Tải bản đầy đủ - 0 (trang)
II. Brief Description of Tropics

II. Brief Description of Tropics

Tải bản đầy đủ - 0trang

.rl



m

P



1



m



cN



TABLE I

Land Areas of Major Climatic Regions of the Tropicso



Tropical climatic regions



Asia and

Pacific Islands



Africa



Australia and

New Zealand



m



6

South

America



North

America



Total



2

C

m

C



rn



OO0 000 ha



............____._............~~~.-.......

..

..

_

. ...._~...._.__......-..

.

....

..

.

.

.

........



Rainy climates (9.5-12 months humid)

Humid seasonal climates (7-9.5 months

humid)

Wet-dry climates (4.5-7 months humid)

Dry climates (2-4.5 months humid)

Semidesert and desert climates (less than 2

months humid)



348

275



197

499



9



601

576



45

51



1191

1410



148

87

131

-



645

487

305



53

114

98

-



98

69

19

__



76

15

7

-



1020

772

560



-



274



1363



194



4953



ga



2



$



ij

b



P



989

a



~



z



2133



Adapted from Table 8-1 of the President’s Science Advisory Committee (1967).



c

l

-4



TABLE I1

Area of Principal Kinds of Soils by Major Tropical Climatic Regionse

~~~~



Tropical climatic regions



Soil groups



Rainy



Humidseasonal



Wet-dry



...................-.C@OC@O



Light colored soils; base rich (Desert; Semidesert),

Aridisols

Dark colored soils; base rock (Chernozemic,

Grumusols, Renddna, Brown Forest), Mollisols

and Vertisols

Moderately weathered and leached soils (noncalcic

Brown, Ando), AIjisoIs and Andepts-lnceptisols

Highly weathered and leached soils (Latosols,

Lateritic, Red-Yellow Podzolic), Oxisofs and

Ultisols

Shallow soils and dry sands (Lithosols and

Regosols), Entisols

Alluvial soils, Entisols



Dry

ha



Semidesert

and desert



?

Total



cd



...............



5



7



98



378



23



56



119



3



211



699



1



292



>

z

4



34



90



75



6



209



93 1



1084



474



49



1



2539



Q



P

>

0



C



v1



81

146

1190



~



~



105



170



153



336



845



124

1410



71

1022



23

77 1



5

560



369

4953



Adapted from Table 8-2 of the President’s Science Advisory Committee (1967).



VJ



rn

r



FERTILIZERS FOR USE UNDER TROPICAL CONDITIONS



179



variation of temperature at 50-cm depth is less than 5OC. Naturally, average temperatures vary considerably in a spatial sense because of differences

in altitude, etc.

In spite of extensive weathering of many of the soils in the humid tropics,

such soils are regarded as having promise for development. Kellogg and

Orvedal (1969) pointed out that most of the potentially arable but unused

land of the world is in the tropics. According to the President’s Science

Advisory Committee (1967), most of the unused but arable land in the

tropics is in Africa and South America. According to Revelle (1974),

much of the potentially arable land in the tropics is of poor quality and

would require major capital investment for development. Consequently,

agricultural use of these lands will be difficult, requiring a good deal of

adaptive research to determine workable systems. But, as populations continue to expand, such lands must be utilized.



B.



TYPESOF AGRICULTURE



The most basic division of agriculture in the tropics is between modern

agriculture and traditional agriculture (Coulter, 1972). Modern agriculture

accounts for industrial products and certain food crops on a largely plantation basis-and with a relatively efficient use of technology. Traditional

agriculture accounts for most of the food produced in the tropics, and the

technology used is relatively primitive.

Accordingly, the modern sector utilizes a disproportionately larger share

of fertilizers and other inputs consumed in the tropics, The larger scale

under which this sector operates makes adoption of such practices much

easier than by traditional farmers who usually operate with limited capital,

high risk aversion, and under the customary socioeconomic restraints.

Cropping patterns common to traditional agriculture vary so widely and

are so diverse that no attempt will be made here to list or discuss all of

them. A few of the more important types are discussed in relation to the

problems of maintaining soil fertility.

1 . Shifting Cultivation

Sanchez ( 1973) defined shifting cultivation as a continuing agricultural

system in which the cropping period is shorter than the fallow period. In

1957, F A 0 estimated that this was the predominant practice on 44% of

the potentially arable and grazing land in the tropics. While several different types of shifting cultivation are practiced, the two types discussed here

are bush fallow in forested areas and grass fallow in savanna areas.

The bush-fallow system of shifting cultivation is quite satisfactory where

population pressures are relatively low. Forest vegetation during the fallow



180



0. P . ENGELSTAD AND D. A. RUSSEL



period is a major buffer against leaching losses of nutrients (Coulter,

1972). Nye and Greenland (1960) indicated that the forest represents

a nearly closed nutrient cycle. The soil is also fairly well protected against

erosion.

Grass fallow is a less satisfactory system for agricultural purposes than

is bush fallow. Grasses are often more difficult to eradicate than is forest;

leaching losses of nutrients in the savanna are also more severe, resulting

in lower fertility levels (Coulter, 1972). Much of the present savanna area

in the tropics has been derived from originally forested land when the bushfallow rotation became too short to allow the forest to regenerate.

2 . Continuously Grown Upland Crops ( N o Fallow Period)

Because of population pressures, the future of shifting cultivation is in

doubt in many areas (Coulter, 1972). Where the nature of the soils permits, systems need to be devised that allow cropping on a sustained basis.

Such systems must solve weed control and erosion problems and also prevent decreases in soil fertility. Organic materials must be returned to the

land where possible for soil improvement and as supplementary sources

of nutrients. Where feasible, legume crops also should be grown as a source

of N.

3. Lowland Rice



In contrast to shifting cultivation generally practiced on humid uplands

of relatively low fertility, lowland or flooded rice is usually grown on the

more fertile alluvial soils of the tropics. In Asia, the population is largely

concentrated in areas where lowland rice can be grown. Although these

soils are generally more fertile, use of fertilizers (primarily N ) for lowland

rice is quite common. This is particularly true where the new dwarf types

of rice are grown in association with good water supply and control. However, good irrigation facilities are by no means prevalent in the tropics.

Problems of effectiveness of fertilizer use in relation to water management

are discussed later.

4 . Pastoral



Permanent pastures occupy about 25% of the total land area in the

tropics, mostly in monsoon areas and in dry areas between 15O and 30°

north and south. The low rainfall in these areas provides limited opportunities for forages and pasture to respond favorably to fertilizers (Russel et

al., 1974). Within the wetter areas, food crops are grown in preference



FERTILIZERS FOR USE UNDER TROPICAL CONDITIONS



181



to forage crops. Use of fertilizer on any crop other than food or export

crops is not an accepted practice. Livestock, although numerous, are expected to use waste lands and these are not fertilized. Returns on a fertilizer

investment for increased forage production may not be realized for several

years.



C.



FERTILIZER-RELATED

INFRASTRUCTURE



In many tropical countries, the infrastructure does not permit extensive

use of fertilizers. Farm-to-market roads frequently are little more than foot

paths, and secondary roads support vehicles only during the dry seasons.

Under these conditions, fertilizers must be transported by headpack or,

at best, by bullock cart. Neither method is conducive to extensive use of

fertilizers. To add to the problem, fertilizers do not always arrive at a district warehouse until after the crop is planted, and those that do arrive

are not always the most suitable grade or material. Fertilizers in the warehouse may be caked, and many bags may be broken. Rebagged fertilizer

may be underweight and/or adulterated. Farmers have no way of checking

either and usually cannot resort to legal action if cheating is suspected.

Few farmers in tropical countries have machines that provide rapid, precise placement of fertilizer for maximum efficiency. Actually, few farmers

have fields large enough to justify ownership of mechanical equipment or

even rental from a cooperative or government machinery pool. Nearly all

fertilizers must be hand-broadcast upon the soil surface; subsequent incorporation is often inadequate or neglected entirely.

Educational personnel (extension service) are few and relatively immobile. Farmers must consult them in extension offices rather than in their

own fields. Many extension people may be no better informed about the

intricacies of fertilizer use than are farmers. The dealers may be even less

knowledgeable about the fertilizers they sell.

Still another major deficiency in infrastructure is the lack of a viable

marketing system. Traditionally, most food commodities are consumed in

the home or in the local area with little intention to produce surpluses

for commercial sales. Consequently, most local markets are not commercially oriented. Use of fertilizers to produce more of a commodity than

needed in the local area cannot be promoted until a new marketing system

is developed. In addition to transportation, the marketing problem involves

local grain storage in vermin- and pest-free silos until adequate transportation is available, marketing credit, marketing intelligence, and a host of

other supporting items.

In contrast to the generalized picture, however, there are many specific

examples where fertilizer use in the tropics is comparable in all respects



182



0. P. ENGELSTAD AND D. A. RUSSEL



to use in the temperate zones. Cash crops frequently are fertilized in accordance with the best recommendations. Farmers with large land holdings

can create large fields and justify their own mechanical equipment. In some

areas, farmers have formed cooperatives that purchase necessary inputs

(seeds, fertilizers, agricultural chemicals) and handle credit arrangements

with the local branch of the agricultural bank. These cooperatives sometimes maintain a machinery pool, but more often they provide the government with adequate justification for locating a machinery pool in the area.

The introduction of high-yielding varieties has been especially helpful in

fostering improvements in agricultural technology and local infrastructure.



111.



History of Fertilizer Use in the Tropics



Low fertility has been a problem with most soils in the humid tropics

almost since man began to farm. Many cures have been discovered through

the centuries, such as ashes, mud, seaweed, compost, and dung, but few

have been widely adopted for a very long period of time (Fussell, 1971).

Shifting cultivation is the major exception.

A.



TIMETRENDS

IN CONSUMPTION

AND PRODUCTION



Chemical fertilizers from the industrialized nations were used first on

the so-called estate crops, such as tea, coffee, rubber, and cacao, and then

on the cash crops, such as cotton and tobacco. Peterson and Frazier

(1964) pointed out that such use reflects the single-crop culture of large

plantations managed by well informed people, the easier access to credit

for cash farmers, and the orientation of cash farming to markets rather

than to home consumption. Despite these islands of fertilizer use in the

midst of vast areas devoted to traditional agriculture and despite the many

years of technical assistance programs, fertilizer generally is not used by

farmers who practice shifting cultivation. The system is too inefficient for

the imposition of modern soil management practices, and the technique

for transforming this traditional system to a more appropriate system has

not yet evolved (Coulter, 1972).

Nearly half of the land area of the world is located in the tropical zone

and so is over 40% of the world’s population. Nevertheless, in 1969 only

a little over 12% of the fertilizer consumed in the world was used in the

tropics (FAO, 1970). This is changing, however. Forecasts are that developing countries (nearly synonymous with tropical countries) will consume

20% of the nitrogen ( N ) , 18% of the phosphorus (P), and 13% of the



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%



t

2



lb.4

0



u)



z



0

I-



-v

a



3



t



W



I

2



0

-I



2



i



I



I



1963



I



I



I



I



I



1965



1967



1969



1971



1973



1



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



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

II. Brief Description of Tropics

Tải bản đầy đủ ngay(0 tr)

×