Agriculture Reference
In-Depth Information
Earthworms
Casting Burrowing
Soil structure
(pore size distribution
and aggregate stability)
Micropores
< 0.2
Mesopores
0.2 to 30
Macropores
>30
µ
m
µ
m
µ
m
Soil physical
functions
Water-holding
capacity
Infiltrability,
aeration
Root penetration and growth
FIGURE 2.5
Diagrammatic representation of ways by which earthworms can affect plant growth via physical
changes in the soil environment by burrowing and casting. (Expanded from Syers and Springett, 1983.)
soils (pores smaller than 30 Hm) and the stability of soil aggregates. However, earthworm species
differ greatly in their ability to modify soil structure, depending on their ecological strategies and
behavior. Plants also differ tremendously in their nutrient and water requirements and rooting
strategies. The minimum pore size for effective penetration of the roots of most crop species is
approximately 200 Hm (Wiersum 1957), so many roots become concentrated in macropores,
although some root hairs may penetrate mesopores 5 to 20 Hm wide (Hofer 1996).
Earthworm Casts
Earthworms produce basically four types of casts (Lee 1985; Lavelle 1988; Edwards and Bohlen
1996):
1.Globular, consisting of coalescent round or flattened units, generally produced by the
larger earthworm species (anecic and endogeic species).
2.Pastelike slurries, mainly produced by endogeic or anecic species and excreted as single
masses of soil without a distinct shape, but that take on irregular shapes once dried.
3.Tall vertical heaps or columns of variable shapes, usually deposited on the soil surface
where they are most visible by endogeic or anecic species. These are usually created by
the sequential deposition of globular casts and, when in tower form, often have a hole
in the middle (Darwin 1881; Edwards and Bohlen 1996).
4.Granular, typically in the form of pellets, produced mainly by smaller earthworm species
(epigeic, small endogeic, and some anecic species) and distributed on or beneath the soil
surface.
Casts from different earthworm species can have very different effects on soil structure. The
first three types of casts tend to be larger, heavier, and more compact and are usually produced by
ÑcompactingÒ earthworm species; the granular casts are normally smaller, lighter, and looser and
break down more easily, and are mostly produced by ÑdecompactingÒ earthworm species (Blanchart
et al. 1997, 1999). Often, the casts of compacting species are consumed by decompacting species,
a process that breaks up the larger aggregates into smaller ones, helping regulate overall soil
aggregation (Blanchart et al. 1997; Decens and Rossi 2001) and liberate nutrients that were
protected in the casts for plant roots (see
Figure 2.6
)
.
