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Definition
Acid tropical soils are often characterized
by a high concentration of Al, low total and available P content
and high P retention capacity (Gaume et al 2001). Application
of Phosphorus to surface soil horizons enhance the ability
of plant roots to penetrate acidic subsoils where a high concentration
of Aluminum would otherwise limit root extension (DeMiranda
and Rowell 1987). P-deficiency in plants suffering from Al-
toxicity are due to a combination of reduced root elongation
and interruption of cell metabolism, as well as to the immobilization
of P by Al on or within the root surface. However, in most
soils P is concentrated at the soil surface while high concentrations
of phytotoxic Al are often found deeper in the profile. Crops
growing on such soils are, therefore, exposed to P and Al
and will interact little with Al in the subsurface. Nevertheless,
surface applied P has been found to improve root penetration
into acidic subsoils (DeMiranda and Rowell, 1987). Phosphorus
is therefore of central importance to agricultural productivity
and sustainability in both developing and high-income economies
(Lynch 1998). The importance of low P to nearly all higher
plants, including mycorrhizal symbioses, include morphological
features such as root hairs, induction of phosphates, RNAase
and other P-scavenging enzymes and P-mobilizing root exudates.
The concentration of free inorganic phosphate in the soil
is typically 60 to 600 times lower than the concentration
of other macronutrients such as K and Mg (Bieleski 1973).
Depending on the type of soil, the organic P content may constitute
20% to 80% of the total P present (Dalal 1977). This pool
of P can be made available through breakdown by micro-organisms,
especially mycorrhizal fungi, or by the enzymatic activity
of secretions from plants (Dalal 1977). The concentration
of P in the soil solution of P-deficient soils is extremely
low result, the mobility of P in the soil solution is far
smaller than that of other major plant nutrients (Barber 1984).
Due to this low mobility, P uptake is generally considered
to be proportional to the surface area of the plant organs
involved in P uptake (Sattelmacher et al 1994).
In general, an evaluation of P efficiency in plants should
include efficient absorption and also an efficient internal
use under conditions of sub-optimal P supply. The movement
of P in soils is governed largely by diffusion. Crop genotypes
with greater P acquisition efficiency would be important contributions
to food security in developing countries where many regions
have low fertility of soils and inadequate fertilizer inputs
(Lynch 1998). A decrease in nitrate uptake is reported under
P deficiency (De Magalhaes et al 1998; Rufty et al 1990).
Price increases of phosphate fertilizer are considered to
be imminent due to the rapid depletion of high grade, low
cost, non-renewable phosphate rock reserve throughout the
world (Carthcart 1980). It has been estimated that at least
5.8 billion hectares of land accounting for about 45% of the
total cultivated land in the world suffered from the problem
(Lu 1987). Plants absorb P from the soil as organic orthophosphate
(Pi) ions. In most soils, the concentration of available Pi
in soil solution is 2µM is several orders of magnitude
lower than in plant tissues (5-20 µM) (Raghothama 1999).
Application of P fertilizers can alleviate this problem but
the lack of locally available P sources and the high cost
of importing and transporting P fertilizers frequently prevent
resource-poor farmers in developing countries from applying
P to their deficient fields (Sanchez and Salinas 1981). To
develop P-deficient tolerant maize cultivars that are characterized
by an improved ability to access soil-bound P or by a more
efficient internal use of P in biomass, production may, therefore,
be a cost-effective solution to this problem. Low P availability
may also restrict nitrogen cycling by limiting symbiotic nitrogen
fixation, a P-intensive process (Norman et al 1984.)
From Year 3 annual report,  Cereal roots (Brazil/East Africa) project.
Relevant CCRP projects
Currently funded projects
Biodiversity and soil (Peru)
Cereal
roots (Brazil/East Africa)
P-efficient
legumes (China/Mozambique)
Sorghum/millet improvement (W. Africa)
Relevant events
July 30-August 1
Workshop: Consultation on breeding for adaptation to P-deficient
soils in Africa
Dundee, NY
Description: This one-day discussion meeting included representatives
of existing CCRP projects involved in P-acquisition: P-efficient
legumes (China/Mozambique) and Cereal
roots (Brazil/East Africa), as well as external resources.
The objective of the workshop was to explore the idea of an
additional project on this theme, more focused on Africa.
English
May 6-11
Workshop: The international workshop on adaptation of crops
to low-Phosphorus soils in the tropics and subtropics: innovative
approaches for sustainable development
Guangzhou, China
Description: CCRP scientists from China and the U.S. involved
in the soil projects met to plan and discuss research activities;
and to renew existing relationships and form new ones among
the participants. A field trip to one of the principal field
research sites showed the results of field screening of soybean
genotypes, showing substantial genetic variation in crop adaptation
to low P soils, which appeared to be correlated with root
architecture and is consistent with the premise of the P-efficient
legumes (China/Mozambique) project.
English
Relevant literature
None listed as of
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Relevant links
None listed as of
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