layer of sand to provide heterogeneity, if not a third dimension (Robinson,
2000). A sand layer on agar stimulates nictation by someSteinernemaspp.
(Campbell and Kaya, 1999) and alters the pattern of movement by
C. elegans(Andersonet al., 1997a,b).Soil texture and moisture
Most plant-parasitic nematodes require thin moisture films for movement,
lack the strength to dislodge soil particles and easily become trapped in
water films (Wallace, 1959c). Consequently, their movement is markedly
influenced by the porosity and moisture of soil. Most of what we know
about the influence of soil texture and moisture on nematode movement
came from the classical experiments of Wallace (1958a,b,c, 1959a,b,c,
1960, 1968a). These experiments are best understood in the context of
a major advance in plant physiology that resulted in the 1950s from
the realization that water movement through plants and soil could be
explained best in terms of the Gibbs free energy of water (the water poten-
tial) at the leaf–air interface, within leaf cells, in roots and in the soil
(Milburn, 1979; Papendick and Campbell, 1981; Kramer, 1983). The water
status of plants was found to be directly affected not by the quantity of
water in soil, but rather by the energy required to extract water, due
primarily to the strong attraction of soil particles for water molecules (the
matric potential). In comparison, the osmotic pressure of soil water was
found to be too small to be of physiological significance to plants in most
cases. Ultimately, concepts and notation from electrical engineering were
incorporated into plant physiology to explain and predict the direction
and rate of movement of water in soil and plants. This notation
partitioned the total Gibbs free energy, or water potential, into matric,
osmotic, gravitational and turgor potential. Today, the water potential in
soil, air and plant tissues is usually expressed as a negative pressure given
in bars or pascals (Pa) (1 bar = 0.1 MPa = 10^6 dyn cm−^2 =c. 1 atm).
Wallace built devices to control and measure soil water potential and
monitor nematode migration in three dimensions. When he compared
movement by nematode species and stages of 15 different mean body
lengths, ranging from 186 to 2000μm, in four soil fractions, with particle
size ranging from 75 to 1000μm, he found that, at equivalent water
potential, large nematodes require soil with larger particles than small
nematodes do and the optimum particle size is linearly related to
nematode length (Wallace, 1958c). This relationship is not apparent if soil
water content rather than matric potential is kept constant, because finely
textured soils can hold several times as much water as coarsely textured
soils at the same matric potential. A second result was that, although the
optimum particle size for movement by a nematode of a given length is
constant in sand, when complex soil containing significant amounts of
colloidal clay and silt is dried and sieved into crumb-size classes and then
rewetted, it is crumb size, not soil particle size, that is critical, i.e. in most92 A.F. Robinson