and Frank Krist of the USFS, who have long been supporters of this project, and to the USFS for research support.

The Drainage Index
What is the Drainage Index?  The DI, originally named the "natural soil wetness index" (Schaetzl 1986), is a measure of the wetness of a soil.  It is designed to represent, as an ordinal number, the amount of water that a soil contains and makes available to plants under normal climatic conditions.  It is not meant to mimic the concept of "plant available water", which is mostly dependent upon soil texture.  The DI only loosely/secondarily takes soil texture into consideration.  The main factor affecting the DI is the depth to the water table and the soil volume available for rooting, for a plant can get at this form of water readily.  The DI concept was first initiated by Hole (1978) and Hole and Campbell (1985), and expanded upon by Schaetzl (1986). 

The DI ranges from 0 to 99.  The higher the DI, the more water the soil can and does, theoretically, supply to plants.  Sites with DIs of 99 are, essentially, open water.  A soil with a DI of 1 is so thin and dry as to almost be bare bedrock.  The DI is derived from the soil's taxonomic subgroup classification in the US system of Soil Taxonomy, and (optionally) its soil map slope class.  Because a soil's taxonomic classification is not (initially) affected by such factors as irrigation or artificial drainage, the DI does not change as soils become irrigated or drained (unless the long-term effects of this involve a change in the soil's taxonomic classification).  Instead, the DI reflects the soil's NATURAL wetness condition.  Each soil SERIES has, in theory, its own unique DI.  Some soil series span two or more drainage classes; in this case the DI that is used is the one that would normally be used for a soil with that subgroup classification. 

To determine the DI of a soil, you must know one or two facts about it:
    (1) its taxonomic subgroup, e.g., Typic Hapludalf, or Vitrandic Torripsamment,
    (2) the slope class within which it is mapped, e.g., 0-2% slopes, or 12-18% slopes.  Slope class is an optional entry.
Once this information is known, you may choose one of three ways to determine its DI, within this web-based platform

2. If you would rather pick from a list of options, starting with order and progressing down to subgroup and then slope class, click here: ______________ and hit RETURN.

3. A third option, for GIS applications only, is to join our master table to your SSURGO table (SSURGO soils data are the county-level soil survey data that can be downloaded from the NRCS directly, from their Soil Data Mart web page.  Join the SSURGO table to our Master DI table, available for download here.  The join should be made on the MUKEY variable.  Once the join has been made you will notice a column in the joined table called "DI", which will then provide the DI for each of your soil map units.  By joining the tables in this way, not only is the DI tallied for each subgroup in your county-level data file, but slope class is also taken into account automatically.


The Fertility Index
What is the Fertility Index? Like the DI, the FI is an ordinal measure of the fertility of a soil. The FI uses family-level Soil Taxonomy information, i.e., interpretations of taxonomic features or properties that tend to be associated with low or high soil fertility, to rank soils from 0 (least fertile) to 19 (most fertile). The index has wide application, because, unlike competing indexes, it does not require copious amounts of soil data, e.g., pH, organic matter, or CEC, in its derivation. GIS applications of the FI, in particular, have great potential. For regionally extensive applications, the FI may be as useful and robust as other fertility indexes that have much more exacting data requirements. As with the DI, users may choose to download the join file from this web site and join it to their SSURGO data. After the join is completed, the FI shows up as a field in the SSURGO data attribute table.

For more details about the DI or FI workings and theory, or to have questions answered, feel free to contact Randall Schaetzl at Michigan State University.  We welcome suggestions and additions to improve the DI and FI.

References::

Hole, F.D. 1978. An approach to landscape analysis with emphasis on soils. Geoderma 21:1-13.

Hole, F.D. and J.B. Campbell. 1985. Soil Landscape Analysis. Rowman and Allanheld, Totowa, NJ 196 pp.

Schaetzl, R.J. 1986. A soilscape analysis of contrasting glacial terrains in Wisconsin. Annals Assoc. Am. Geogs. 76:414-425.

PDF of the DI paper.