Larger root systems with greater surface area, typical of tropical grasses, are generally believed to
be better for acquiring P per unit soil surface area than are the smaller roots typical of tropical legumes.
Larger root systems are often associated with plants that are better competitors for nutrients, water,
light, etc., such as tropical grasses [213], and grasses may competitively exclude companion species of
legumes. Legumes with smaller root systems, however, may have a greater capacity to absorb P rapidly
than grasses with larger root systems [234]. The higher P uptake efficiency in legumes could be at-
tributed to the ability of legume root systems to modify the chemistry of the rhizosphere by exuding
organic acids, Al and Fe chelators, reducing agents, or enzymes such as phosphatases. The superior
compatibility of certain forage legumes (e.g., Arachis pintoi) with the aggressive grasses may be due
to their ability to acquire P from less available forms of P (aluminum-bound P and organic P) in the
low-fertility acid soils.
- Adaptive Responses to Low Phosphorus Supply
In highly weathered acid soils, such as the Oxisols and Ultisols of tropical America, P is often the most
limiting nutrient for pasture establishment and production [238,239]. For pasture establishment in tropi-
cal soils, yield increases following P applications are common [238].
Rao et al. [232] showed that the grass B. dictyoneuraand legume A. pintoidiffer significantly in their
responsiveness to soluble-P fertilizer application in acid soils. When grown either as monocrop or in as-
sociation, the grass responded more than the legume to an increase in P supply on either sandy loam or
clay loam soil. The most striking effects of low P supply on shoot growth and development are reductions
in leaf expansion and leaf surface area [232]. Leaf expansion is strongly related to the extension of epi-
dermal cells, and this process may be particularly impaired when the P content of epidermal cells is low
[240]. However, the P concentration in legume epidermal cells may be greater than that in grass leaves,
which would contribute to the legume’s greater leaf expansion.
Root attributes such as length, surface area, fineness (radius), and density of root hairs are consid-
ered to influence strongly plant adaptation to low-P soils [18]. This is because soil P is supplied to plants
mainly by diffusion and the P diffusion coefficient is very low [241,242]. The most prominent root char-
acteristic of the grass is high root length that results in a large root surface area and a high ratio of root
surface to shoot dry weight. These two parameters greatly enhance P acquisition and P supply to the
shoots. This is because a finely divided and rapidly developing root system provides better access to less
mobile soil nutrients, such as P [243].
The response of B. dictyoneurato applied P was greater than that of A. pintoiin terms of both shoot
and root biomass production [232]. This increased response to P supply in the grass was associated with
higher P use efficiency (grams of forage produced per gram of total P uptake). However, P uptake effi-
ciency (milligrams of P uptake in shoot biomass per unit root length) was several times higher in the
legume than in the grass [234]. These differences in P acquisition between the grass and legume could re-
sult from their differences in ability to use sources of less available P from low-fertility acid soils.
Rao et al. [236] tested this hypothesis and found marked differences between the legume and grass
in their ability to use sources of relatively less available inorganic and organic P. The legume produced
greater amounts of leaf area when grown on sources of sparingly soluble P than did the grass. Increased
availability of P from calcium-bound P (Ca-P) was of greater advantage to the grass than to the legume,
enabling the grass to dominate when grown in association [232,244]. Grass and legume growth, as mea-
sured by shoot and root biomass per unit soil surface area, responded very differently to different P
sources. The response of the two plants to sources of relatively less available aluminum-bound P (Al-P)
and organic P was similar.
Increasing P supply to an Oxisol improves fine root production of the grass to a greater extent than
in the legume [232]. Under monoculture, although source of P and soil type affected the grass’s total root
length but not that of the legume [236,237], root length values were several times higher for the grass than
for the legume. Specific root length (SRL) values were also greater for the grass than for the legume, re-
gardless of soil type and P source. Under association, the proportion of legume roots was greater under
no P supply and lower under Ca-P supply. The ratio of root length to shoot biomass was also markedly
greater for the grass than for the legume. These results indicate that the grass was more efficient than the
legume in producing root length and root biomass, regardless of P source and soil type. When grown in
association with legumes, the grass can effectively compete with the legume by exploring a greater vol-
ume of soil to acquire nutrients and water.
600 RAO