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III. History of Fertilizer Use in the Tropics

III. History of Fertilizer Use in the Tropics

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FERTILIZERS FOR USE UNDER TROPICAL CONDITIONS



183



potassium ( K ) (or 17% of the total plant nutrients) by 1980 (Harre et

al., 1974). Traditionally, the developing countries (i.e., tropical countries)

have obtained most of their fertilizer supplies from developed countries.

There was considerable fertilizer plant construction in developing countries

during the 1960s. Many of these countries sought to exploit indigenous

supplies of raw materials by processing them into intermediate and finished

fertilizers. Other countries imported virtually all raw materials needed for

fertilizer production. Thus, by 1972-1973, there were one or more N fertilizer plants in 24 tropical countries; similarly there were P plants in 18

countries and K facilities in 2 countries. As Figs. 1, 2, and 3 show, fertilizer

production in the tropics, even after a decade of construction, was not

adequate to match consumption. Fertilizer plants located in tropical countries produced only enough fertilizer to supply the equivalent of 42% of

the fertilizer actually used.

Fertilizer plants located in developing countries were designed to produce more fertilizer than was needed in these countries. All were conventional plants producing ammonium nitrate, ammonium sulfate, urea, ammonium phosphate, superphosphate, and potash. They were identical or

very similar to plants producing the same fertilizers in the developed countries; yet, they produced less than half the fertilizer that was used in tropical

countries, and only a third as much as they were designed to produce.

Although there was an apparent surplus of production in the world in

1972-1973, plants in the developed nations were operating at 85% to 95%



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1967



1969



1971



1973



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Fro. 1. Trends in the production and consumption of fertilizer N in the tropics.



184



0. P. ENGELSTAD AND D. A. RUSSEL



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PHoSPHoRUS



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CONSUMPTION



PRODUCTION



1963



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1965



1967



1969



1971



1973



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FIG.2. Trends in the production and consumption of fertilizer P in the tropics.



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1965



1967



1969



1971



1973



FIG.3. Trends in the production and consumption of fertilizer K in the tropics.



of capacity. These difficulties in handling of industrial technology in developing countries appear comparable to difficulties that have been experienced in the handling of agricultural technology. The low production relative to need and capacity is not due to inefficient plant personnel, since



FERTILIZERS FOR USE UNDER TROPICAL CONDITIONS



185



these are usually well trained. Rather, production stoppages are due to

a multitude of seemingly small interferences-power interruptions, failure

of essential raw materials to arrive on time, lack of spare parts, etc.

B.



SHIFTSI N PRODUCTION

AND CONSUMPTION

OF

NUTRIENT

SOURCES



Prior to the 1950s and 1960s. fertilizer use in the tropics was attuned

to estate and cash crops. Fertilizers were obtained through contacts that

generally had been established during colonial days. Materials used chiefly

were Chilean nitrate (in Latin America), ammonium sulfate, ordinary

superphosphate, and low-analysis NPK compound fertilizers. During the

1960s, however, and largely as the result of technical assistance programs,

fertilizer use on food crops was greatly expanded.

In contrast to low-analysis fertilizers generally used on estate crops,

emphasis began to shift to high-analysis fertilizers for food crops. The reasons were mainly economic. Shipping costs per unit of plant food were

reduced up to 65% and so also were import duties and handling fees that

were based on tonnage rather than value. Physical handling problems, especially within a country, became acute as the demand for fertilizer for food

crops began to strain public and commercial transport facilities. High

analysis helped some. Thus, urea, triple or concentrated superphosphate,

and diammonium phosphate became the tropical fertilizers of note during

the early part of the 1970s.

It seems likely that high-analysis fertilizers will continue to dominate

the international trade for many years. The only reasons now foreseen for

shipping low-analysis materials are fulfilling demand during periods of

acute shortages and for specific agronomic situations.

Indigenous production for strictly local consumption may not follow the

same trends as for imported fertilizers. The difficulties experienced in operating complex chemical plants and the consequent high cost of production

per unit of fertilizer manufactured could result in a change to rather simple

plants. Such plants would be designed to operate on a batch basis or at

most on a continuous basis that could be interrupted easily. Bulk blending

of imported intermediates and steam granulation of mixtures of ammonium

phosphate and potash offer considerable promise as suitable processes.

Such plants should be designed to substitute local labor and some local

materials for higher priced foreign materials. The net result should be a

product that is comparable in nearly all respects, except analysis, to imported products. Savings incurred in local production would be offset by

the somewhat higher distribution costs. But, local production provides

superior opportunities to utilize excess labor, to supply fertilizers to farmers



186



0. P. ENGELSTAD AND D. A. RUSSEL



on a more timely basis, and to more closely conform to the varied needs

of a country’s crops and soils.



IV.



A.



Agronomic Considerations



SOMECAUSESOF Low FERTILIZER

EFFECTIVENESS

AND POSSIBLE

SOLUTIONS

1. Leaching Losses of Nutrients



This is one of the most common soil fertility problems in tropical agriculture. Leaching occurs most rapidly in soils of coarse, sandy texture or with

high contents of hydrous oxides of Fe and Al. Such soils have low cation

exchange capacity and therefore a low capacity to hold bases. Leaching

is enhanced also by an excess of rainfall over evapotranspiration that occurs generally in the rainy season. Poorly managed irrigation systems also

contribute significantly to leaching losses.

Susceptibility to leaching also is related to nutrient mobility. Of the

macronutrients, N as nitrate is generally most susceptible to leaching losses,

with K being intermediate and P least susceptible. In reviewing literature

primarily from temperate regions, Allison ( 1966) concluded that leaching

commonly constitutes the main mechanism of loss of N (as nitrate) from

field soils. Wetselaar (1962) found a high correlation between nitrate

movement and rainfall in tropical soils of Australia.

Leaching can be very important in the tropics even during cropping periods. In experiments on alluvial soils influenced by volcanic materials in

Costa Rica, Gamboa et a!. (1 971 ) found leaching losses of N and K to

be 65% and 50%, respectively, of that added over a period of 3 years

in which five crops of maize were grown.

Boyer (1972) concluded that while losses of K by leaching in the tropics

are small in soils under natural vegetation (forest or savanna), losses are

considerable in cultivated soils. For example, Roose et al. ( 1970) reported

50% to 60% of added K leached from a sandy Oxisol under a yearly

rainfall of 190 cm in the Ivory Coast. Boswell and Anderson (1968) studied the leaching losses of K from two Ultisols of the southern United States.

They concluded that normal applications of K will not leach beyond the

root zone during the growing season of most field crops. This is in general

agreement with conclusions drawn by Munson and Nelson (1963), Nolan

and Pritchett (1960), Doll et al. (1959), Lutrick (1958), and Kilmer

(1974). This points up differences in leaching potential between the

weathered soils of the tropics and of the temperate regions during the cropping season.



FERTILIZERS FOR USE U,NDER TROPICAL CONDITIONS



157



Generally, NO,,-N and SO,-S are not lost in significant amounts in temperate areas during cropping periods. Rhue and Kamprath (1973) found

leaching of SO,-S to be quite rapid from a coarse-textured Ultisol only

during the winter months in the southeastern United States, when percolation of water is highest.

While nitrate and sulfate anions are susceptible to rapid leaching in many

soils, anion retention is also important in some soils. Thomas (1970) reports that NO,,-N is weakly adsorbed in soils that have appreciable contents

of Fe and Al oxides, coupled with pH values of 6 or below. He also found

that adsorption of nitrate was much less than adsorption of sulfate.

Kamprath et al. ( 1956) reported that sulfate adsorption increased with

soil content of kaolinite, with increase in soil acidity, and with decreasing

concentration of P in solution. Similar results were reported by Ensminger

(1954) and Harward and Reisenauer (1966).

These results indicate that anion retention would be of significance in

more heavily weathered soils of the tropics; i.e., those containing kaolinite

and Fe and A1 oxides. Also, Keng and Uehara (1973) point out the importance of pH-dependent charge that operates for much of the colloidal material found in tropical soils, particularly Ultisols and Oxisols. Research is

needed on the fertilizer management of soils where this mechanism is

dominant.

Reduced mobility of nitrate apparently can be physical as well as chemical in nature. Balasubramanian et al. (1973) concluded that diffusion of

nitrate into aggregate micropores in a well aggregated Oxisol in Hawaii

resulted in decreased downward movement with flood irrigation.

Many soils of the tropics exhibit rather high acidity. This not only increases anion retention but also slows the rate of biological reactions such

as nitrification. Using lysimeters filled with soil from an Oxisol in Malaysia,

Bolton ( 1968) found that NO,-N derived from urea or (NH,) S O , leached

more slowly with increase in soil acidity. This was interpreted as an indication of slower nitrification rate at lower pH levels. There was also a strong

adsorption of SO,-S in this soil.

There is another important facet to leaching losses of anions; such losses

result in concomitant losses of bases as well. Raney (1960) concluded

that leaching losses of bases are related primarily to the nitrate content

of the drainage water. He found little or no correlation between losses

of bases and chloride or sulfate apparently because contents of these anions

in the drainage water were too low.

Phosphorus is subject to leaching in coarse soils of low sesquioxide content when rainfall is high. Spencer (1957) and Humphreys and Pritchett

( 1971 ) found added P moved substantially in sandy soils in Florida.

Spencer reported that most of the added P accumulated within the 30-



188



0. P. ENGELSTAD AND D. A. RUSSEL



to 90-cm depth but some leached to a depth of over 200 cm in a Lakeland

fine sand.

There is also evidence that P leaches rather easily in organic soils (Larsen et d.,1958; Fox and Kamprath, 1971). With the exception of such

soils, however, leaching of P is not considered a problem. Doll et al.

(1959) found topdressed P to move downward no more than 7.5 cm in

a Kentucky study. Gamboa et al. ( 1971 ) found that added P was retained

in the top 60 cm of the soil in a study in Costa Rica. Most of the P had

formed reaction products with A1 and Fe.

Where leaching potential exists, conventional N and possibly K fertilizers should be applied as split or postplant applications to minimize the

time period between application and crop uptake. On Ultisols and Oxisols

in Puerto Rico, Fox et al. (1974) found that postplant application of

urea-N to maize and sorghum was more effective than preplant application.

Rainfall was sufficient on these soils to leach NO,-N from the top 75 cm

of soil at least once a year.

Where postplant applications of conventional fertilizers are still subject

to serious leaching of N or K, one can resort to less soluble or slowly

soluble forms. To reduce the rate of initial dissolution, several approaches

have been tried. One is to use N or K sources that have a low rate of

dissolution in water. The rate of dissolution of such compounds decreases

as the surface area exposed decreases; hence, large particles dissolve more

slowly than do finer particles. Examples of such N compounds are oxamide

(DeMent et al., 1961 ) and isobutylidene diurea (IBDU) (Hamamoto,

1966). One K compound that has some potential for slow release is potassium calcium pyrophosphate ( K,CaP,Oi) (Engelstad, 1968). Of these,

only IBDU is being produced in commercial amounts at present.

Another approach to lowering the rate of dissolution is to coat soluble

N or K compounds with materials that slow or delay the release of the

nutrient source. An example is sulfur-coated urea (SCU), developed by

the Tennessee Valley Authority (TVA) (Rindt et al., 1968). The soluble

urea prill or granule is coated with a combination of elemental S and wax

or S alone to reduce the rate of dissolution. The actual rate of dissolution

is affected by such factors as soil temperature, soil moisture, placement,

coating weight, and imperfections in the coating. Granular KC1 and K,SO ,

also have been coated in the same way to form a slowly dissolving source

of K. Sulfur-coated urea has been primarily useful for forages, turf, sugarcane, and for rice grown under intermittent flooding (Young, 1974;

Sanchez et al., 1973). It is not generally suitable for maize or similar crops

with a high demand for N during vegetative growth. This was verified by

Fox et al. (1974) in Puerto Rico.

Plastics, asphalt, and other types of granule coatings have been tried



FERTILIZERS FOR USE UNDER TROPICAL CONDITIONS



189



as well as perforated plastic bags. In research on the latter, Attoe et al.

( 1 970), found that release rates are directly related to number and size

of bag perforations. Lowering release rates by these means supplies nutrients more uniformly to the crop and reduces seedling injury and luxury

nutrient uptake. However, an admixture with uncoated material would be

necessary to provide an initial supply of nutrient. Further information on

slow-release materials is provided by Hauck ( 1972), Hauck and Koshino

( 197 1 ) , and Allen and Mays ( 1974).

Another approach to reducing losses of N is to restrict the process of

nitrification-that is, to prevent the biological transformation of NH,-N

to NO,-N. In soils where there is substantial cation exchange capacity,

this should theoretically reduce leaching losses. Several experimental nitrification inhibitors have been described by Hauck and Koshino (1971),

Hauck ( 1972), and Prasad et al. ( 1 97 1) . Generally, however, these inhibitors have been more effective in greenhouse and laboratory systems than

in the field.

Sulfur can be added in the elemental form as prills or granules to reduce

the rate of leaching loss. The S must be biologically oxidized to the sulfate

form before it is available for plant uptake or subject to leaching losses.

Generally the rate of oxidation is very slow for S in prilled or granular

form. While this in itself reduces leaching losses, the rate of supply of S

to the plants is also very slow.

2. Gaseous Losses of N from Flooded Soils

Lowland rice production is unique; flooding imposes a completely different chemical regime on the soil and soil nutrients. Flood water management

has much to do with the utilization of added N by lowland rice. This occurs

largely through the potential for nitrification of NH,-N to NOs-N. Continuous flooding prevents the nitrification of NH,-N contained in the reduced

layer of the soil. However, even under good water control, rice may take

up only 30% to 40% of added N as compared to 50% to 60% for upland

crops (Patrick and Mahapatra, 1968).

Under continuous flooding, conventional ammonium N fertilizers are

usually as effective as slow-release N fertilizers. However, the effectiveness

of conventional N fertilizers can be improved by deep placement. De Datta

et al. (1969), working in the Philippines, reported a recovery of 68%

of deeply placed fertilizer N and of 38% from N broadcast and incorporated. This indicates that a rather large portion of N is left near the surface

with the broadcast-incorporation system and thus subject to nitnficationdenitrification losses. Under California conditions, Mikkelsen and Finfrock

(1957) reported that N placed at 6-10 cm in dry soil before flooding in-



190



0. P. ENGELSTAD AND D. A. RUSSEL



creased plant recovery of N about 20% and increased yields over broadcast

treatment by 25-35%.

However, continuous flooding with good water control is the exception

rather than the rule in the tropics. Often the flooding is either delayed

or intermittent after fertilizer application. Delay in flooding after fertilizer

N application permits nitrification to proceed; upon flooding, the NO,-N

formed is subject to either denitrification losses as N, or N,O or to leaching

losses where percolation rates are significant.

The same N-loss sequence can occur with intermittent flooding, a “system” that occurs often in the tropics as a result of inadequate water supply

or control. The flooding-drying sequence may occur several times per season with intermittent flooding-often resulting in serious N losses. Sanchez

et al. (1973) obtained recoveries of only 20-30% of N added as urea

under this system in Peru. Tusneem and Patrick ( 1971 ) and Prasad and

Rajale (1972) found quite severe losses of N from laboratory soil systems

subjected to alternative flooding and drying cycles.

In both of these water management systems, topdressing of conventional

NH,-N materials should enhance the effectiveness of added N. The timing

for this topdressing is rather critical, as indicated by Ishizuka (1965) and

Matsushima (1965). If the most appropriate time for such topdressing

is difficult to predict in advance, an alternative is to use slowly dissolving

N material applied either as a basal dressing or shortly after planting or

transplanting. Sanchez et al. (1973) found that SCU as basal dressing was

usually more effective than uncoated urea added either as basal dressing

or as topdressing for rice grown under intermittent flooding in the dry coastal area of Peru. Losses of N through denitrification and/or leaching are

serious in this area with this system of water management. Table I11 shows

data for eight experiments conducted in Peru (Sanchez et al., 1973). Additional data from use of SCU for flooded rice are presented by Engelstad

et al. ( 1972).

There are situations also where fertilizer application after flooding is

very difficult. An example is large deltas which are flooded rather deeply

for a long period of time. Under such conditions, Matsuo and Suthdani

( 1972) tested fertilizers containing urea and several slow-release N compounds, such as IBDU, crotonylidene diurea (CDU), and guanyl urea

phosphate (GUP), over a 3-year period at six rice experiment stations

in Thailand. Fertilizer containing IBDU proved most effective for enhancing rice yield under these conditions, followed by GUP. Rajale and Prasad

(1974) also found that IBDU was useful for rice.

Since nitrification of NH,-N leads to losses of N from flooded soils,

nitrification inhibitors have been tested also. Under laboratory conditions,

several chemicals have been shown to be effective in retarding formation



FERTILIZERS FOR USE UNDER TROPICAL CONDITIONS



191



TABLE 111

Rice Grain Yield Response in Peru as Affected by N Source and

Time of Application (Averaged Over All N Rates)"

Yield

Expt.

No.



No N



SCUb



Ureab



UreaC



1

2

3

4

5

6

7

8

Mean



2.21

3.26

3.88

4. I9

4.40

4.82

5.88

7.03

4.45



3.66

4.45

5.04

4.47

5.65

3.72

2.53

2.97

4.06



2.59

2 . I9

1.83

3.12

3.41

2.86

2.16

2.27

2.55



2.28

2.90

4.33

4.33

4.50

3.61

3.27

3.30

3.37



Adapted from P. A. Sanchez, A. Gavidia, G. E. Ramirez, R. Vergara,

and F. Minguillo, SoilSci. Soc. Amer., Proc. 37,789-792 (1973).

* Applied as basal dressing at transplanting.

Applied as topdressing, one-half at tillering and one-half at panicle

initiation.



of nitrate (Prasad et al., 1971; Prasad and Rajale, 1972; Patrick et al.,

1968). The effectiveness of inhibitors under field conditions has been less

clear. They have not been very effective under field conditions in Louisiana

(Patrick et al., 1968); however, Rajale and Prasad (1974) found urea

treated with inhibitors to be superior to urea alone in field experiments

in India.

While denitrification may generally result in more serious losses, direct

volatilization of NH,, from flooded soils also may occur. Blasco and Cornfield ( 1966) reported that considerably more ammonia volatilized from

flooded soils than from nonflooded soils. MacRae and Ancajas (1970)

found significant losses of NH:,-N from four flooded soil systems in the

laboratory. Incorporation of the N fertilizers decreased losses as compared

with surface application. Losses were greater from urea than from ammonium sulfate, apparently because of a higher local pH produced upon hydrolysis of the urea. Willis and Sturgis (1944) found that high temperatures and high pH increased volatilization loss of applied NH,. Such losses

reinforce the contention that conventional N fertilizers should be deepplaced for maximum effectiveness for flooded rice.

3. Volatilization Loss of NH,-N from Nonflooded Soils



Topdressing of at least a portion of the N applied to a crop is a very

common practice in the tropics. Ordinarily this practice should enhance



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0. P. ENGELSTAD AND D. A. RUSSEL



the effectiveness of the fertilizer N by applying near the time of need. However, there also may be losses unique to this system of application, particularly if certain sources are applied to the surface and not incorporated.

Volatilization losses are greatest on soils of low cation exchange capacity

(Gasser, 1964b); low exchange capacities are typical of the more heavily

weathered or sandy soils of the tropics. Some volatilization loss occurs

from topdressing of most ammonium or ammonium-producing N sources

(Larsen and Gunary, 1962), but the greatest losses are usually associated

with urea (Volk, 1959; Fernando and Bhavanadan, 1971). Since urea has

become the dominant N source in many areas of the tropics, this concern

is of special urgency. Losses from urea are promoted by the local alkalinity

produced by the formation of NH, by the enzyme urease. This can occur

in both acid and calcareous soils; however, it appears that losses are greater

from the latter (Terman, 1965). Losses are also very high from urea topdressed on grass sod (Volk, 1959). In fact, any plant material or residue

contains urease and enhances the rate of urea hydrolysis. Shankaracharya

and Mekta (1971) found that volatilization losses of NH, were much

greater with the heavier rates of urea. Losses were also greater from dry

soil than from freshly irrigated soil.

Incorporation of urea with the soil reduces losses (Ernst and Massey,

1960) ; however, this is difficult to achieve during a topdressing operation.

One precaution that could be taken where irrigation facilities are available

is to add water immediately after application. This will move the urea into

the soil before hydrolysis and ammonia release.

An alternative to attempts to control volatilization losses from urea is

to use a source such as ammonium nitrate for topdressing. This fertilizer

usually suffers relatively little loss of N as ammonia when applied in this

way. Ammonium nitrate has been used infrequently in developing countries

because of its potential explosive hazard and possible use by dissident

groups. Use of ammonium sulfate or calcium ammonium nitrate may be

feasible alternatives that have little or no explosive hazard. Also, these

carry secondary nutrients that may be needed.

A slow-release N source may be of real benefit where volatilization of

conventional sources is serious. Another approach that might be of value

is to use a urease inhibitor to retard the rate of urea hydrolysis (Hauck,

1972). In relation to losses from urea, addition of ammonium sulfate or

a P compound to the urea melt might have an effect of reducing the pH

in the granule site and thereby reduce losses. Some evidence of such an

effect of P was found by Terman and Hunt ( 1964).

4. Fixation of P



As indicated above, P is not ordinarily lost from the soil except by erosion and crop removal. Instead, applied P upon dissolution reacts with



FERTILIZERS FOR USE U,NDER TROPICAL CONDITIONS



193



other chemical elements in the soil to form less-soluble compounds. There

is, therefore, a decrease in solubility rather than a physical loss from the

system. This change to less-soluble forms is usually referred to as “P fixation.” In a strict sense, fixed P refers to that portion that is not extractable

in dilute acids. Fixation of P in soil appears to result from several possible

mechanisms. The research on these processes is voluminous and will not

be covered here. Reviews on the subject have been written by Dean

( 1949), Huffman ( 1962), and Larsen ( 1967).

There is a tendency to label tropical soils in general as being capable

of fixing large amounts of added P. However, the capacity to fix added

P varies widely among soils of the tropics. Kamprath (1973) in a review

of Latin American research, concluded that many of the Oxisols, Ultisols,

and Andosols fix large amounts of added P; also that fixation of added

P generally increases with content of amorphous Fe and A1 oxides and

decreases with content of crystalline material. Pratt et al. (1969) reported

that P fixation was related to the Fe oxide content of selected groups of

soils in Sao Paulo, Brazil.

There are also soils in the tropics in which calcium phosphate compounds predominate. Fassbender er al. (1968) fractionated soil P in 110

Central American soils using the Chang and Jackson procedure. In the

80 soils with pH above 5 . 5 , calcium phosphates were dominant; in the

remainder, aluminum or iron phosphates were dominant.

Rates of P fertilizers required to satisfy P deficiencies are markedly

higher on soils of high P-fixing capacity than on soils of low P-fixing capacity. Pichot and Roche (1972) concluded that 32-44 kg of P/ha is adequate

for most tropical soils of Africa; however, on soils of high P-fixing capacity,

applications of 130 to 175 kg of P/ha are required. Shelton and Coleman

(1968) found that added P converted rapidly to aluminum and iron phosphates when added to a soil of high-fixing capacity. They found further

that P-fixing capacity could be largely satisfied with high rates of P; this

results in a much slower decline in available P with more prolonged residual effects. They also concluded that residual effects in acid soils were

related more to aluminum phosphate content than to iron phosphate content. As might be expected, the residual effects of modest applications of

soluble P to soils of high P-fixing capacity are very poor (Monsalve and

Lotero, 1972).

In view of the high potential for fixation of added P in some.soils of

the tropics, there are at least three alternatives that can be considered in

adding P to such soils: ( 1 ) Apply sufficient soluble P at one time to satisfy

fixation capacity. ( 2 ) Band or row-place soluble P at rates adequate only

for immediate crop needs. (3) Use less-soluble P fertilizers either alone

or together with soluble P fertilizers.

Alternative 1 would be largely academic unless the farmer received



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III. History of Fertilizer Use in the Tropics

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