Compared
with the other major nutrients, P is the least mobile and available to plants
in most soil conditions and therefore it is commonly a major limiting factor
for plant growth. Even in well-fertilized soils, the P concentration rarely
exceeds a few hundred parts per billion (ppb) and is commonly less than 50 ppb
in the soil solution.
Up to
half of the soil P is commonly found in organic forms, derived from materials
such as plant residues and soil organisms. However, organic P must be
mineralized to inorganic phosphate (the form found in most fertilizers) before
it can be taken up and used by plants for growth.
Because
of its strong reactions with soil components, P is principally supplied to
plant roots by diffusion rather than mass flow. Phosphorus uptake occurs
primarily at the young root tip, into the epidermal cells with root hairs, and into
cells in the outer layer of the root cortex.
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| The P depletion zone in soil |
The young
root tips, continually expanding into fresh soil, are exposed to P
concentrations found in the bulk soil solution (Figure 1). While P is rapidly
taken up along the root surface, a P depletion zone of 0.2 to 1.0 mm (about the thickness of a dime) develops surrounding
the root. Root hairs help expand the surface area available for P absorption.
Mycorrhizal fungi, growing in association with root cells and extending up to
several centimeters into the soil, can also transfer P to the root. Various
crops have been shown to have different abilities to extract P from the soil
and to make these beneficial associations with mycorrhizal fungi that extend
their effective root system.
Rhizosphere is the soil zone (within
2 mm of the root) that is influenced by heightened biological activity, nutrient
uptake, and chemical changes that result from root activity.
Getting P into the Plant: Apoplastic
and Symplastic Transport
The apoplasm comprises
the root walls, the cortical cells and the open spaces between these tissues
(Figure 2 ). This “dead space” consists of interlaced fibers that form an open latticework
in roots that serves to filter the soil solution. The soil solution moves
through these spaces and pores until it reaches the tough “Casparian strip”
that surrounds the core (stele) of the root. A net negative charge associated with
the cell wall fibers repels anions...such as phosphate and nitrate (NO3 - ) in
solution and confines their transport to larger pores within the apoplasm.
Movement of nutrients through the apoplasm into the root is greatest near the
root tip. Mucilage, a complex mixture of organic materials excreted around the
root, also carries negatively charged hydroxyl groups which can further repel
the movement of anions towards the root.
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| Root cross section: pathways of uptake |
The symplast , by contrast, is a living
system within the plant that connects each growing organ. It consists of thin walled
cells called parenchyma, which have small openings acting as “tunnels” where
the content of one cell connects with the contents (protoplasm) of the adjacent
cell. The symplast is very fragile and requires the rigid framework of the
apoplast to hold it in place.
Although
the soil solution P concentration may be less than 50 ppb, the concentration in
plant cells is much higher…50 to 500 parts per million (ppm). For plants to boost
their P concentration by over a thousand times, active transport (requiring
energy) across the root membranes (the plasmalemma) is required.
The
movement of P from the apoplast across root cell membranes is the critical step
in the transport of nutrients into the plant, requiring an energy-driven
transport mechanism to move P through the membranes into the plant root cells
(phosphate transporters). Researchers
are currently working on ways to increase the uptake of P by stimulating these
nutrient transport proteins found in the root.
Other
changes take place in the rhizosphere to improve P uptake and recovery. Many
roots exude organic acids (primarily
citrate and oxalate) into the rhizosphere, which enhance P availability. While
this response has been frequently measured, the major effect may be due to the organic
compounds displacing P held by the soil (ligand exchange), rather than a direct
effect of the organic acids on the rhizosphere pH. Some microorganisms living
in the rhizosphere have been shown to help solubilize soil P.
These
mechanisms can help improve P recovery, but they all require that P be present
in the soil to begin with. No amount of excreted organic acid, root-zone
microorganisms, or mycorrhizal fungi can allow a plant to recover P from a soil
that does not contain P to begin with.
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| Nitrate uptake increases pH |
Plant
roots can also have a considerable effect on rhizosphere pH. This pH change
comes from the release of H+ or OH- (as HCO3 - ) as plants balance
their uptake of excess cations or anions. Nitrogen nutrition plays the most important
part in this ion balance since it is the mineral taken up in the largest
quantity by most plants and it can occur as either an anion (NO3 -
) or a cation (NH4 + ). Research suggests that greater P uptake that
occurs as plants are fertilized with a source of NH4+ may be the
result of rhizosphere acidification that occurs with this N source. However, depending
on the buffering capacity of the soil, only little change in pH may be observed
in some soils.
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| Ammonium uptake decreases soil pH |
There are
many complex reactions occurring under our feet as roots invisibly work to meet
the nutritional needs of the growing plants. Research is underway in many areas
to improve the efficiency of P fertilizer use. Advances in improving root
function may someday lead to better ways of meeting the nutrient demands of
crops. For now, the focus remains on maintaining adequate soil fertility by using
regular plant and soil testing, proper fertilization techniques, and
appropriate management to keep the nutrients where they are needed for the
growing plant.
This article originally appeared as a newsletter of the International Plant Nutrition Institute. The pdf is available here.
