Fragipan Horizon - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 148

Chapter 15
Fragipan Horizon

15.1 Introduction

Soils with restricting layers occur in all parts of the world. Some of these layers
are saturated with water or permanently frozen. In many soils, the restricting layer
has a geogenic origin due to limited weathering or lithological discontinuities.
The pedogenic accumulation of clay, carbonates, gypsum, iron, silica, or a combi-
nation of these can also lead to the formation of a hard and dense horizon. Some
root-restricting layers are the result of high concentrations of salts or other
substances that affect plant growth. Soils with restricting layers occur in most of
the soil orders of Soil Taxonomy (Soil Survey Staff 2010 ). Examples are petrocalcic
and petrogypsic horizons, duripans, ortstein, placic horizons, plinthite, and
fragipans. The formation of these horizons and layers is fairly well-understood
except for fragipans.
In Soil Taxonomy (Soil Survey Staff 2010 ), a fragipan (from the Latin fragilis ,
meaning “brittle”) is defined as a layer that is 15 cm or more thick; contains a very
coarse prismatic, columnar, or blocky structure (or is massive); has a firm or firmer
rupture-resistance class (brittle) in more than 60 % of its volume; and contains
virtually no roots. Fragic soil properties are the essential properties of a fragipan,
but not all soils with fragic properties have a fragipan. A characteristic of a fragipan
is that more than 50 % of air-dry fragments disintegrate and crumble (slake) in
water, and the fragipan material is not effervescent in HCl; thus, it is not cemented
by carbonates. Fragipans have a high bulk density, a relatively low clay percentage,
and a pronounced, reversible induration (Grossman et al. 1959a , b , c ).
In the World Reference Base for Soils (IUSS Working Group WRB 2006 ),
a fragipan is defined as a “natural non-cemented subsurface horizon with pedality
and as porosity pattern such that roots and percolating water penetrate the soil only

Bockheim, J.G., Hartemink, A.E. 2013. Soils with fragipans in the USA. Catena 104:233-242.
Acknowledgment is given to Elsevier Publishers for allowing a revision of this article to be
published herein.