Research Progress Report 184
Project 910 April, 1965

Fragipans--What Are They?

A. L. Zachary and H. P. Ulrich, Agronomy Department
Soils with an extremely impermeable subsoil are commonly known as "fragipans." They occur on the Southern Indiana Forage Farm and throughout southern Indiana.  Most commonly, fragipans have distinct upper boundaries at depths of 15 to 40 inches below the original soil surface (see Figure 1). Soils of the fragipan group are developed in several types of material in southern Indiana, such as (1) deep loess or wind blown silty material, (2) thin loess over Illinoian age till, and (3) thin loess over residuum material that are products of the disintegration and decomposition of limestone and interbedded sandstone and shale.

	The Forage Farm fragipan soils are Zanesville, Tilsit and Johnsburg. The soil materials from which these soils have developed are a thin loess mantle and materials weathered from interbedded sandstone and shale. These soils have slow internal drainage, shallow rooting and winter heaving of tap-rooted plants. Such soils are commonly found to be either extremely wet or extremely dry. Farmers with similar soils experienced high risks with so-called normal corn belt cropping practices."

	A fragipan is a special kind of dense, firm and brittle subsoil or B horizon that has a strong development of structure, relatively low clay percentage and reversible cementation with iron and aluminum. The fragipan layer or horizon consists of rounded or sharp pointed wedge shaped blocks, prisms or polygons with broad and indistinct bases. The tops have a thin white silt cap of a few inches, if strongly developed, that extends downward and covers the faces of the structural units or peds. The light gray polygonal cracks extend 1 to 3 feet or more and taper off into the underlying material. These cracks are filled with light colored silts and some clay. The interior of the blocks or prisms are so dense that roots rarely grow. Water, air and root movement occurs mostly along the cracks and not through the dense prisms.

	Fragipans are commonly medium texture with silt loam as the predominant texture class. Because of the low clay content, the fragipans are only slightly plastic when wet and are hard to extremely hard when dry. They crush suddenly and completely under severe pressure. Change in consistence with change in moisture content is a striking feature of fragipans.

How Fragipans Develop
	The development and formation of a fragipan is not clearly understood. It is

PURDUE UNIVERSITY • Agricultural Experiment Station • Lafayette, Indiana