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Phosphorous (P) is essential element to all living
things and plays vital role in many physiological and chemical process and
cannot be replaced by any other elements. The ultimate sources of Phosphorous
in soils are the primary minerals. Africa is endowed with numerous phosphate
ore deposits which are potential sources of phosphate fertilizer. Mixing
phosphate rock (PR) increases soil available-P. Adequate supply off soil
available-P is critical for normal plant growth. Phosphorous deficiency is a
major constraint in the humid and sub-humid zones of Africa. Frequent
application of P-fertilizer can build up P-stock in the soil which through time
could be available to plants. It is important to select and develop P-efficient
genotypes. Phosphorus recovery can be defined as a socio-technical system. In
Africa phosphorous sources are not well managed and the P-cycle is broken. Soil
and water conservation practices reduce P-loss by water and wind erosion. Most
soil types in Africa are low in P and expose the farmer to use inorganic
fertilizer to produce crops. Excess P-fertilizer application risks phosphorous
losses via run-off which may result in eutrophication of water bodies. To
improve the lifespan of existing PR deposits mining use of renewable P-sources
in Africa, integrated soil fertility management and use of new crop germplasm
with high P-use efficiency is needed. Launching waste treatment Phosphorous
recovery from urban waste disposal is crucial for Africa. Finally this review
work aims at creating awareness among policy makers, investors, farmers and
scientists in Africa about the role of phosphorous in Agriculture for poverty
reduction.
Keywords: Acid
soils, Available phosphorous, Phosphorous deficiency, Rock phosphate,
Phosphorous recovery, Eutrophication
INTRODUCTION
Phosphorus (P) is
essential to all living things and plays vital role in many physiological and
biochemical processes, and cannot be replaced by any other element [1].
Phosphorus is very reactive in the environment and in soil solution, it is mainly
found as orthophosphate forms (PO43-, HPO42-
or H2PO4-) depending on the acidity of the
solution. Soils might have a high overall P content, but only the P that is
soluble in water is useful for plants [2]. Phosphorus inputs to farmer fields
in Africa consist primarily of inorganic fertilizers and organic sources such
as biomass, manures, and composts gathered from outside the field. However, the
P content of plant residues and manures is normally insufficient to meet crop
requirement [3]. However, Africa has numerous phosphate ore deposits, which are
a potential source of phosphate fertilizers. In Senegal, Tunisia and Morocco
few of the developed deposits have been targeted for export. Thus, use
Phosphate Rocks (PRs) in Africa should be studied to replenish the lost loop in
highly weathered, low P and acid soils [4]. Mixing PR with compost increased
the availability of African PRs in Kodjari PR in Burkina Faso and Minjingu PR
in Tanzania [5]. Particularly, PR application in P deficient soil has advantages
of P investing the capital stocks which in the long run which available to the
plant and may future reappointment to the invested PR in the soil [6]. High
P-Sorption, clayey, red soils of East and southern Africa will therefore have
different replenishments strategies as compare to low P-sorption, sandy soils
of the Sahel, where smaller and more frequent applications are required [6].
Soil-fertility depletion is the main biophysical root cause for declining per
capita food production particularly in sub-Saharan Africa [7]. Understanding,
huge soil loss from agricultural lands many African Countries are launching
soil conservation strategy to conserve soil and water loss. This may lead to
the most effective long-term solution to prevent phosphorus losses and water
pollution.
with phosphorus and
decrease the erodible soil to water bodies [8]. Microbes assimilate phosphate
ions in the soil solution into organic forms in their biomass, a process
referred to a P immobilization. Mineralization of soil organic P, including
recently immobilized biomass P, releases it once again to soil solution P,
which is readily available to plants, thus providing an additional service flow
[9]. The by-product in phosphate rock process is phosphogypsum, which contains
significant amounts of cadmium, uranium and of fluoride [10]. Long-term
phosphate fertilizer application accumulates cadmium in soil and could increase
the risk of uptake by crops and transfer through the food chain [11,12].
Agronomic practices such as crop rotation, fertilization, and tillage affect
the extent of mycorrhizal colonization and arbuscular mycorrhizal fungi (AMF)
facilitated nutrient uptake of crops and management of arbuscular mycorrhizal
fungi has the potential to improve the profitability and sustainability of
agricultural systems. Though urban agriculture (UA) is a small subsystem, it
may prove to be a valuable asset to increase urban P sustainability by becoming
a catalyst of increased city recycling. Sewage sludge represents one
possibility of permanent P-supply, but heavy metal contamination and high
pathogenic risks; the usage of untreated sewage sludge in agriculture has to be
regarded critically. Different sources present opportunities to recover
phosphates such as sewage sludge ashes (SSA), meat and bone meal ashes (MBMA)
and struvite [13]. The by-product in phosphate rock process is phosphogypsum,
which contains significant amounts of cadmium, uranium and of fluoride [10].
Long-term phosphate fertilizer application accumulates cadmium in soil and
could increase the risk of uptake by crops and transfer through the food chain
[11,12]. At last, this paper reviews the great concern of Phosphorous in Africa
and proposes further study of this non-renewable element.
ROLE OF PHOSPHOROUS FOR LIVING THINGS
Phosphorus (P) is
essential to all living things and plays vital role in many physiological and
biochemical processes, and cannot be replaced by any other element [1].
Phosphorus occurs in complex DNA and RNA structures which hold and translate
genetic information and so control all living processes in plants, animals and
man, transport of energy system in all cells. Phosphorus does not occur by
itself in nature always but combined with other elements to form phosphates.
Hence, African counties must pay attention to use phosphate resource and
reserves for the benefit of their nation [14].
PHOSPHORUS IN SOILS
The ultimate
sources of phosphorus are primary minerals, such as apatite (calcium
phosphate). Phosphate-bearing minerals are found in many different rocks and
soils; through mineral weathering process and phosphorus is released into the
soil. Phosphorus is very reactive in the environment and in soil, solution is
mainly found as orthophosphate forms (PO43-, HPO42-
or H2PO4-) depending on the acidity of the
solution. If plants or soil microorganisms do not quickly take up
orthophosphate, it will react with other compounds (such as calcium, iron,
aluminum and Manganese) associated with the soil, making it unavailable to many
plants [15]. Hence, phosphorus is the limiting nutrient in many agricultural
soils [16]. Along with nitrogen (N) and potassium (K), phosphorus (P) is one of
the most important nutrients for crops. Organic P sources in the soil include
decomposing plant matter, microorganisms, and compost or animal manure. Inorganic
P is found in the mineral matter component of the soils and in fragments of
P-containing rocks and minerals. Soils might have a high overall P content, but
only the P that is soluble in water is useful for plants [2].
SOURCE OF PHOSPHOROUS
Africa is endowed
with numerous phosphate ore deposits, which are a potential source of phosphate
fertilizers. In Senegal, Tunisia and Morocco few of the developed deposits have
been targeted for export (Figures 1-6). Here limited domestic markets
and low phosphate prices in the global market does not justify foreign
investment and operating costs [4]. Phosphorus inputs to farmer fields in
Africa consist primarily of inorganic fertilizers and organic sources such as
biomass, manures and composts gathered from outside the field. The P content of
plant residues and manures is normally insufficient to meet crop requirement
[3].
The primary source
of P is from geological resources such as phosphate rocks [17-19]. Both organic
and inorganic forms of P exist in the soil and are important for normal plant
growth [20]. Complex and several forms of phosphate are found in soils, water
and living things. However, the P mineral fertilizer is derived from
non-renewable resource is dependent exclusively on mined rock phosphates [21].
Several studies in East Africa showed that direct rock P application increased
soil P availability and crop yield in acid soils [3,22,23]. However, some of
the low-quality rock phosphate from igneous rock showed low in P availability.
Vaccari and Strigul [19] noted that though the problem of potential scarcity of
rock phosphate is less urgent, but phosphate rock is a finite resource subject
to eventual depletion under improper use.
Phosphate rocks
(PRs) from diverse origins, reserves and characteristics are widely distributed
in Africa. When, PRs is applied directly it has the potential to supply the
commonly limiting P nutrient to crops and improved yields and food security.
Use of PRs in Africa should be revisited to replenish the lost loop in highly weathered,
low P and acid soils [4].
COMBINING INORGANIC AND ORGANIC PHOSPHORUS
INPUTS
One of the problems
of P replenishment in Africa is that acidifying agents are needed to facilitate
the dissolution of PR. Many P-depleted African soils have pH values above 6.2,
which are too high for rapid dissolution of reactive PR. The decomposition of
organic inputs produces (i) organic acids that may help acidify PR or (ii)
chelating agents that bind Ca dissolved from PR and thus stimulate further
dissolution of the PR.
Mixing PR with
compost has been shown to increase the availability of African PRs in Kodjari
PR in Burkina Faso and Minjingu PR in Tanzania [5]. Finely-grounded PR mixed
with poultry or cattle manure prior to application to nonacid soils increased P
availability. Organic anions produced during the decomposition of plant
materials temporarily reduces the P-fixation capacity of soils by binding to
the Fe and Al oxides and hydroxides at surfaces of clay particles. Nziguheba
and Smolders [12] found that the rapid decomposition of tithonia dry biomass
reduced the P sorption and increased the available P pools of an acid. This was
attributed to the blocking of P sorption sites by organic anions produced
during biomass decomposition. The integration of locally available organic
resources with commercial P fertilizers may be the key to increase and sustain
levels of P capital in smallholder African farms [2,24].
SUPPLY OF PHOSPHOROUS TO PLANTS
Adequate supply of
phosphorus is crucial for normal plant growth, and low level of phosphorus will
result in a reduction of crop yield [25,26]. Studies in Africa showed that
phosphorous is a limiting factor in agriculture, because most of the African
soils are low in phosphorous content compared to other continents [27]. Highly
weathered Ferralsols, Acrisols and Luvisols soils are generally deficient in phosphorus
[28,29]. Phosphorous deficiency is common on Ultisols and Oxisols, because of
fast fixation of soluble P into insoluble forms through reactions with iron and
aluminum oxides.
The acid soils in
Africa fix P and the best solution is to apply liming and addition of organic
matter to increase crop yield [17,30]. In Ruanda application of lime alone in
acid soils increased P availability by 3 mg/kg and maize grain yields by 13.5
times compared to control treatment [17]. The more acid the soil, the more rapid
the dissolution rate of PR, and high P sorption enhances the dissolution of PR
by reducing the concentration of P in solution around the PR particle.
PHOSPHOROUS DEFICIENCY IN SOIL AND PLANTS
Phosphorus
deficiency is a major constraint to crop production on highly weathered, low
activity clay soils in the humid and sub humid zones of sub-Saharan Africa [7].
For acidic soils, it is assumed that the sorption of phosphorus (P) by the
hydrous oxides of iron (Fe) and aluminum (Al) binds added P and that the reverse
reaction controls the concentration of P in soil. Understanding, the
contribution of different soil properties to P sorption and the interactions
among their effects could lead to new ways to control the concentrations of P
in soil [31]. Amount of phosphorus adsorbed by soils is highly correlated with
exchangeable aluminum, total iron, organic matter, and low pH. In highly
weathered soils, phosphorus is mainly in the organic and combined forms;
iron-phosphate is dominant in an active form [20,32].
FREQUENT APPLICATION OF P FERTILIZER
Fertilizer
consumption in Africa is very low compared to other continents (Figure 2).
Frequent application of P fertilizer can build P in the soil which trough time
could be available to plants. Particularly PR application in P deficient soil
has key advantages of P investing the capital stocks which in the end available
to the plant and may future reappointment to the invested PR in the soil
[6,25]. Use of inorganic fertilizers is a fast and immediate way to avail P mostly
for plant uptake and boost crop yields. Organic fertilizers either alone or
combined with inorganic fertilizer have also shown its importance in rising up
soil available P and other adjacent contribution to promote nutrient uptake and
lastly increase crops yield [33]
The current rate of
consumption of P fertilizers, the fast exhaustion of high-grade phosphate ores
worldwide clearly challenges the sustainability of current P fertilizer use in
Africa in the coming decades [27]. Using lower quality resources and paying a
greater cost clearly requires a major shift in P fertilizer use [34].
RESIDUAL EFFECTS
Crop yield response
to P applications depends on the duration of P amount applied, the soil's P
sorption and cropping intensity. The larger the P application rates the longer
the residual effect. Low P-sorption soils have shorter residual effects than
high P-sorption soils [35]; the higher the number of crops harvested per year
the shorter is the residual effect. High P-sorption, clayey, red soils of East
and southern Africa will therefore have different replenishments strategies as
compare to low P-sorption, sandy soils of the Sahel, where smaller and more
frequent applications are required. Given these variables, as well as
logistical, financial and infrastructure considerations, the choice of P
fertilizer source and the rate used for replenishment is site and situation
specific.
ORGANIC RECYCLING OF P
Plants convert
inorganic P absorbed from the soil solution into organic forms in their
tissues. The addition of plant material grown in situ to the soil as litter
fall, root decay, green manure incorporation, crop-residue returns and animal
excreta (in grazing systems) and its subsequent decomposition results in the
formation of organic forms of soil P [36,37]. Microbes assimilate phosphate
ions in the soil solution into organic forms in their biomass, a process
referred to a P immobilization. Mineralization of soil organic P, including
recently immobilized biomass P, releases it once again to soil solution P,
which is readily available to plants, thus providing an additional service flow
[9,33].
P EFFICIENCY GENOTYPES
For smallholder
farmers in Africa to fill the scarcity of phosphorus (P) resources, available
technologies are needed that increase efficiency of P fertilizers [38,39]. This
should focus for P-fixing soils where P fertilizer use efficiency may remain
low due to strong P adsorption and fixation. One of such technologies is the
selection and development of P-efficient genotypes [31,40]. P efficiency may be
the result of either the development of certain root traits leading to enhanced
P uptake or of certain mechanisms leading to enhanced P utilization [41,42]. In
Africa already different soybean genotypes showed high P efficiency and P efficiency
correlates with P uptake. P utilization efficiency may play a more important
role at higher levels of P supply, and breeding programs aiming at increasing
plant growth at suboptimal P supply should exploit genotypic variation in P
uptake [40,43,44].
BROKEN P-CYCLE
The main causes of
broken phosphorous cycle in the ecosystems are extensive crop production
systems, deforestation, and soil erosion by both water and wind [45].
Traditional African fallow system practice has been forgotten due to small land
holdings and arable land fragmentation attributed to rapid population increase
[26]. Crop-livestock system has been disturbed by overgrazing and grazing land
encroachment that supplements with dung and manure to crop land [46-48]. Farm
management practices, intensification of agriculture, and other human
activities also caused phosphorus deficiencies (Figure 3). In Africa
whenever, crops are harvested, P is removed from the soil system. Burning and
removing crop residues, failing to return organic matter to the soil, and
allowing soil erosion have all led to loss of phosphorus from soils [36,41,49]
indicated that phosphorus recovery and re-use can be defined not just as a
technology, but also as a socio-technical system involving collection and
storage, treatment and recovery, transport, refinement and reuse. Animal manure
and other parts of animals such as blood and bones, is widely used as a source
of phosphorus fertilizer in many regions of the world [36,50] and the collected livestock
wastes must not too far away from arable land for transport to be
economically viable [51].
MANAGEMENT OF PHOSPHOROUS SOURCES
Phosphorus sources
are not well managed in African it seems the P-cycle is broken and farmers do
not understand the role of phosphorous from organic and in organic sources (Figure
4). Regarding nutrient cycle the policy in many African countries is not
well defined. As the result widespread malnutrition and severe land degradation
are direct consequences of inappropriate policy that resulted in large scale
soil nutrient mining [52-55]. Soil-fertility depletion is the main biophysical
root cause for declining per capita food production particularly in sub-Saharan
Africa [7]. Phosphate ore deposits are a potential source of phosphate
fertilizers with good agronomical valuable and high quality phosphate rock
deposits exist in Africa, which can be applied to the soil with simple
processing or directly to very acid soils [56,57].
Therefore,
knowledge of drought tolerance and nitrogen fixation mechanisms induced by
phosphorus may contribute to improve the management practices for the farmers’
land. Further, selection of plant genotypes that produce good yield under low-P
soil or those with high-P response efficiency can be a low-input approach to
solving this problem [20]. Clearly, an integrated approach combining plant
traits improvement and optimum land management needed to revitalize the crops
performance under climate change and degraded land conditions [58].
LOSS OF PHOSPHORUS
Significant
phosphorous loss from African agricultural is field driven by cultivation on
nutrient-poor soils; a breakdown of traditional soil-fertility practices and
poverty in rural Africa, which does not permit effective fertilizer management
practices, hinders agricultural production [58]. Land ownership is also the
major cause for soil fertility maintenance in some African Countries as the
land belongs to the government and the farmers are not interested to improve
the soil fertility of their farm. Extensive farming practice is causing the
depletion of soil-P and hinders to use new technologies and innovations.
Application of too
much P fertilizer to the soil can increase amounts of phosphate in the
insoluble form and the soluble form also increases. This increases the risk that
phosphate will be lost via soil run-off or leaching through the soil [20]. Soil
conservation practices such as reduced tillage without removal of crop
residues; terracing on sloping land; cultivation and planting along the
contour; and, maintaining a soil cover of actively growing vegetation or plant
residues can harness soil loss [20,31,38,50]. Understanding huge soil loss from
agricultural land many African Countries are launching soil conservation
strategy to conserve soil and water loss (Figure 5). This may lead to
the most effective long-term solution to preventing phosphorus losses and water
pollution with phosphorus and decrease the erodible soil to water bodies [8].
FORMS OF INORGANIC PHOSPHORUS
Most inorganic P
compounds in soils fall into one of two groups: those containing calcium (Ca)
and those containing iron (Fe) and aluminum (AI). The availability of P in
alkaline soils is determined largely by the solubility of the Ca-compounds in
which the P is found. In acid soils, Fe and Al minerals control solubility of
inorganic P [20]. When soluble P is added to soils, insoluble phosphates are
formed with Ca or Fe and AI. With time, continuous interactions of soluble P
with Fe and Al minerals (in acid soils) and Ca-compounds (in alkaline soils)
results in formation stable phosphate minerals. This process results in
significant decrease in bio-available forms of P. The inorganic P fractionation
schemes identified the following pools: (i) exchangeable P, (ii) Fe and AI-P,
(iii) Ca- and Mg-bound P, and (iv) residual P.
SOIL AVAILABLE PHOSPHOROUS
Due to low
solubility of natural phosphorus-containing compounds and the slow natural
cycle of phosphorus, the agricultural industry in Africa is heavily reliant on
imported mineral fertilizers containing concentrated phosphoric acids [59-62].
Low phosphorus availability in soils decreases the amount of food farmers can
grow to feed their families. Small-scale farmers often lack the resources to
buy mineral fertilizers [63]. P availability is opposite to total P due to Al
and Fe content of the soil, which reflects P sorption capacity. In light
textural soils, such as sandy or silty soils, P is more readily available for
plants and for lateral transfers. Including pH, clay or organic matter content
which depends from land use can have impact on P availability.
Soil available P is
the fraction of total P in soil that is readily available for absorption by
plant roots. It is estimated in the laboratory using extracting solutions that
rely on the dual contact time between the soil and the extracting solution (kinetic
reaction) to capture inorganic P from the soil solution and the soil solid
phase during a predetermined period of time (minutes to hours). The dominant
inorganic P form extracted from soil is orthophosphate (HPO42-
and H2PO4- ions) that can be absorbed directly
by plant and microbial cells. Polyphosphates (including pyrophosphate) are
another form of inorganic P that may be present in soils, of biological origin
and generally in low concentrations relative to orthophosphate [6].
PHOSPHOROUS DEFICIENCY
Most soils in
sub-Saharan Africa naturally have low fertility. Over long periods of time, the
nutrients have been lost due to rain and leaching, hot temperatures and
chemical weathering processes. Sandy loam soils and soils derived from granite,
common in Africa, have low nutrient levels and low water holding capacity. In
much of sub-tropical soils in Southern Africa, the predominant soils are sandy
with low P fixing potential and P deficiency is mainly due to low inherent
fertility and low soil organic matter contents. Soil phosphorus deficiency is
recognized as a major factor limiting maize production in smallholder farming
systems in East and Southern Africa. P deficiency is particularly acute in the
highly weathered and acidic tropical soils in East Africa, which have a high
P-fixing capacity [6,21].
Farm management
practices, intensification of agriculture, and other human activities also
caused phosphorus deficiencies. Whenever crops are harvested, P is removed from
the soil system. Poor farm management techniques, such as burning crop
residues, failing to return organic matter to the soil, and allowing soil
erosion, have all led to the loss of phosphorus from soils [64,65].
Unfortunately farmers are removing the P that has been transferred to the plant.
Eventually P levels get low when crops are harvested. Hence the nutrient needs
to be replenished for future plant needs [30].
Use of inorganic
fertilizers is a fast and immediate way to correct P deficiency mostly for
plant uptake and boost crop yields. Organic fertilizers either alone or
combined with inorganic fertilizer is important in buildup soil available P,
promote nutrient uptake and increase crops yield. Improving the crop
productivity requires efficient use of limited P resources available to farmers
through selection of the right P sources and applies them at right rate, time
and place. Mineral P fertilizers offer the best option to reduce P deficiency
but their use is restricted by poor accessibility and unaffordable to farmers.
Use of reactive P rocks, partial acidification of low reactive P rock and the
application of various organic matter sources can improve agricultural
production at small holder level [20].
PHOSPHORUS AND CLIMATE
Applied phosphorus
fertilizer in excess to soils makes more available to crop plants, but
increases the risk of phosphorus loss via run-off, leaching or soil erosion,
finally accumulates in lakes, rivers and oceans. It may result in
eutrophication by phosphorus and nitrogen fertilizers and use of highly soluble
inorganic P in fertilizers and feeds, specialization and regionalization of
farming systems, deforestation and increased urbanization are key activities
promoted greater losses of P in dissolved and particulate forms from land to
rivers and the oceans. Cadmium can accumulate in crops leading to
concentrations in the edible portions of the crop that may be harmful for human
health [66,67].
After phosphorus
fertilizers are applied, only a small proportion of it is immediately available
to plants. The rest is stored in soils in varying degrees of availability to
plants. Applied phosphorus fertilizer in excess to soils makes more available
to crop plants, but increases the risk of phosphorus loss via run-off, leaching
or soil erosion, finally accumulates in lakes, rivers and oceans [43,68]. This
represents a financial loss and environmental damage. An excess of nutrients in
water systems – eutrophication is a major and common problem worldwide, driven
mostly by overuse of phosphorus and nitrogen fertilizers [10].
The by-product in
phosphate rock process is phosphogypsum, which contains significant amounts of
cadmium, uranium and of fluoride [10]. Long-term phosphate fertilizer
application accumulates cadmium in soil and could increase the risk of uptake
by crops and transfer through the food chain [11,12] (Photos 1 and 2).
PHOSPHORUS CYCLE IN THE PLANT-SOIL SYSTEM
The P cycle in a
cropped field is characterized by transformations among several P chemical
forms. Pool sizes of these P forms vary by five to six orders of magnitude.
Soil P compounds can be categorized as follows: (1) soluble inorganic and
organic P in the soil solution; (2) weakly adsorbed (labile) inorganic and
organic P; (3) insoluble P; which is associated with Ca in calcareous and
alkaline soils or bound to Fe and Al in acidic soils; (4) P strongly adsorbed
and/or occluded by hydrous oxides of Fe and Al; and (5) insoluble organic P in
decomposed plant, animal, and microbial residues within the soil organic matter
(SOM) [63,69]. In the plow layer of cropped soils, about 70% of the total P is
present in inorganic forms, more than 20% is in organic forms, and only few
percentages are in the soil microbial biomass (bacteria and fungi).
LIFESPAN OF THE EXISTING PR DEPOSITS
To improve the
lifespan of existing PR deposits mining use of renewable P sources in Africa,
integrated soil fertility management and use of new crop germplasm with high
P-use efficiency is needed. Use of human excreta (urine and feces) is potential
renewable sources, which contains P, N and K, has a readily available form of
P, provided safety measures and perception issues are addressed [57,70]
revealed that in Zimbabwe and Sweden nutrients emerging in one person’s urine
are sufficient to produce 50-100% of the food requirement for another person.
Based on this the large populations of urban areas of Africa can produce
substantial amounts of excreta which contains considerable P.
USE OF ORGANIC RESOURCES
Plants and animal
parts added to soils have a wide role in agricultural production system [71];
they increase soil organic matter content and improve soil physical properties.
Soil organic matter (SOM) leads to slow release of macro elements to crop and
improve buffering capacity of the soil and cation exchange capacity [71].
Manures improve soil structure which in return improve water storage,
infiltration capacity and reduce erosion and loss of nutrients [65,72,73]. Crop
residues incorporated in the soil replenish nutrients and requires presence of
soil microorganism to decompose the organic material and make P through
mineralization process [31]. Manure applied to the soil may form complex with
ions of Fe and Al in soil and affect availability of P [74,75].
PHOSPHORUS-SOLUBILIZING BACTERIA AND
MYCORRHIZAS
Enhancement of
microorganism activity by regularly returning organic material to the soil
increases cycling and best use of phosphorus [20,76]. Addition of inoculants
increase phosphorus availability in pastures and cropland soils from reserves
in the soil and applied rock phosphate. Organic farms will often be using
practices that promote higher levels of mycorrhizas, which in turn can help
uptake of phosphorus by crops [69]. Mycorrhizal symbiosis is a highly evolved
mutually beneficial relationship found between arbuscular mycorrhizal fungi
(AMF) and vascular plants and they increase plant uptake of phosphorus and other
non-mobile soil nutrients such as zinc and copper [75]. Crop plants differ in
their dependence on AMF nutrient uptake, ranging from flax which is highly
dependent on AMF for the uptake of phosphorus, to canola, a completely
non-mycorrhizal crop [76,77]. Agronomic practices such as crop rotation,
fertilization and tillage affect the extent of mycorrhizal colonization and AMF
mediated nutrient uptake of crops. Proper management of arbuscular mycorrhizal
fungi has the potential to improve the profitability and sustainability of
agricultural systems [78].
RECOVERY AND REUSE OF PHOSPHORUS FOR AFRICA
Recovery and reuse
of phosphorus is very important for Africa. It is better to establish waste
water treatment which consists of an activated sludge system without primary
sedimentation and chemical phosphorus removal with iron salts [79,80]. Several
streams digested sludge, and fecal deposit from any source should be
established at every urban area to catch free phosphate content using standard
methods. High free phosphate and high quality struvite granules (MgNH4PO4.6H2O)
can be harnessed [13,59,81]. The recovery overproduced struvite will replace
artificial fertilizer. The recovered phosphorus can be reused locally as a
fertilizer, contributing to a reduction of the environmental P pollution.
Urban agriculture
(UA) contributes to increase P-recycling by recycling cities’ high-P waste into
very local food production [82]. Though UA is a small subsystem, it may prove
to be a valuable asset to increase urban P-sustainability by becoming a
catalyst of increased city recycling. Sewage sludge represents one possibility
of permanent P-supply, but heavy metal contamination and high pathogenic risks;
the usage of untreated sewage sludge in agriculture has to be regarded critically.
Different sources present opportunities to recover phosphates such as sewage
sludge ashes (SSA), meat and bone meal ashes (MBMA) and struvite [13]. The main
challenge is to include these streams in the existing production processes
without jeopardizing quality and plant-availability. Direct sewage sludge
application in agriculture would be the most simple and appropriate method to
recover P from waste water. But due to potential environmental risks as heavy
metals (HM), acceptance is decreasing. Thus, numerous technologies have been
developed to recover ideally great amounts of plant available P with reduced
environmental risk [83]. Considerable amount of phosphorus entering the Earth
systems is lost in human urine and excreta [36]. African countries should create real sustainable
facilities to recovery P and Nitrogen form in urine and feces which could
recovered and used to fertilize agriculture lands [80].
SUMMARY
Phosphorous is very
important element for all living things. Phosphorous is found on earth only
combined with other elements. This makes phosphorous scarce element and less
available to plants. Few scientists believe that phosphorous is not renewable
hence everybody should use wisely this scarce element. The main sources of
phosphorus fertilizer are the rock phosphate (RP), organic matter and weathered
minerals. Africa endowed with the deposits of rock phosphate rock (RP). The
huge amount of global rock phosphate is found in Africa (Morocco). Besides the
west, central, south and east Africa have several phosphate deposits. Few
phosphate fertilizer processing from RP are emerging to improve the phosphate
fertilizer availability in the continent. However, the yearly demand of
Phosphorous fertilizer is growing at higher rate to produce more agriculture
crops to feed the rapid growing population. With this huge P-resources use of
phosphate fertilizer ruse in Africa is very low. African soils are very low in
phosphorus attributed to very acidic soils, very high content of Al, Fe and low
soil organic carbon content. Thus most soils are deficient in soil-P which
resulted in low agricultural productivity. Loss of phosphorous from
agricultural land is aggravated by removal of crop residue and very low organic
phosphorous addition to the soil. Several studies are going on in developing P
observant plant genotypes and P-solubilizing plant genotypes under stressed P
and weather conditions. Studies are going to promote Phosphorus-solubilizing
bacteria and mycorrhizas. Care should be taken around rock phosphate process
plant to minimize the Cadmium and Uranium contamination of the water and
surrounding soil, because these elements are highly toxic to all living things.
Though the eutrophication level appears to be low in general, but the big lakes
like Victoria Lake are exposed to rapid eutrophication. Urban wastes, faces and
urine could be additional P-sources if properly processed to extract P with
appropriate technologies [84,85].
IMPORTANT SUGGESTIONS
The following
suggestion may be helpful to alleviate some of the land degradation problems
encountered in Africa. Phosphorous is not a renewable element, hence we must
create awareness among farming community, policy makers and scientists about
the role phosphorous fertilizer for all living things. Use phosphate resources
efficiently and launch appropriate technologies for P-processing plants.
Improve the Phosphorus soil balance by using organic sources such as livestock
manure. Protect the phosphorus soil reserve and protect it from any loss by
runoff and poor management of agricultural land. Utilize the local available RP
mixing with other organic matter, develop and look for plant genotypes which
solubilize fixed soil-P and use efficiently. Acid soil of Al and Fe content fix
the available-P hence it is important to improve such soils through improved
agricultural practices, soil and conservation practices. Availability and
behavior of soil phosphorous have been done, however management of phosphorous
element for agriculture needs more attention. Population in African is growing
at faster rate and this needs food growth symmetrically, which needs more
fertilizer use and processing factories.
Studies revealed that most of the RP in Africa can be mixed with other
organic sources simply which is a promising venture to overcome the huge
investment cost and foreign market opportunities [86].
Early planning to
protect the water bodies, soils and environment from RP processing is very
important. Protection of rivers, lakes, beaches and oceans from eutrophication
is very important for Africa. Phosphorous extraction from, animal manure,
forest debris and urban wastes could improve the broken P-cycle if we use the
available technologies [87].
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