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 potential) 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 = 106 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 2000 |im, in four soil fractions, with particle size ranging from 75 to 1000 |im, 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 most natural soils where fine particles can stick together to form much larger crumbs, soil structure is the primary determinant.

A perhaps more important finding of Wallace's was that movement by nematodes is fastest not in water-saturated soil, but rather when soil water content is near field capacity (approximately -0.05 bar = 50 cm water), i.e. when soil pores have drained just to the extent achievable by gravity. In fact, when movement was measured at various water potentials in a series of soils with particle sizes ranging from fine to coarse, the optimal water potential for movement was the same (-0.05 bar) regardless of soil texture or moisture content. This effect was shown with infective juveniles of both Heterodera schachtii and the much larger D. dipsaci (Wallace, 1956, 1958a,b). Finally, in any soil at soil matric potentials drier than c. -0.5 bar, nematode movement essentially stopped.

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