capacity are independent of the nutrient
root, metabolites for cell wall construction, and
concentration.
growth hormone to loosen the bonds within the
Parameters required for the Claassen-Barber
cell wall constituents (Lockhart 1965). Water
model along with one additional parameter re-
flows radially into the elongating root cells only
quired for the Cushman model are
when the cell's total water potential is lower than
1. Root morphology
the combined osmotic and matric potentials of
a) Initial root length
the soil (Rendig and Taylor 1989). Furthermore,
b) Rate of root growth
lowered soil water content can shrink root diame-
c) Average root radius
ter (Cole and Alston 1974), which reduces root/
2. Root uptake kinetics
soil contact, increases root senescence, lowers soil
a) Maximum influx Jmax of the nutrient
hydraulic conductivity, reduces the water poten-
b) External nutrient concentration Km to ob-
tial of the soil surrounding a specific root, and
tain one-half Jmax
decreases root hairs, thereby affecting the water
c) External nutrient concentration cmin
and nutrient uptake.
where net nutrient uptake is nil
Water absorption is influenced by the geomet-
3. Soil nutrient supply
ric distribution of viable roots in the soil profile
a) Rate of water influx
and the availability of water at the root surfaces
b) Concentration of the nutrient in the soil
(Smucker and Aiken 1992). Absorption of soil
solution at the start of the plant growth
water is not always linear with greater root
period
growth. Incomplete root contact with the soil
c) Apparent diffusion coefficient of the nu-
and/or declining soil water potentials reduce the
trient
water absorption efficiencies, resulting in the pro-
d) Differential soil buffer capacity b for the
duction of excessive plant root surfaces. Weak
nutrient
root contact with the soil matrix results as roots
e) Mean half-distance between root axes
are clustered within the macropores of soils and
The CushmanBarber model, written in Pascal
smaller roots grow along soil aggregates or in
and compiled for IBM PC, is available for educa-
pores larger than the diameter of the roots (Lafo-
tional purposes (Oates and Barber 1987).
lie et al. 1991, van Noordwijk et al. 1992). Greater
root/soil contact occurs as roots are exposed to a
compacted soil environment (Kooistra et al.
ROOT GROWTH UNDER PHYSICAL
1992).
EDAPHIC CONSTRAINTS
Lascano and van Bavel (1984) calculated the
partition of the water uptake rate over the root
Soil water
The most crucial edaphic factor in a plant's life
zone as
is water, which links it to the soil via roots and
Rj = (Ψsj Ψe,i )RDj/PHR
serves as a vehicle for nutrient transport. Water
(16)
also controls the exchange of gases and moder-
ates soil temperature changes (Clothier and Scot-
where Rj = rate of water extraction from the root
ter 1985). Soil absorbs, stores, and releases water
zone or compartment j
Ψsj = soil water potential in that compart-
depending upon volume and size distribution of
pores as determined by texture, structure, organic
ment
Ψe,i = effective leaf water potential
matter content, and the depth of the soil. Plant
available water is held in the soil between 0.01 or
RDj = relative root density in the soil com-
0.03 MPa potential (in coarse-textured and fine-
partment
textured soils, respectively), known as the upper
limit or field capacity, and 1.5 MPa potential,
The summation of Rj from each soil compartment
called the lower limit or permanent wilting point.
equals the total water uptake, provided there is
Water availability may not be limited to 1.5 MPa
no change in water content of the plant. In nature,
potentials; plant roots can extract water at lower
this is not true because the plant water content
potentials depending upon the plant type and the
changes diurnally with radiation load and chang-
aerial environment (Musick et al. 1976).
ing soil water content. Lascano and van Bavel
Root growth rates are controlled by the pres-
also assumed that the plant hydraulic resistances
ence of continuing supplies of water to maintain
are constant; conversely, the significance of plant
hydrostatic pressure in the elongating cells of the
10
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