Agriculture Reference
In-Depth Information
accumulate along the root and the movement of water, exudates and ions creates specif-
ic conditions. The soil of the rhizosphere is relatively dry and nutrient-poor compared
with the soil external to this zone.
Roots can induce pH changes at the root-soil interface and gradients of concentra-
tion in the rhizosphere can be very steep (
e.g.,
Nye, 1981; Marschner and Römheld, 1983;
Kotyk
et al.,
1991; Gahoonia and Nielsen, 1992). Plants maintain neutral electric condi-
tions in the rhizoplane and thus induce unequal uptakes of anions or cations depending on
pH of the bulk soil (Hedley
et al.,
1982). Release of through root respiration,
enhanced efflux of resulting from iron deficiency and the secretion of organic acids are
other mechanisms whereby roots change the pH of their rhizosphere. The pH is often, but
not always, higher in the root tip region than close to root hairs, it becomes more acid when
N is supplied as ammonium rather than nitrates and differs greatly between plant species
and with soil type (Gijsman, 1990; Marschner and Römheld, 1983; Marschner
et al.,
1986;
Youssef
et al.,
1989; Dormaar, 1988). Haynes (1990) and Jaillard
et al.
(1996) particular-
ly emphasise the effects of proton excretion into the soil solution during cation
absorption (the 'proton pump'): where more cations than anions are absorbed, the soil is
acidified; where more anions are absorbed, the opposite pattern occurs (Figure IV.32).
