Otani and Ae’s [245] field and greenhouse studies indicated that P uptake by crops in soils where
P availability is high is strongly related to root length, but this relationship is lost in soils with low
P availability or where soil volume is limited. Their results also suggested that peanut (Arachis hy-
pogaeaL.) uses additional mechanisms beyond root length to increase P uptake. The forage legume A.
pintoi, a perennial peanut and wild relative of the cultivated peanut, shows remarkable adaptation to
less available P forms, such as Al-P and organic P, by producing greater leaf area with less root length
[236,237].
Rao et al. [237] showed that A. pintoiis more efficient in acquiring Al-P and organic P from acid
soils than is B. dictyoneura. They also showed that the association of the two forages could increase the
total P acquisition from low-fertility acid soils. The legume has at least three attributes that are important
for its efficiency in acquiring P from acid soils: (1) high storage capacity for inorganic P, (2) a favorable
ratio of P uptake per unit root length, and (3) high activity of acid phosphatase in the root and a capabil-
ity for using P from organic P sources. These attributes could form a self-controlling system for acquir-
ing P from low-P acid soils. The authors speculated, furthermore, that the superior compatibility of A. pin-
toiwith aggressive grasses such as Brachiariaspecies may be due to its ability to acquire P from less
available forms. The mechanism by which A. pintoiaccesses the sparingly soluble inorganic P remains
unknown.
Rao et al. [237] tested the relationships between root and shoot attributes and showed a significant
positive correlation between the level of inorganic P in the legume’s roots and key shoot attributes such
as leaf area production, shoot biomass, and shoot P uptake. This observation indicates that measuring the
level of inorganic P in roots may serve as a selection method to evaluate differences in adaptation of trop-
ical forage legumes to P-deficient acid soils.- Selecting for Improved Adaptation to Other Soil Constraints
Defining nutritional requirements of different grass and legume ecotypes can help to reduce the amount
of fertilizer needed to establish pastures rapidly and to maintain productivity over time. Research con-
ducted at CIAT and elsewhere generated valuable information on both internal (plant) and external (soil)
critical nutrient requirements for several tropical forage species [225–227,246–248]. Internal P and Ca re-
quirements for plant growth of Brachiariaspecies are much lower than those of P. maximum(Table 4).
Among the Brachiariaspecies,B. humidicolaappears to require lower internal concentrations of P, Ca,
and K. The external P requirements of B. brizanthawere much higher than those of B. decumbenswhen
grown in low-fertility Latosol in Brazil [228]. Although nutrient requirements of A. pintoi, compared with
other tropical forage legumes, are relatively low to moderate [227], it has higher critical levels for P and
Ca (Table 4).
Diagnosis of mineral nutrient disorders using visual symptoms has been developed for B. decumbens
andA. pintoi[227,228]. Several greenhouse and field experiments on Brachiariaspecies demonstrated
striking responses in terms of forage yield to P applications but not to lime applications [228]. Responses
ADAPTATION OF BEANS AND FORAGES TO ABIOTIC STRESSES 601
TABLE 4 Values or Ranges of Critical Concentrations of Various Nutrients in Shoot Dry Matter of
Different Tropical Forage Grasses and Legumes, Compared with Animal Requirements
NPKCaMgSZnCu
Forage species (g kg^1 ) (mg kg^1 )
Grasses
Andropogon gayanus 13.0 1.0 9.0 2.5 2.0 1.4 18 6.0
Brachiaria brizantha 0.9 8.2 3.7
B. decumbens 1.0 8.3 3.7 1.6
B. humidicola 0.8 7.4 2.1 1.4
Panicum maximum 1.7 — 6.0 1.5
Legumes
Stylosanthesspp. 21–31 1.7–2.5 7.8–9.2 8–14 2.3–3.1 1.1–1.5 16–24 4–6
Centrosema pubescens 1.6–2.0 7.5–14 6–13 2.4–4.6 1.5–1.9 20–25 4–6
Arachis pintoi 2.3 5.0 18 1.1 16 5
Animal requirements 1.2 6.0 1.8 1.0 1.0 20 7
Source: Adapted from Refs. 225–228 and 248.