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IV. New Potentially Arable Soils

IV. New Potentially Arable Soils

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3.2 billion by adding roughly 1.8 billion new hectares of potentially

arable soils that are not now used for crops.*

For some countries data are more abundant and more reliable than for

others. Yet at least some reliable data are available for nearly every

large part of the world. As these data have increased over the years, so

have our estimates of potentially arable soils.

Most of the 1.8 billion hectares that could be developed for farming are

well or moderately well watered. That is, additional irrigation was not

assumed beyond the existing potential from wells and streams. No additional estimates were made for irrigation water by desalinization of

seawater. Of course, with water available at reasonable cost, additional

soils could be made arable and other tropical soils in wet-dry areas could

have 2 to 4 crops each year.

Thus the world has the physical and biological potential for very much

more food. A bit over one-half of the unused potential lies in the Tropics.

Of this, roughly one-third has abundant rainfall for crops, one-third has

seasons of abundant rainfall alternating with seasons that are relatively

dry, and one-third is semiarid or semidesert with short rainy seasons or

scanty and erratic rains.

As agricultural research continues, doubtless these estimates will appear low to soil scientists of the year 2000. Many have assumed that the

effect on farming of accelerating science and technology is to increase

yields per acre and to reduce the onfarm labor required. They have these

effects along with parallel improvement in the nonfarm service and industrial sectors of agriculture. In the United States today, for example,

only about one-fifth of the people employed in agriculture work on farms.

But today’s science and technology have an even more dramatic effect

on farming. They make possible highly effective new systems of soil use

on many kinds of soil that are unresponsive to the only possible systems

of a few decades ago. These effects are especially great with well-watered

soils of the Tropics. Doubtless these trends will continue.




The estimates of arable soil, already mentioned and shown in Table I,

were made originally for a recent study of the President’s Science Advisory Committee, “The World Food Problem,” 1967. They are based

on data about soils and related resources accumulated by the Soil Survey

*After the development of these estimates, we noted that with far fewer data the late

distinguished agricultural economist and geographer, Dr. 0. E. Baker ( 1 923) some 45 years

ago estimated 2.590 billion hectares of potentially arable soils in the world, of which less

than one-half was used for crops at that time.




Estimated Area of Potentially Arable Soils in the World

Both Cultivated and Not Cultivated

Total area

Potentially arable

of map unit

soils in map unit

(hectares, (percent) (hectares,



Prairie soils and Degraded Chernozems

Chernozems and Reddish Chestnut soils (inclusions of

darkgray and black soils of the subtropics and


Dark gray and black clay soils of the subtropics and

Tropics (inclusions of Chernozems, Reddish Chestnut

soils, and hydromorphic soils)

Chestnut, Brown, and Reddish Brown soils

Sierozems, Desert, and Red Desert soils (inclusions of

Lithosols, Regosols, ans saline soils)

Podzols and weakly podzolized soils (inclusions of

Bog and Half-Bog soils)

Gray-Brown Podzolic soils and Brown Forest soils

Latosols, Red-Yellow Podzolic soils (inclusions of

hydromorphic soils, Lithosols, and Regosols)

Red-Yellow Mediterranean (including Terra Rossa) soils

mostly mountainous (including many areas of

Rendzina soils)

Soils of mountains and mountain valleys (many


Tundra soils

























I 1 1.8




459. I







over many years, especially by the World Soil Geography Unit in the

last 25 years, and synthesized on maps at the scale of 1 : 1,000,000. To

give some suggestions of the distribution of soils in the world, a highly

generalized soil map is included as Fig. 8.

Estimates of potentially arable soils were related to a similar map in

1964 (Kellogg, 1964). Those were somewhat lower than the current

estimates more carefully made for “The World Food Problem.” Since

more people are familiar with the older great soil group names, such as

Podzol and Chernozem, than with those in the new and less familiar system of soil classification (Soil Survey Staff, 1960, 1967), we have used

the old names. Table I1 gives the names of the approximate equivalents

in the new classification.

FIG.8. A small-scale soil map of the world.




Names of Great Soil Groups Shown on the Soil Map or Discussed in

the Text and Their Approximate Placement in the Orders of the New

System of Soil Classification (Soil Survey Staff, 1960, 1967)

Great soils groups

Alluvial soils

Brown soils

Brown Forest soils


Chestnut soils

Dark Gray and Black soils

of subtropics and Tropics

Degraded Chernozem

Desert soils

Gray-Brown Podzolic soils

Gray Wooded soils

Hydromorphic soils



Organic soils


Prairie soils

Reddish Brown soils

Red Desert soils

Reddish Chestnut soils

Red-Yellow Mediterranean soils

Red-Yellow Podzolic soils



Saline soils


Terra Rossa


Weakly podzolized soils

Orders in new classification


Aridisols, Mollisols





Alfisols, Mollisols


Alfisols (mainly suborder Udalfs)


(Aquic taxa of various orders)

Oxisols, Ultisols, lnceptisols

(Lithic subgroups of several orders)



Mollisols (mainly suborder Udolls)

Aridisols, Alfisols


Mollisols, Alfisols

Alfisols (mainly suborder Xerall's)


Entisols (mainly suborder Psamments)

Mollisols (suborder Rendolls)



Alfisols (mainly suborder Xeralfs)


Spodosols, Inceptisols, Alfisols

Potentially arable soils are those that give an acceptable production of

cultivated crops adapted to the environment. In making the estimates,

the average level of agricultural technology used in the United States was

assumed. The figures in hectares include all arable soils, both cultivated

and uncultivated. Part of the uncultivated soils need practices for water

control - runoff control, irrigation, drainage, or some combination of

these-clearing of trees, stone removal, and so on. It was assumed that

the cost of these measures, however, would not be excessive in relation

to anticipated returns. All arable soiIs have potential for grazing. Those

with adequate rainfall can be used for forests.



1. Soil Descriptions

This section gives descriptions of the soils shown on the map in Fig. 8. Since no area o f a

single kind of soil is large enough to be shown by itself on such a small-scale map, the map

units are made up of several great soil groups. The principal kinds of soil are named for each

map unit, but others in it are not named. Some areas of Alluvial soils, Lithosols, and Regosols, for example, occur in all map units. Also, Bog and Half-Bog soils occur in several of


The areas of soils shown in Table I are based on measurements of units shown on this map.

The figure for any map unit, therefore, includes areas of both the named kinds of soil and the

inclusions. From studies of maps of much larger scale and of detailed soil surveys of sample

areas, the areas of inclusions were estimated to get a total of potentially arable soils for

each map unit.

Prairie soils and Degraded Chernozems make up only about 122 million hectares or

slightly less than 1 percent of the land surface of the world. Yet their high responsiveness

makes them important. A high proportion are already used for food production. They occur

mainly in the United States and the Soviet Union. Prairie soils in the United States are used

especially for high-yielding corn (maize), although forages and other cereals are grown successfully too. In the Soviet Union and eastern Europe, where Degraded Chernozems are

more extensive than Prairie soils, the soils are used mainly for small grains. Soil moisture

for crops is commonly abundant, although shortages occur now and then. Since the soils are

cold in winter, double cropping is not possible.

Prairie soils, developed under tall grass vegetation in moderately temperate regions, are

usually adjacent to regions that were originally forested. They have dark brown or dark

grayish brown surface soils grading into brown subsoils that are somewhat richer in clay

than the surface soils. Although somewhat leached, Prairie soils are high in fertility; they

are intermediate between Gray-Brown Podzolic soils and Chernozems. Degraded Chernozems have a gray, leached layer between a black or dark brown surface soil and a brown


Chernozems and Reddish Chestnut soils (some inclusions of dark gray and black soils of

subtropics and Tropics). A high proportion of this group is used for farming: but soil moisture, although enough for sustained annual production of wheat and other small grains, is

commonly too low for high yields in some years. Yet considerable corn and sunflowers are

grown on them, and sugar beets are important on those with the best supply of moisture.

The group of Chernozems and Chestnut soils and inclusions provide an estimated 282

million arable hectares. The principal areas are in the United States, the Soviet Union,

eastern Europe, and Argentina.

The Chernozems are black soils rich in humus to a depth of some 45 to 90 centimeters.

Beneath this dark material is a layer of calcium carbonate accumulated from leaching of the

soil above. In their natural state, Chernozems are covered with grass and the soils are

granular and high in fertility.

Chernozems were the great wheat-producing soils of the world. Now corn, sunflowers,

and sugar beets are also grown. Despite their granular structure and high fertility, yields

usually are not high because of limited soil moisture. Nevertheless, substantial progress has

been made through better tillage practices. improved varieties, and strategic use of modest

amounts of nitrogen and phosphatic fertilizers. In fact, we may be on the threshold of grain

yields twice those of two decades ago.

The Reddish Chestnut soils have about the same general fertility as the Chernozems and

about the same soil moisture limitations, but, since they are commonly warm for longer



periods, a wider range of crops can be grown. Sorghum and cotton are common crops in

addition to small grains.

With irrigation crop production can be increased on both Chernozem and Chestnut soils.

Dark gray and black soils of the subtropics and Tropics (inclusions of Chemozems,

Reddish Chestnut, and hydromorphic soils) cover nearly 500 million hectares. About onehalf of the map unit is estimated to be potentially arable. The principal areas are in Africa,

South America, Australia, and India. These soils occur in several other places, including

the southern part of the United States, in areas too small to be shown on the soil map.

Dark gray and black soils of this group go under a variety of names, such as black cotton

soils, Grumosols, Regur, “self-mulching” soils, “cracking” clays, and tropical black clays.

In the new United States system, they are called Vertisols (turning soils). The mature uncultivated Vertisols have a distinctive microrelief called “gilgai.” Most areas of soils in this

group are in the warm or tropical regions having alternating wet and dry seasons. Some of

them are moist for several months each year. Most are nearly flat.

The soils of this group have poor structure, are high in clay, are plastic, and swell and

shrink. When dry, vertical cracks extend downward 30 centimeters to 1 meter. Although

these soils are moderate to high in fertility, they are hard to work. In fact, they can scarcely

be cultivated at all with only hand tools. For this reason, some areas are still available for

settlement by people using power machinery and modern techniques for drainage, irrigation,

tillage, and fertilization.

Chestnut, Brown, and Reddish Brown soils cover nearly 1,204 million hectares. Substantial areas are on all continents. About 50 percent is estimated to be potentially arable.

These are the soils of the warm to cool temperate grasslands. The Chestnut soils have a

somewhat drierclimate than Chemozems. The Brown soils are still drier but are not desertic.

The Reddish Brown soils, which extend into the Tropics in some places, are drier than the

Reddish Chestnut soils but not so dry as the Red Desert soils.

The surface soils have moderate supplies of organic matter, and commonly at about 20 to

30 centimeters there are layers of accumulated calcium carbonate leached from the soil

above. Although moderately high in fertility, insufficient soil moisture is a serious limitation common to all soils of this group. Crop yields are generally low. Yet, with methods to

conserve moisture and to prevent soil blowing, many areas are used for producing small

grains and some other crops. More hectares are used for grazing than for cropping.

With irrigation, the productive capacity of these soils can be increased severalfold and

the range in crops that can be grown substantially enlarged, especially in warm areas. Under

irrigation, nitrogen and phosphatic fertilizers are needed for sustained high yields. Such

micronutrients as iron and zinc may also be needed.

Sierozems, Desert, and Red Desert soils (inclusions of Lithosols, Regosols, and saline

soils) cover nearly 2,800 million hectares, slightly more than 2 1 percent of the land surface

of the world. Large areas are found on all continents, especially in Africa, Asia, and Australia. Dryness is common to all the soils and irrigation is essential for crop production.

Only about 0.5 percent of the large area is estimated to be potentially arable, yet this

amounts to about 14 million hectares. Our estimate is based in part on judgment about the

availability of irrigation water at reasonable cost and does not include water that may become available from desalinization of seawater or from other expensive sources. Even if

unlimited water supplies at reasonable cost were to be assumed, the estimated proportion of

potentially arable soils would not be high, although it would be several times the present

estimate. A substantial proportion of the areas shown on the map include soils too shallow,

stony, steep, cold, sandy, or saline to be considered as potentially arable under the assumptions used in this paper.



The Desert soils have scanty vegetation and most are gray. The Red Desert soils are

similar except for their reddish color; they are mainly in the tropical and subtropical deserts.

The Sierozems are gray soils of dry temperate regions that have somewhat more vegetation

and organic matter than the Desert soils. Most of these soils, except for the very sandy ones,

are well supplied with the primary plant nutrients except nitrogen, but the balance is not

always favorable.

Because irrigation is expensive, only the soils reasonably free of limitations other than

dryness should be selected for crop use; and even these need to be managed well to make

production economically feasible. With good soil selection and good management, including

heavy fertilization, adapted and responsive crop varieties, and control of weeds, insects,

and diseases, production can be very high. This is especially true in the Tropics where,

with the moisture limitation removed, two to four crops can be grown on the same hectare

each year. Hence, at reasonable cost, modest increases in irrigated hectares in tropical and

subtropical deserts could substantially increase the world food supply.

Podzols and weakly podzolized soils cover nearly 1,295 million hectares. The large areas

are in northern North America, northern Europe, and Siberia. Between I29 and 130 million

hectares are estimated as potentially arable.

Podzols are leached soils of the humid cool-temperature to cold forested regions. On the

soil surface is a mat of acid, peatlike organic matter from a few to 30 centimeters thick.

Under it, the surface layer of humus-rich mineral soil is only 1 or 2 centimeters thick. Directly beneath is an ashy gray, leached layer from which these soils get their name. The subsoil

is a brown layer richer in humus and iron oxide than the gray layer above it.

The weakly podzolized soils also have at the surface a mat of acid peatlike organic matter,

but the horizons (or soil layers) below are less clearly developed than in the Podzols.

Included in the southern fringes of the large areas in North America, and presumably in

Eurasia too, are some Gray Wooded soils, which resemble the Podzols but are less acid and

have clayey subsoils (Fig. 9). Also included throughout the entire area of Podzols, as shown

on the map, are innumerable small areas of Bog and Half-Bog soils.

In addition to the limitation of short freeze-free seasons, many local kinds of soils are too

stony, hilly, sandy, or swampy for crop use.

Crops have been produced on the least cold fringes of the large areas of Podzols and

associated podzolized soils for a long time. Liming and the application of fertilizers containing nitrogen, phosphorus, and potash are necessary for satisfactory yields. With development of plant varieties that mature in shorter growing seasons and practices, including

liming and fertilization, designed to make the best use of the solar energy available, cropping

can be pushed onto many millions OF hectares now in forest. The major proportion of areas

delineated as Podzols and podzolized soils, however, is not potentially arable in the foreseeable future.

Gray-Brown Podzolic soils and Brown Forest soils: This group of soils, although not extensive, is highly important. As shown on the map, this group of soils occupies about 605

million hectares or between 4 and 5 percent of the land area of the world. About 393 million

hectares are estimated to be potentially arable, most of which are already under cultivation.

Gray-Brown Podzolic soils in their natural state have thin organic-rich surface layers

underlain by grayish brown or yellowish brown leached layers over brown subsoils. These

brown subsoils are richer in clay than the surface soils from which part of the clay has come.

The less extensive Brown Forest soils are similar, but their subsoils are not enriched with

clay and the soils are somewhat less acid and more permeable.

The Gray-Brown Podzolic soils like other podzolic soils are leached, acid, and relatively

poor in most plant nutrients in available form. They also are low in organic matter, especially



after a few years of cropping. These soils commonly have an abundance of soil moisture and

are highly responsive to management. With lime and fertilizer they support a wide range of

crops, for both food for people and feed for livestock. The soils are too cold in winter for

plant growth. Commonly only one crop can be grown during the year.

FIG.9. New clearing of Gray-wooded soil in northern Saskatchewan. (In the region of

Podzol soils on the soil map.)

It was on these soils and their close relatives that much of the research on soils and plants

and their relationships was conducted in the 19th and early 20th centuries. From the fall of

Rome nearly to the French Revolution, grain yields in Europe were low, between 350 and

600 kg. per hectare. With the adoption of crop rotations they nearly doubled, and elimination

of the fallow year gave more hectares for harvest. The application of chalk and farm manures

was found to be beneficial and became a general practice. By 1850, wheat yields had risen

to about 850 kg. per hectare in France, about 1200 in Germany, and somewhat over 1200

in the United Kingdom. By 1906, they had gone to 1200 kg. in France, 1800 kg. in Germany,

and beyond that in the United Kingdom. Now they stand at about 2100 kg. in the United

Kingdom. Somewhat more than half the increases in the United Kingdom and Germany

came before the common use of fertilizers after 1850 (Ignatieff and Page, 1958).

This remarkable increase in production on Gray-Brown Podzolic soils has been attained

through improved technology, much of which grew out of research of the past 160 years or


On no other great group of soils has a comparable amount of research been done. By expanding the methods of research first learned on the Gray-Brown Podzolic soils, the

technology for doubling or tripling yields has already been developed for more kinds of

soils and is in early stages for others. The limit has not yet been reached on any kind of

soil, not even on the Gray-Brown Podzolic ones.

Latosols and Red-Yellow Podzolic soils (inclusions of hydromorphic soils, Lithosols, and



Regosols) cover nearly 32 14 million hectares or between 24 and 25 percent of the land area

of the world. The humid and wet-dry Tropics and subtropics are covered mainly by these

soils. The estimated potentially arable hectares come to 1382 million of which today less

than one-fourth are under cultivation; in the vast humid and wet-dry tropical regions of

South America and Africa lie the great areas of unused or but little used potentially arable


Latosols, the principal component of this group, are extensive under the rain forest of the

humid Tropics and under both woodland and anthropic savanna of the seasonal wet-dry

Tropics. The name Latosol covers a wide group of soils, including Red, Reddish Brown,

Reddish Yellow, and Yellow Latosols (Kellogg, 1950). Included also are relatively young

soils from basic rocks and volcanic ash. The minerals in the Latosols are highly weathered.

The soils have been subject to strong leaching with warmer water than in temperate regions.

Yet in the humid Tropics abundant plant nutrients are held in the living trees. These nutrients make a continuing cycle from the soil into the plants and back to the soil as the twigs,

fruits, leaves, and old wood decay. On many of the mature Latosols after the forest is removed and crops are grown without fertilizer 1 to 5 years, the supply of plant nutrients is

too low for economic production. But under shifting cultivation the nutrient supply can be

restored by 6 to 12 years of forest fallow.

In the Reddish Brown Latosols plant nutrients are not so scanty, but they become so

after a few years of cropping without fertilizers, compared to Chernozem or well-managed

Gray-Brown Podzolic soils.

Latosols vary widely in the amount of available phosphorous for crop plants. Some are

exceedingly low indeed (Fig. 3).

The small-scale soil map includes soils with wide differences in native fertility. Over the

years they receive varying amounts of volcanic ash, dust from dried foam blown in from the

sea, and dust from the deserts. Thus, speaking generally, the Latosols in Africa, on the

whole, have higher initial amounts of calcium and other mineral plant nutrients than those

in South America. Yet there are exceptions in both continents. Apparently enough calcium

and other material to be beneficial moves south from the Sahara Desert and north from the

Kalahari Desert.* Many of the Latosols under forest are higher in organic matter than they

appear to be because the organic matter is brown, not black. Some of the Reddish Brown

Latosols have as much organic matter as the nearly black soils of the American Midwest.

Yet decomposition is rapid in warm humid climates. Unless measures are taken to maintain

good cover, the organic matter drops to low levels with cropping.

Most Latosols are permeable to water and air. Most are free of physical barriers for crop

penetration, are easy to maintain in good structure or tilth, and have at least moderate

moisture-holding capacity. Perhaps as much as 75 percent of the total area of Latosols and

associated soils has smooth relief suitable for the use of farm machinery. Yet some ofthose

inherently most productive are on strong slopes and require measures for runoff control to

have good supplies of moisture.

With important local exceptions, most Latosols are responsive to modern management

systems and can be highly productive of many food and industrial crops. Although practical

ways have not yet been found for efficient farming on the most infertile Latosols, such as

some in Brazil, at a high level of productivity, research can be expected to find ways (Figs.

* Interestingly a common tree of the Africa rainforest, Chlorophora excelsia, accumulates

calcium from the soil, even acid soil, and from dust on the leaves. In the odd tree are stones

of calcium carbonate up to several kilograms in weight that local people use traditionally to

make whitewash for their walls-people living hundreds of kilometers from any other source

of limestone.



10 and 1 I). Already systems are in use for making some Latosols, such as many in Hawaii

and northern Queensland, highly productive, perhaps too high for use as a standard for

estimating productivity of most Latosols during the next two decades. But certainly these

examples of success justify optimism about the technical feasibility of high production on

millions of hectares of Latosols.

FIG.10. View of stunted cover (cerrodo) on uncultivated "dark red Latosol," probably

Ustox in the new classification, near Jatai, Brazil. The stunting is probably due to mineral

nutrient deficiences, especially of calcium, and seasonal dryness.

Scattered throughout most areas where Latosols occur, especially in the wet-dry Tropics,

are some that contain laterite. This iron-rich material of Ground-Water Laterite soils in the

wet-dry Tropics is soft when formed under some 40 to 100 centimeters of soil. Yet if the

climate becomes very dry or if the soil is removed through natural or accelerated erosion,

the laterite hardens irreversibly. We have seen road cuttings in laterite that are now like

ferroconcrete (Alexander and Cady, 1962). Once laterite hardens, it weathers only very

slowly. It is found in spots under many kinds of soil developed from materials deposited

above it, such as alluvium and volcanic ash. Soils with hard laterite have severe limitations

for crop production because this material commonly restricts root penetration and because

the surface soil is subject to serious erosion if unprotected from heavy showers. Yet some

soils with soft laterite are used with reasonable success by local cultivators who have

learned not to plow but to maintain a mixed culture of palms, food crops, trees, and the like.

Curiously, the cultivators in Kerala of southern India discovered many centuries ago that

these soils could be used for mixed cultures without plowing. We should also point out that

there are small areas of soils containing hard laterite now in places having a different climate

from that under which the soils formed. That is, climates do shift but once laterite is formed,

it changes extremely slowly. Such soils, although dramatically conspicucius locally, make up

less than 10 percent of the Latosols.



FIG. 1 I . Closer view of trees shown in Fig. 10.

Red-Yellow Podzolic soils differ from Latosols mainly in having a subsoil higher in clay

than the surface soil. The clay is somewhat more active than the clay in Latosols although

it is less active than that in Gray-Brown Podzolic soils. Because the subsoil of Red-Yellow

Podzolic soils is less permeable than that of Latosols on comparable slopes, the erosion

hazard is greater.

The largest single area of Red-Yellow Podzolic soils is in the southeastern part of the

United States. The steadily increasing success in developing ways to manage these soils for

sustained high production of a variety of crops suggests that corresponding or even greater

success should be possible with Latosols on which double cropping or even triple cropping

is technically possible over extensive areas. After all, soils in the Tropics are always warm

enough for plant growth, except in the high mountains, and this is a unique advantage over

all other regions, including the subtropical southeastern part of the United States.

Red-Yellow Mediterranean (including Terra Rossa) soils, mostly mountainous (including many areas of Rendzina) cover slightly less than I12 million hectares. Of this total,

somewhat less than 15 million hectares are estimated to be potentially arable because

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IV. New Potentially Arable Soils

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