Agriculture Reference
In-Depth Information
on the total number of macropores in soils (see
Chapter 10
this volume). Furthermore, casting on the
soil surface may open new pores in the soil and can even break surface crusts, thereby helping
germinating seedlings reach the soil surface (Kladivko et al. 1986).
Compacted soils may also benefit from the activity of decompacting earthworm species (Blan-
chart et al. 1997, 1999), the incorporation of OM (aggregating agent) by anecic species, and the
burrowing strength and stable aggregate formation by endogeic species (Zund et al. 1997; Larink
and Schrader 2000). For example, the introduction of various endogeic and deeper-burrowing anecic
species of lumbricid earthworms into New Zealand pastures aided the rates of decomposition of
accumulated thatch and physical incorporation of lime, fertilizers, and pesticides into the soil,
reducing physical, chemical, and biological limitations to root growth and pasture productivity
(Stockdill 1982; Springett 1985). However, excessively loose soils or soils with greater proportions
of sand that are prone to water stress, may actually benefit from the aggregating action of compacting
earthworms.
Not all the effects of earthworms on soil structure help plants to grow better. First, the deposition
of fresh earthworm casts on the soil surface and the burial of protective surface litter by anecic
earthworm species can expose soil particles to splash erosion ( Darwin 1881; Sharpley and Syers
1976; Sharpley et al. 1979; van Hoof 1983; Binet and Le Bayon 1999), promoting their downhill
soil movement if the area is sloping. In particular situations and over long time periods, this could
reduce the topsoil layer upslope considerably and increase its downslope, as well as change its
texture (Nooren et al. 1995) and suitability for plants.
In addition, when soils are prone to compaction and a single earthworm species of the com-
pacting type dominates the community, reaching large populations, biomass, and activity levels,
the ultimate effect of the earthworms on plant growth may be negative. Hence, Puttarudriah and
Shivashankara-Sastry (1961), Blackemore (1994), Barros et al. (1996, 1998), Chauvel et al. (1999)
and Ester and Rozen (2002) all observed increased soil compaction and ÑcloddingÒ caused by
earthworm (
P. corethrurus
and various other species) activities and related the lower soil porosity
and water infiltration rates that occurred with decreased plant (radish, carrot, bean, pasture, sorghum,
and potato) productivity. Excessive casting on the soil surface and base of plants by lumbricid
earthworms in England caused difficulties in harvesting cereals and hay (Stephenson 1957; Edwards
and Bohlen 1996), and large amounts of casts on the soil surface of grazed pastures led to ÑpoachingÒ
from cattle trampling, decreasing grass growth in the Netherlands (Hoogerkamp 1984) and New
Zealand (Lee 1959).
Earthworm Burrows
Macropores usually represent only a very small part of the total soil porosity (particularly in clayey
soils), yet they are very important in hydraulic conductivity and water infiltration rates when
connected with the soil surface and in increasing aeration (Kretzschmar 1998, see
Chapter 11
, this
volume). The positive effects of earthworms on water infiltration may help decrease runoff rates
(Roth and Joschko 1991), thereby allowing more water to enter the soil and reducing overall erosion
(Hopp 1946, 1973; Sharpley et al. 1979), as well as increasing the potential for water storage in
the soil. Thus, the effect of earthworms on soil porosity and infiltration, as well as on organic matter
breakdown, has been associated consistently with increased yields in New Zealand pastures (Stock-
dill 1959, 1982) and reclaimed Dutch polders (e.g., van de Westeringh 1972; Hoogerkamp 1984)
and with greater hay and bean yields in large container experiments (Hopp and Slater 1948, 1949),
although the interactions with incorporated or surface OM (another aggregating agent) are also
likely to be implicated (Cogle et al. 1994) in some responses observed by these authors.
Earthworm burrows can serve as preferential pathways for root elongation (Ehlers 1975;
Edwards and Lofty 1980; Kirkham 1981; Ehlers et al. 1983; Wang et al. 1986; Kladivko and
Timmenga 1990; Hirth et al. 1997; Jimnez 1999), especially in compacted zones found typically
in deeper soil layers. In open, abandoned earthworm burrows, the greater aeration and the small
