Academic Research Journal
of Agricultural Science and Research Vol. 2(1), pp. 1-5,
ISSN: 2360-7874 ©2014 Academic Research
Phosphorus in Sub-Sahara African Soils - Strategies and Options for
improving available Soil Phosphorus in Smallholder Farming Systems: A
Benvindo Verde1, Jossias Matusso2
researcher, MSc Integrated Soil Fertility Management, BSc Agricultural
Engineering, Mozambique, Kenya.
author’s E-mail: email@example.com
researcher, ICRISAT, Mozambique
In the Sub-Saharan Africa,
soil-fertility depletion in smallholder farms is the fundamental
biophysical root cause for declining per capita food production. In
the soils of this region, Phosphorus deficiency is widely considered
the main biophysical constraint to food production in large farmland
areas. Therefore there is a need to adopt measures in order to build
up soil P capital especially among smallholder farmers who are
economically poor. It has been reported that use of inorganic
fertilizers is a fast and immediate way to avail P mostly for plant
uptake and boost crop yields. Meanwhile 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. This review paper explores the different alternatives which
may be adopted by the smallholder farmers in SSA in order to build
up soil available P and increase the yields of their crop.
Key words: Soil fertility, Soil available P, Fertilizer, Crop
For some time, the
research community has recognized low soil fertility, particularly N and
P deficiencies, as one of the major biophysical constraints affecting
African agriculture (Sanchez et al., 1997). However, soil fertility in
Africa has seldom been considered a critical issue by the development
community, who until very recently have focused primarily on other
biophysical constraints such as soil erosion, droughts, and the need for
improved crop germ plasm (Sanchez et al., 1997). From nutrient balance
studies and some field observations across Africa, it was concluded that
soil fertility depletion in smallholder farms is the fundamental
biophysical root cause of declining per capita food production in
Africa, and soil fertility replenishment should be considered as an
investment in natural resource capital (Sanchez et al., 1997).
Phosphorus deficiency is
widely considered the main biophysical constraint to food production in
large areas of farmland in Africa. Phosphorus dynamics in soils are
complex, because they involve both chemical and biological processes and
the long-term effects of sorption (fixation) and desorption (release)
processes. The low concentration and low solubility of P in soils
frequently make P a limiting factor. Phosphorus (P) is one of the major
essential plant nutrients which is required in relatively high amount
since it contributes not only for the growth and uptake of water and
other plant nutrient as also is involved in maturity phase of most
crops. However, smallholder farmers are faced with low crop yields,
income and food scarcity due to low soil available P in most of
Sub-Sahara African (SSA) soils. This is mainly due to diverse causes
which include inherent poor soil fertility, soil acidity, over
cultivation with no adoption of good management measures, soil erosion
among others (Buresh and Smithson, 1997). These factors affect soil
available P in different manners which include sorption of P by Al, Fe
oxides and clay materials (Buresh and Smithson, 1997), physical lose
through erosion and last by removal by crop through over cultivation (Jama
et al., 2000).
The soil P pools in the
soil include inorganic P and organic P which influence the labile forms
of P in the soil and therefore the uptake by plants. The lower the
amount of labile P in the soil solution the lower the uptake of crops
and consequently the yields are low. It has been reported that use of
inorganic fertilizers as also organic fertilizer have contributed to
increase soil available P and yields of crops. Another alternative is
the bio-fertilizers which include symbiotic mychorizas which contribute
to P fertilization. The present review aims summarizing different
alternatives which may be adopted by the smallholder farmers in SSA in
order to build up soil available P and increase the yields of their
Soil Phosphorus pools
In nature P pools include inorganic and organic phosphorus.
The original soil source of soluble inorganic P is dissolution of
primary P minerals, mainly apatite. Primary P minerals decrease in soil
with increasing soil weathering and are relatively unimportant in highly
weathered soils (Buresh and Smithson, 1997). The fate of inorganic P can
be (i) taken up by plants, (ii) immobilized by soil micro-organisms and
then converted to organic P, and (iii) sorbed onto soil minerals.
Inorganic P in soil solution is partly replenished by mineralization of
Total organic P decreases with continuous cropping without P
fertilization (Adetunji, 1994), and total organic P as a fraction of the
total soil P tends to increase with soil age. Total organic P is lower
in sandy soils common in the semi-arid tropics than in medium-and
fine-textured soils. A small fraction of total organic soil P is labile
in the short term; the vast majority of soil organic P occurs in
stabilized soil organic matter (SOM) and is not rapidly mineralized. Use
of available soil P by plants and soil biota is the driving force for
the conversion of inorganic P to organic compounds and the subsequent
mineralization of organic P (Buresh and Smithson, 1997). Much of the P
associated with soil biota is contained in bacteria and fungi. Amoebae,
nematodes, and soil fauna generally contain only a small fraction of the
P in soil biota, but they can be very important in the mineralization of
organic P and the availability of P.
Options for rising up soil available P
Use of organic resources
Organic fertilizers are derived from plants and animal parts and have a
wide role in agricultural production system (Gachene and Kimaru, 2003).
When added to the soil they increase its organic matter content and
improve soil physical properties. Furthermore, improvement in soil
organic matter (SOM) leads to slow release of crop nutrients Nitrogen
(N), Phosphorus (P) and Potassium (K)); improve buffering capacity of
the soil and cation exchange capacity (Gachene and Kimaru, 2003). In
addition, manures contribute to improve physical soil conditions
contributing then for improved structure which in return improve water
storage, infiltration capacity and reduce erosion and loss of nutrients
(Rasoulzadeh and Yaghoubi, 2010; Liang et al., 2011; Salahin et al.,
2011). However, manure quality and its P content depends more on the
quality of manure which is then related to the feeding regime of the
animal (Risse et al., 2006). Manure obtained from animals fed with
supplements and grains are more expected to provide manure of high
Apart from animal
manures, crop residues are incorporated in the soil in order to
replenish the up taken nutrients by the crop. The use of organic
resources requires presence of soil microorganism to decompose the
organic material and make P contained in them available through
mineralization process. Though it has been a common practice in most
smallholder farmers to collect and burn crop residues. This practice in
medium and long term contributes for reduced soil fertility through soil
degradation and high risk for soil erosion.
Studies conducted by
Maerere et al. (2001) and Odedina et al. (2011) have reported that
application of manures increased soil available P in Tanzania and
Nigeria, respectively. Manure applied to the soil affects the soil
available P in different ways which include, forming complex with ions
of Fe and Al in soil solution, preventing the precipitation of phosphate
(Suge, Omunyin and Omami, 2011), gradually neutralization of the soil
acidity hence makes fixed phosphorus available in the soil solution (Onwonga
et al., 2008; Mwangi et al., 2002), and create conditions for
mineralization (Toor, 2009). Manure also may provide more favourable
environments for microbial activities and possibly results net
mineralization of soil organic P. In a study conducted in UK Hooda et
al. (2001) reported that high P accumulation was observed high under
manured treatments than in mineral fertilized ones.
solution, preventing the
precipitation of phosphate (Suge, Omunyin and Omami, 2011), gradually
neutralization of the soil acidity hence makes fixed phosphorus
available in the soil solution (Onwonga et al., 2008; Mwangi et al.,
2002), and create conditions for mineralization (Toor, 2009). Manure
also may provide more favourable environments for microbial activities
and possibly results net mineralization of soil organic P. In a study
conducted in UK Hooda et al. (2001) reported that high P accumulation
was observed high under manured treatments than in mineral fertilized
Inorganic sources of P include mineral fertilizers and Rock Phosphate
(RP). Mineral fertilizers used to supplement soil P and are of immediate
availability to the plant uptake. Most of P fertilizers used in SSA
include Triple Super Phosphate (TSP), Mavumo, Single Super Phosphate (SSP),
Diammonium Phosphate (DAP) which are mostly manufactured and
commercialized by agricultural input companies; and the Rock Phosphate.
Some of these fertilizers, such DAP, apart from supply of P they also
contain Nitrogen nutrient. Most researches have reported the influence
of mineral P fertilizers on crop yields but less changes are observed in
the soil available P. In part this is due to the application method
adopted which consists on band placement that is justified by the low or
immobility of P in the soil. Therefore such application method does not
contribute with significant increase in soil available P (Kamara et al.,
2008). It has an explanation which stands on the fact that when the
fertilizer P is applied as placement and the nutrient is immobile within
the soil but the sampling for soil P analysis is done between the row of
crops naturally will result in very low P than when it is done within
the row with the risk of sampling the fertilizer.
However, the efficiency of mineral fertilizers may be affected by the
application method, clay content of the soil and soil acidity (Buresh
and Smithson, 1997). Under acid soils with high content of Al and Fe
oxides, P is held and its availability reduced. Also under high clay
content soils P is sorpted. RP is a naturally occurring rock containing
P which may be used to supply P to the soil. This is more adequate under
soils with moderate to high acidity level to ensure rapid dissolution
and release of P to the soil. Nekesa et al. (2005) when working in acid
soils of Western Kenya reported that RP increased significantly soil
available P (Table 1).
Adoption of Integrated management practices
Integrated management of soil to rise up soil available may be seen as
the best option. The use of small amounts of inorganic P fertilizers (in
band placement) combined with organic resources as broadcast and
incorporated in the soil. The use of organic will also promote
micro-organism development with increased mineralization. Moreover,
organics will contribute to increased soil physical conditions which
will result in improved soil conditions and reduced risk of erosion.
This combined effects will result in the increased soil available P in a
sustainable manner. As result of this the soil will not only benefit
from P but also other crop nutrient required.
As stated before that soil acidity contributes for low soil available
and to the success on programmes implemented with the objective of
promoting soil available P. Under this conditions it’s recommended the
use of lime to raise soil pH to an adequate level and (i) increase
availability of P which was immobilized by Al and Fe oxides, (ii) create
soil conditions for micro-organism’s development to enhance then organic
matter decomposition and mineralization of nutrients, (iii) development
of good conditions for root development.
Thus, integrated application of mineral fertilizer with lime as resulted
in improved soil available P (Table 1). However it is expected to cause
a slow increase in soil P when lime is combined with RP which may be due
to the need of acidic soil conditions to enhance RP dissolution.
Combined use of organic and inorganic soil amendments has been
investigated, recommended and adopted. It has shown the positive effects
on soil fertility management such as rise up of soil available P.
Panneerselvam et al. (2000) reported increased soil available P under
application of manure together with inorganic fertilizers.
Soil P is a controversial nutrient required by the crop which is
affected mainly by soil conditions. In order to build up the soil P
farmers should adopt the use of organic resources which supply nutrients
in a long term manner. Since smallholder farmers lack financial
resources to purchase inorganic fertilizer they can successfully replace
them with organic manures. Although, where the financial resources are
available the integrated use of organic and inorganic P resources may be
the best option.
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