When humans first transitioned from hunting and foraging to
farming, soil P depletion began as crops were harvested and removed from their
fields. Early farmers learned to enrich soils with animal manure or adopt
shifting cultivation. However, as cities developed, nutrients were
systematically withdrawn from the field and concentrated near the city.
Plant nutrient depletion and agricultural sustainability has
been addressed in various ways by different civilizations. Newman (1997)
describes how P depletion as a result of crop production was handled in the
U.S. Prairie (by exploiting P from organic matter mineralization), on a typical
medieval English Farm with declining wheat production (running a P deficit of
0.7 to 0.9 kg P/ha/yr), for Egyptian fields which remained in P balance from
annual flood water, and in Northern China, where P deficits occurred even with
the traditional spreading of human excreta (with the accompanying fecal-borne
diseases).
Slash and burn agriculture was commonly employed to clear land
and enrich the soil with nutrients from the residual ash. One study reported
that forest ash contained 11 kg P/ha and 27 kg N/ha after burning, of which
more than half was blown from the field in wind (Giardina et al., 2000).
Additionally, in medium to high-intensity fires, heat-induced reactions can
increase P sorption by soil minerals, leading to reduced P recovery by crops.
During the U.S. colonial period, slash and burn techniques forced inland
migration from the Atlantic Coast as agricultural fields were successively
exhausted of their nutrients with no means of restoring the fertility. When
added to soil, the liming effect from ash and the input of mineral P and K made
it a good amendment for growing a N-fixing crop.
In the early 1800’s, it was discovered that P is beneficial
for plant growth. As the value of “pounded” bones was recognized as a P source,
the demand grew quickly in the early 19th century. Unprocessed bones
(hydroxyapatite; Ca5F(PO4)3OH) were crushed and applied to the soil at a rate of
1 t/A or more. In England, the demand for bones outstripped the domestic supply
and by 1815, bones were imported from the Continent, reaching a maximum of
30,000 t/yr (Nelson, 1990). This led the famous plant nutritionist Justus von
Leibig to complain:
“England is robbing all other countries for their fertility.
Already in her eagerness for bones, she has turned up the great battlefields of
Liepsic, and Waterloo, and of Crimea: already from the catacombs of Sicily she
has carried away the skeletons of many successive generations. Annually she
removes from the shores of other countries to her own the manuerial equivalent
of three million and a half men…. Like a vampire she hangs from the neck of
Europe” (Liebig).
The observation that not all bones were equally effective as
a plant nutrient source led to experimentation to acidify the bones before
adding them to soil. One early innovator, John Lawes applied raw bone to his
farm fields without seeing any
additional crop growth. This led him to experiment with treating bones with
sulfuric acid, which proved to be very effective. In 1842 he was granted a
patent for “superphosphate of lime”, composed of calcium hydrogen phosphate and
calcium sulfate. The manufacturing of superphosphate quickly spread around the
world and marked the beginning of the modern fertilizer industry.
2 Ca5F(PO4)3 phosphate rock + 7 H2SO4 →
3 Ca(H2PO4)2 [superphosphate] + 7 CaSO4 + 2 HF
The manufacturing of superphosphate consisted of placing
ground bones into a pit and then stirring in sulfuric acid as the mixture solidified
for several hours. The solid paste was then allowed to mature in a curing pile
for a few weeks until it was ready be broken apart with picks, crushed,
screened, and bagged. The lumpy texture could make it difficult to spread
uniformly in the field. This simple process also encouraged farmers to make
their own superphosphate for on-farm and local use (New England Farmer: July
1869).
The name “superphosphate” is thought to have first appeared
in a pamphlet by Joseph Graham who explained how “phosphate of lime (as it
exists in bone) is totally insoluble in water…when deprived of a portion of the
lime constituting its base, (it is) reduced into a state of superphosphate,
becomes soluble…” (Cooper and Davis, 2004). The “super” likely refers to its
superiority over ground untreated animal bones. In addition to making
fertilizer, much of the bone-derived P was calcined and reduced in a furnace to
elemental P for use in making matches.
The eventual shortage of bones led to the exploration of
other potential P sources. Guano, which had accumulated from dried bird manure
in large quantities in the arid islands off the coast of Peru and in the South
Pacific, became an important source of P fertilizer between 1840 and 1870.
However, the most nutrient-rich guano deposits (typically 4 to 5% P) were
quickly depleted and it’s use declined in the latter half of the 19th century
as low-grade mineral deposits were discovered around the world.
When Peruvian guano first became available in the U.S., it
quickly began to substitute for bulky, locally derived recycled organic
materials and led to the development of the commercial fertilizer industry in
the U.S. Not surprisingly, the major U.S. meat processing companies and
slaughterhouses were also major fertilizer manufacturers, distributing both N
and P-based products for crop production.
Mineral deposits of phosphate rock (apatite) were later
developed and substituted for bones in the production of superphosphate. The P
fertilizer industry entered the modern era as phosphate rock sources became
readily available and accessible from geologic deposits around the world (e.g.,
England, 1847; Norway, 1851; France, 1856; USA, 1867; Tunisia, 1897, Morocco,
1921; Russia, 1930).
All common P fertilizers are now produced from phosphate rock
as the starting material. Most sources of phosphate rock are too insoluble for
direct use as a P source for plants. Phosphate rock from a few geologic
deposits are suitable for direct application, especially if used for perennial
crops growing in acidic soils, where the acidity and low Ca concentrations help
speed rock dissolution and the release of P.
Superphosphate became the dominant P fertilizer in the world
for over 100 years, but is no longer widely used and traded (with the notable
exception of pastures in Australia and New Zealand). Other P sources remained
available in limited quantities (such as manure, guano, ground phosphate rock
and basic slag) and new P fertilizers were tested (such as triple
superphosphate, ammoniated phosphates, nitric phosphates), but they were not
commercially competitive for many years.
The additional advantage of treating the phosphate rock with
phosphoric acid instead of sulfuric acid was discovered in the 1870’s. This
process resulted in the production of fertilizer with soluble P concentrations
almost three times higher than superphosphate, named triple superphosphate
(TSP). However, TSP did not gain widespread usage until much later. This new
concentrated P source greatly reduced fertilizer transportation costs and the
manual labor required to spread powdered P fertilizer on the field, as
granulation technology did not become widespread until 1950’s.
Ca5F(PO4)3 phosphate rock + 7 H3PO4 →
5 Ca(H2PO4)2 [triple superphosphate]+ 2 HF
The nitrophosphate (Odda) process was developed in Norway in
the late 1920’s. This reaction involves mixing phosphate rock with nitric acid
to produce calcium nitrate and phosphoric acid. A compound fertilizer
containing both N and P (and K is frequently added) is also commonly produced
from this process.
The Modern Era
In 1933, the National Fertilizer Development Center (NFDC)
of the Tennessee Valley Authority (TVA) was given the mission of improving the
efficiency of fertilizer manufacturing and fertilizer use on farms. This
organization was pivotal in advancing global fertilizer technology and use. The
majority of fertilizers produced in the world are still made with processes
first developed by the TVA. The successor organization, the International
Fertilizer Development Center (IFDC) still continues research and development
projects on new fertilizer technology.
Diammonium phosphate (DAP) became the dominant P fertilizer
following the introduction of the TVA process in the early 1960’s where
phosphoric acid is reacted with ammonia, using a pipe-cross reactor. TVA also
introduced processes for manufacturing nitric phosphate, solid ammonium
polyphosphate, and urea phosphate. The popular fluid ammonium polyphosphate
became widespread after TVA introduced a method for combining superphosphoric acid
(a mixture of phosphoric acid and polyphosphoric acid), with ammonia in the
T-pipe reactor. The polyphosphate in superphosphoric acid keeps metal
impurities from precipitating from solution.
As fertigation becomes more common, introducing soluble P
fertilizer into irrigation systems requires careful management to prevent
precipitation with constituents in the water that can lead to fouling and
plugging of the irrigation system (Mikkelsen, 1989). A variety of excellent
water-soluble P sources can be used for fertigating crops (such as
monopotassium phosphate or urea phosphate), but close attention to the system
chemistry is required.
The most common P fertilizers in the world are currently
DAP, monoammonium phosphate (MAP), and TSP. A large amount of P is traded as
phosphoric acid, of which 80 to 85% is used in the production various P
fertilizers.
The current global outlook is for steadily declining
positive growth rates for P fertilizer. However, this global average masks
specific regional trends such as the slowing P demand in China and increasing
demand for P fertilizer in Africa (IFA, 2018).
The Future
Phosphorus fertilizers have achieved farmer acceptance by
being: 1) efficient to manufacture, 2) affordable, and 3) agronomically
effective. New P fertilizer materials will additionally need to satisfy various
environmental criteria (such as during mining and reclamation, manufacturing,
and field use), social demands (such as energy consumption, greenhouse gas
production, phosphogypsum management), and consumer expectations (such as
minimizing trace elements in fertilizer, using sustainable mining practices,
minimizing water quality impacts). These new considerations place additional
constraints on the development of new fertilizer products.
Improved recovery of P that is directly consumed in human
food and in animal feed will certainly gain more importance as P recycling from
various waste streams is emphasized. Future efforts to more effectively reuse
and recycle P derived from waste streams will likely include:
1. Manure-based fertilizers and composts: Phosphorus may be
separated by solid-liquid processing and the products may be further
concentrated by drying, composting, fortifying, or pelletizing.
2. Combustion products and ash from manures and sludges:
Incineration at 800 to 900°C concentrates the mineral fraction without cause
significant P volatilization losses. Heating P-containing waste products to
higher temperatures will vaporize elemental P which can be condensed and
oxidized to phosphoric acid.
3. Extract P from organic waste streams: A variety of useful
P fertilizers can be produced from various waste products, including struvite
and calcium phosphate minerals such as brushite and hydroxyapatite.
Additional work has recently focused on the behavior of
organic P materials in the soil, and manageable factors that control the value
of these P sources for plant nutrition. The use of microbial inoculants and
biofertilizers is under investigation to improve fertilizer P recovery by plant
roots. While recent attention has focused on root-fungi interactions for
enhancing P uptake, other plant-growth promoting organisms may significantly
contribute to P solubility and rhizosphere activity.
Rapid advances in the field of material sciences also offer
new matrices and delivery mechanisms for supplying P to crops. Many new
approaches have been suggested, but the economic barrier has so far prevented
widespread adoption of new P fertilizer technologies.
Conclusion
The development of the modern P fertilizer industry has
provided farmers with easy and safe access to effective and affordable crop
nutrients. These products replace P that is removed from the field during
harvest and enhance the fertility of nutrient-depleted soils. The commonly used
P fertilizers have their origins in chemistry and processes that are well
established.
Emerging insights into material science and engineering may
provide breakthroughs in innovative P fertilizer sources. Closer integration of
new fertilizer products and root biology may also improve recovery of applied
P. The development of innovative P fertilizers that sustain agricultural
productivity and minimize off-site environmental impacts would make a
significant contribution for agricultural science.
Dr. Mikkelsen is Vice President, IPNI Communications based
in Merced, California; e-mail: rmikkelsen@ipni.net.
References and Additional Reading
Ashley, K. et al. 2011. Chemosphere 84: 737-746.
Chien, S.H. et al. 2009. Adv. Agron. 102: 267-322.
Cohen, Y. et al. 2011. Integrated Waste Management 2:
247-268.
Cooper, M. and J. Davis. 2004. The Irish Fertiliser
Industry. Irish Academic Press, Dublin.
Emsley, J. 2002. The 13th element: The sordid tale of
murder, fire, and phosphorus. J. Wiley
Fussell, G.E. 1971. Crop nutrition: science and practice
before Liebig. Coronado Press, Lawrence, KS.
Giardina, C.P. et al. 2000. Soil Sci. Soc. Amer. J. 64:
399-405.
IFA. 2018. IFASTATs and Medium-Term Outlook for World
Agriculture and Fertilizer Demand 2017/18 - 2022/23.
McKinley, S.W. 2014. Stinking Stones and Rocks of Fold:
Phosphate, Fertilizer, and Industrialization in Postbellum South Carolina.
Univ. Press Florida, Gainesville, FL.
Mikkelsen, R.L. 1989. J. Prod. Agric. 2: 279-286.
Mikkelsen, R.L., and T.W. Bruulsema. 2005. HortTech 15:
24-30.
Nelson, L.B. 1990. History of the U.S. Fertilizer Industry.
Tennessee Valley Authority, Muscle Shoals, AL.
Newman, E.I. 1997. J. Applied Ecology 34: 1334-1347.
Russel, D.A. and G.G. Williams. 1997. Soil Sci. Soc. Amer.
J. 41: 260-265.
Smil, V. 2000. Annual Rev. Energy Environ. 25: 53-88.
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