United States

Department of

Agriculture

Natural

Resources

Conservation

Service

Soil Quality Institute

Grazing Lands

Technology

Institute

Introduction to

Microbiotic Crusts

July 1997
2

Foreword

Introduction to Microbiotic Crusts provides information on a soilassociated component of many plant communities that has not

been widely recognized or characterized. The majority of the

research on these crusts is limited to the Great Basin and Colorado Plateau regions of the United States. There is validity in

generalizing the basic functions of the crusts to wherever crusts

are found, given that their gross compositions are similar

(cyanobacteria, algae, mosses, lichens, etc.). However, it would

not be valid to estimate the general importance of these functions

in other regions because species composition does differ between

crusts, particularly within the larger components (i.e., lichens,

mosses) (39). In addition, the plant composition and functions of

associated plant communities where crusts occur differ between

regions. Understanding the role of microbiotic crusts in total

resource management is an ongoing challenge.

This document was written by Roxanna Johnston, botanist, and

includes the comments of numerous reviewers.

Cover

Top photo - mature crust in the Colorado Plateau

Bottom photo - Area without crust

Credits: Jayne Belnap / USGS-Biological Research Division

More information is needed about the functions that crusts perform and the effect

of crust disturbance or elimination on the total plant community and production,

the soil and the environment.

The United States Department of Agriculture (USDA) prohibits discrimination in its programs on the basis of race, color,

national origin, sex, religion, age, disability, political beliefs and marital or familial status. (Not all prohibited bases apply to

all programs.) Persons with disabilities who require alternative means for communication of program information (Braille,

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1-800-245-6340 (Voice) or (202) 720-1127 (TDD). USDA is an equal employment opportunity employer.
3

Introduction to

Microbiotic Crusts

Microbiotic crusts are commonly found in semiarid and

arid environments throughout

the world. Areas in the United

States where crusts are a

prominent feature of the

landscape include the Great

Basin, Colorado Plateau (19),

Sonoran Desert (12), and the

lower Columbia Basin (23).

Crusts are also found in

agricultural areas (21), native

prairies (36), and sandy soils

in Glacier Bay, Alaska (42).

Outside the United States,

crusts have been studied in the

Antarctic (13), Australia (33),

and Israel (28), among other

locations. In fact, microbiotic

crusts have been found on all

continents and in most habitats, leaving few areas crust

free (39).

Microbiotic crusts are

formed by living

organisms and their

by-products, creating

a surface crust of soil

particles bound

together by organic

materials.

Many names and many forms

Microbiotic crusts are also known as cryptogamic,

cryptobiotic, and microphytic, leading to some

confusion. The names are all meant to indicate

common features of the organisms that compose

the crusts. The most inclusive term is probably

‘microbiotic’ (38), referring to the small size of

the organisms and not limiting crust components

to plants. Whatever name used, there remains

an important distinction between these formations

and physical or chemical crusts.

Microbiotic crusts are formed by living organisms

and their by-products, creating a crust of soil

particles bound together by organic materials.

Chemical and physical crusts are inorganic

features, such as a salt crusts or platy surface

crusts.

Figure 1—Utah

The general appearance of crusts in terms of color, surface topography and surficial

coverage varies in different regions.(Jayne Belnap / USGS-Biological Research Division)
4

Characteristics and

formation

Microbiotic crusts are formed

by living organisms and their

by-products, creating a surface

crust of soil particles bound

together by organic materials.

Aboveground crust thickness

can reach up to 10 cm (39).

The general appearance of the

crusts in terms of color, surface

topography, and surficial

coverage varies (figs. 1-4).

Mature crusts of the Great

Basin and Colorado Plateau are

usually darker than the surrounding soil. This color is

due in part to the density of the

organisms and to the often dark

color of the cyanobacteria,

lichens, and mosses. The

presence or absence of a crust

is partly determined by soil

texture and conductivity, pH,

moisture, and possibly temperature (15, 21, 22). Crust

coverage varies greatly, from

less than 10 percent to nearly

100 percent (39).

Figures 2, 3, and 4

The general appearance of crusts in terms

of color, surface topography and surficial

coverage varies in different regions.

Fig. 2 Santa Barbara Island, California;

Fig. 3 southern Arizona;

Fig. 4 Salmon, Idaho)

(Figs 2 and 3: Jayne Belnap / USGSBiological Research Division.

Fig 4: Julie Kaltenecker/USDI-BLM)
5

Some crusts are characterized

by their marked increase in

surface topography, often

referred to as pinnacles or

pedicles (3). Other crusts are

merely rough or smooth and flat

(22). The process of creating

surface topography, or

pinnacling, is due largely to the

presence of filamentous

cyanobacteria and green algae

(fig. 5). These organisms swell

when wet, migrating out of

their sheaths. After each migration new sheath material is

exuded, thus extending sheath

length. Repeated swelling

leaves a complex network of

empty sheath material that

maintains soil structure after the

organisms have dehydrated and

decreased in size (7). A contributing mechanism is frost

heaving and subsequent uneven

erosion, leaving soil mounds

bound by crust organisms.

Lack of frost heaving has been

used to explain the absence of

pinnacles in warmer regions

(39).

Figure 5

Pinnacles are formed by sheaths of cyanobacteria as they extend in length and bind soil

particles together. Frost-heaving also causes sheath-bound particles to rise.

(Jayne Belnap / USGS-Biological Research Division)
6

Composition

Microbiotic crusts are predominantly composed of

cyanobacteria (formerly bluegreen algae), green and brown

algae, mosses, and lichens (figs.

6-8). Liverworts, fungi, and

bacteria can also be important

components. Cyanobacteria or

green algae make up a large

component of microbiotic

crusts in semiarid and arid

regions of the United States.

Glossary

algae nonvascular photosynthetic plantlike organisms, they are informally

divided into groups by their

dominant pigments (i.e., green,

brown, red, etc.).

bacteria microscopic, single celled

organisms.

cyanobacteria photosynthetic bacteria formerly

called blue-green algae, their

growth forms tend to be filamentous.

fungi nonphotosynthetic multicellular

organisms that are either

saprophytic or parasitic.

hyphae single strands of a fungus.

lichen a composite plant consisting of fungi

living symbiotically with algae or

cyanobacteria.

liverworts and mosses – nonvascular

plants of small stature, the two are

similar with the exception of

reproductive methods.

rhizines/rhizoids root-like structures of lichens

and mosses respectively, they

are used for attachment.

sheaths external coating formed by some

filamentous cyanobacteria, those

discussed in the article are formed

from polysaccharides.
7

In the Great Basin and the

Colorado Plateau, Microcoleus

vaginatus (a cyanobacteria)

composes the vast majority of

the crust structure (10, 3).

Lichens of the genera Collema

spp. and mosses from the

genera Tortula spp. are also

common (3, 4, 26). In hot

deserts, such as the Sonoran,

Schizothrix species (another

cyanobacteria) are more common (12). Lower Columbia

Basin crusts tend to be dominated by green algae (23).

Shifts between green algal and

cyanobacterial dominance have

been attributed to changes in

pH, with decreasing alkalinity

(pH) favoring green algae (23,

27). Crusts from other regions

can be dominated by lichens

and/or mosses. The organism

that dominates the crust is

partly determined by microclimate and may also represent

different successional stages

(39).

Figures 6, 7, and 8

Microbiotic crusts may include

cyanobacteria, green and brown algae,

mosses, and lichens.

(Figs 6, 7: Mike Pellant/USDI-BLM

Fig 8: Pat Shaver/NRCS)
8

Functions

Crusts contribute to a number

of functions in the environment.

Because they are concentrated

in the top 1 to 4 mm of soil,

they primarily effect processes

that occur at the land surface or

soil-air interface. These include

soil stability and erosion,

atmospheric N-fixation, nutrient

contributions to plants, soilplant-water relations, infiltration, seedling germination, and

plant growth.

Soil stability

Crust forming cyanobacteria

and green algae have filamentous growth forms that bind soil

particles (figs. 9-10). These

filaments exude sticky polysaccharide sheaths around their

cells that aid in soil aggregation

by cementing particles together

(13, 7). Fungi, both free-living

and as a part of lichens, contribute to soil stability by binding

soil particles with hyphae (1,

19, 36). Lichens and mosses

assist in soil stability by binding

particles with rhizines/rhizoids,

increasing resistance to wind

and water action (2, 36). The

increased surface topography of

some crusts, along with increased aggregate stability,

further improves resistance to

wind and water erosion (33, 40,

41).

Figure 9

Polysaccharide sheaths of cynobacteria and green algae bind soil particles together.

(Jayne Belnap / USGS-Biological Research Division)

Figure 10

Sheaths are at the soil surface. Soil particles are attached to the sheaths.

(Jayne Belnap / USGS-Biological Research Division)
9

Microbiotic crust functions include:

-soil stability and erosion

-atmospheric N-fixation

-nutrient contributions to plants

-soil-plant-water relations

-infiltration

-seedling germination

-plant growth

Nutrient contributions

Microbiotic crusts can increase

available nitrogen as well as

other nutrients in the soil. This

process is almost solely based

on the cyanobacterial component of the crust, whether freeliving or as part of lichens. It

has been estimated that microbiotic crusts fix 2-41 kg N/ha/

yr, though these numbers may

be inflated due to the method of

measurement (39). Crusts can

be the dominant source of fixed

N in semiarid ecosystems (37,

17), and this nitrogen appears to

be available to higher plants

(32). Part of the increasing

nutrient availability might be

due to the ability of the

cyanobacterial sheaths to

directly bind positively charged

molecules (8). Phosphorus

levels are also increased in soils

with well developed crusts.

This increase is accomplished

by the binding of soil fines,

which are relatively high in

phosphorus content (19).

Increased nutrient levels are

most evident near the soil

surface due to the dependence

of the organisms on light.

Maximum input of nitrogen and

other minerals occurs when the

organisms are most active.

Photosynthesis and nitrogen

fixation optimal temperatures

are 75 to 86 degrees F and 51 to

61 degrees F respectively (10,

34, 35). Photosynthesis in

green algae has been shown to

be particularly sensitive to high

temperatures (24). Moisture

levels are also important.

Photosynthesis maximizes

when the soil surface is near

saturation, and nitrogen fixation

maximizes when the plant

moisture level is between 60

and 80 percent (19, 10, 15).

Water relations

Crust organisms are quickly

able to utilize moisture from

dews (10) and, in the case of

green algae, water vapor (37).

An investigation of

cyanobacteria and green algae

in Death Valley determined that

certain species of algae could

retain water against an osmotic

pull of 50 atmospheres (-50.7

bars) (16). This ability to retain

water under high tension might

be beneficial to survival in dry

habitats. Many crust organisms

are extremely drought tolerant,

but this does not ensure continuous growth and functioning.

Crust samples from Idaho

(predominantly Microcoleus

vaginatus) were shown to be

particularly sensitive to moisture levels. Photosynthesis and

growth in cyanobacteria dominated crusts were inhibited at -

18 bars and -7 bars respectively

(10). Lichens do not appear to

be as sensitive to moisture

levels (19).

The water holding capacity of

crust organisms has been

proposed to benefit surrounding

vegetation by slowing evaporation. It has also been proposed

that this ability to hold water

may be so strong as to prevent

vegetation from accessing it,

thereby decreasing available

water. So far, a conclusion has

not been reached on this issue.
10

Infiltration

Microbiotic crusts can alter

infiltration. Some studies have

shown increases in infiltration

in the presence of crusts (11,

30); this is usually attributed to

increased aggregate stability.

Other studies found either

decreases in infiltration or no

effect (18, 40). Differences in

findings seemed to be site

specific and were often related

to soil texture and chemical

properties of the soil.

Effects on plant germination and growth

Studies investigating the role of

crusts in plant germination have

had varied results. Increased

surface relief is presumed to

provide safe sites for seeds

while darker surface color

increases soil temperatures to

those required for germination

earlier in the season, coinciding

with spring water availability

(6, 19). While the above

conditions should favor seed

germination, not all studies

have supported this conclusion.

Conflicting results might be

reconciled by these considerations: 1) seeds that become

worked into the crust will more

likely be able to benefit from

the crust environment than

those that remain on the surface, and 2) seed size and

degree of crust pinnacling may

determine whether the crust

environment is beneficial to

germination and establishment

(29).

Studies on plant health are more

clear-cut. Many studies have

shown increases in survival

and/or nutrient content in crust

covered environments as

opposed to bare soil (8, 19, 29),

though these results are not

universal (19). Nutrients shown

to increase in plant tissues

grown in the presence of crusts

are nitrogen, phosphorus,

potassium, iron, calcium,

magnesium, and manganese (5,

8). Some of the plants benefited by crust presence include

Festuca octoflora (sixweeks

fescue), Mentzelia multiflora

(desert blazing star) (5, 8),

Arabis fecunda (rock-cress)

(29), Kochia prostrata (prostrate summercypress),

Linum perenne (blue flax),

Lepidium montanum

(mountain peppergrass), and

Sphaeralcea coccinea (scarlet

globemallow) (20).

Response to

disturbance

Microbiotic crusts are well

adapted to severe growing

conditions, but poorly adapted

to compressional disturbances.

Domestic livestock grazing, and

more recently, tourist activities

(hiking, biking, and ORV’s) and

military activities place a heavy

toll on the integrity of the crusts

(fig. 11). Disruption of the

crusts brings decreased organism diversity, soil nutrients, and

organic matter (9).

Direct damage to crusts usually

comes in the form of trampling

by humans and livestock.

Trampling breaks up the

sheaths and filaments holding

the soil together and drastically

reduces the capability of the

soil organisms to function,

particularly in nitrogen fixation

(9, 6, 17). Changes in plant

composition are often used as

indicators of range health. This

indicator may not be sensitive

enough to warn of damage to

microbiotic crusts (31). Studies

looking at trampling disturbance have noted that losses of

moss cover, lichen cover, and

cyanobacterial presence can be

severe (1/10, 1/3, and 1/2

respectively) (2), runoff can

increase by half, and the rate of

soil loss can increase six times

(20) without apparent damage

to vegetation. Adding nitrogen

to the soil can retard natural

nitrogen fixation by soil organisms (19).
11

Other disturbance impacts are

indirect. Several native rangeland shrubs (Artemisia

tridentata, Atriplex

confertifolia, and Ceratoides

lanata) may have allelopathic

effects on the nitrogen fixing

capabilities of crusts, potentially lowering nitrogen fixation

by 80 percent (35). Actions that

increase the shrub component,

such as excessive grazing, can

have an unexpected impact on

crust functioning.

Another indirect disturbance

occurs through crust burial.

When the integrity of the crust

is broken through trampling or

other means, the soil is more

susceptible to wind and water

erosion. This soil can be

carried long distances, covering

intact crusts. Crusts tolerate

shallow burial by extending

sheaths to the surface to begin

photosynthesis again. Deeper

burial by eroded sediment will

kill crusts (37) (fig. 12).

Fire is a common component of

many regions where microbiotic crusts grow. Investigations

into the effects of fire on crusts

show that fires can cause severe

damage, but that recovery is

possible (25). The degree to

which crusts are damaged by

fires apparently depends on the

intensity of the fire. Low

intensity fires do not remove all

the structure of the crust allowing for regrowth without significant soil loss (fig. 13).

Shrub presence (particularly

sagebrush) increases the intensity of the fire, decreasing the

likelihood of early vegetative or

crust recovery (23).

Full recovery of microbiotic

crusts from disturbances is a

slow process, particularly for

mosses and lichens (4). There

are means to facilitate recovery.

Allowing the cyanobacterial

and green algae component to

recover will give the appearance of a healthy crust. This

visual recovery can be complete

(with the exception of lichens

and mosses) in as little as 1 to 5

years given average climate

conditions (14, 4). Limiting the

size of the disturbed area also

increases the rate of recovery

provided that there is a nearby

source of inoculum (4).

Figure 11

Crust disturbance along a trail breaks up the sheaths and filaments that bind the soil

together. (Jayne Belnap / USGS-Biological Research Division)

Figure 12

Burial by wind blowing sand will kill crusts. (Jayne Belnap / USGS-Biological Research

Division)
12

Future research

Information on microbiotic

crusts is based on a small

amount of research—most of

which is from arid or semiarid

regions. More studies are

needed, especially those that

expand into other ecological

regions. Most pressing is the

need to learn more about the

functions of the crusts, such as

soil stability, nutrient contributions, soil-plant-water relations,

infiltration, seedling germination and plant growth. Information on the relative importance

of these functions in different

ecosystems is also needed. This

understanding is necessary to

determine the management

strategies needed to protect or

favor the development and

functions of the crusts. Additional areas of research are 1)

learning how crust composition

and functions vary with climate,

soil texture, soil chemical

composition, and plant community, 2) how function correlates

to differences in the composition and appearance of crusts,

and 3) the effect of management practices on crusts.

The land where crusts occur is

used for a wide range of purposes—from grazing and

recreation to military uses, and

in some places, crops. Ultimately, land managers want to

know how the functions of

crusts change under different

practices. Where the functions

of crusts are impaired or eliminated because of land use

practices, and are essential to

the health of the ecosystem,

land managers need guidelines

to adapt their practices to

protect or restore the functions

of crusts.

Figure 13

Microbiotic crust in a 1983 seeding (crested wheatgrass, Siberian wheatgrass and

bluebunch wheatgrass) following a 1996 fire. The crust remained intact between the

burned bunchgrass clumps. (Julie Kaltenecker/USDI-BLM)

Where the functions of

crusts are impaired or

eliminated because of

land use practices, and

are essential to the

health of the ecosystem,

land managers need

guidelines to adapt their

practices to protect or

restore the functions of

crusts.
13

For further reading see the References section and:

Harper, K.T., and J.R. Marble. 1988. A role for nonvascular plants in management of arid and semiarid

rangelands in Vegetational Science Applications for Rangeland Analysis and Management. P.T. Tueller

(ed.). Kluwer Academic Publishers. Dordrecht, Netherlands. Articles in the Great Basin Naturalist,

volume 53, 1993.

Johansen, J.R. 1993. Cryptogamic crusts of semiarid and arid lands of North America. Journal of

Phycology 29:140-147.

References:

1. Anderson, D.C., K.T. Harper, and R.C. Holmgren. 1982a. Factors influencing development of

cryptogamic soil crusts in Utah deserts. Journal of Range Management 35(2):180-185.

2. Anderson, D.C., K.T. Harper, and S.R. Rushforth. 1982b. Recovery of cryptogamic soil crusts

from grazing on Utah winter ranges. Journal of Range Management 35(3):355-359.

3. Anderson, D.C., and S.R. Rushforth. 1977. The cryptogam flora of desert soil crusts in southern

Utah, U.S.A. Nova Hedwigia 28(4):691-729.

4. Belnap, J. 1993. Recovery rates of cryptobiotic crusts: inoculant use and assessment methods.

Great Basin Naturalist 53(1):89-95.

5. Belnap, J. 1994. Potential role of cryptobiotic soil crusts in semiarid rangelands. USDA, Forest

Service, Intermountain Forest and Range Experiment Station. General Technical Report. INT

313:179-85.

6. Belnap, J. 1995. Surface disturbances: their role in accelerating desertification. Environmental

Monitoring and Assessment 37:39-57.

7. Belnap, J., and J.S. Gardner. 1993. Soil microstructure in soils of the Colorado Plateau: the role of

the cyanobacterium Microcoleus vaginatus. Great Basin Naturalist 53:40-47.

8. Belnap, J., and K.T. Harper. 1995. Influence of cryptobiotic soil crusts on elemental content of

tissue of two desert seed plants. Arid Soil Research and Rehabilitation 9:107-115.

9. Belnap, J., K.T. Harper, and S.D. Warren. 1994. Surface disturbance of cryptobiotic soil crusts:

nitrogenase activity, chlorophyll content, and chlorophyll degradation. Arid Soil Research and Reha

bilitation 8:1-8.

10. Brock, T.D. 1975. Effect of water potential on a Microcoleus (cyanophyceae) from a desert crust.

Journal of Phycology 11:316-320.

11. Brotherson, J.D., and S.R. Rushforth. 1983. Influence of cryptogamic crusts on moisture relation

ships of soils in Navajo National Monument, Arizona. Great Basin Naturalist 43(1):73078.

12. Cameron, R.E. 1960. Communities of soil algae occurring in the Sonoran Desert in Arizona.

Journal of Arizona Academy of Science 1(85-88).

13. Cameron, R.E., and J. R. Devaney. 1970. Antarctic soil algal crusts: scanning electron and optical

microscope study. Transactions of the American Microscopical Society 89(2):264-273.

14. Cole, D.N. 1990. Trampling disturbance and recovery of cryptogamic soil crusts in Grand Canyon

National Park. Great Basin Naturalist 50(4):321-325.

15. Dunne, J. 1989. Cryptogamic soil crusts in arid ecosystems. Rangelands 11(4):180-182.

16. Durrell, L.W. 1962. Algae of Death Valley. Transactions of the American Microscopical Society

81:267-273.
14

17. Evans and Ehleringer. 1993. Broken nitrogen cycles in arid lands: evidence from N15 of soils.

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20. Harper, K.T., and L.L. St. Clair. 1985. Cryptogamic soil crusts on arid and semiarid rangelands

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21. Hilton, R.L., and F.R. Trainor. 1963. Short communication: algae from a Connecticut soil.

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23 Johansen, J.R., J. Ashley, and W.R. Rayburn. 1993. Effects of rangefire on soil algal crusts in

semiarid shrub-steppe of the lower Columbia Basin and their subsequent recovery. Great Basin

Naturalist 53(1):73-88.

24. Johansen, J.R., and S.R. Rushforth. 1985. Cryptogamic soil crusts: seasonal variation in algal

populations in the Tintic Mountains, Juab County, Utah. Great Basin Naturalist 45(1):14-21.

25. Johansen, J.R., and L.L. St. Clair. 1984. Recovery patterns of cryptogamic soil crusts in desert

rangelands following fire disturbance. The Bryologist 87(3):238-243.

26. Johansen, J.R., and L.L. St. Clair. 1986. Cryptogamic soil crusts: recovery from grazing near

Camp Floyd State Park, Utah, USA. Great Basin Naturalist 46(4):632-640.

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28. Lange, O.L., G.J. Kedion, B. Budel, A. Meyer, E. Kilian, and A. Abeliovich. 1992. Taxonomic

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29. Lesica, P., and J.S. Shelly. 1992. Effects of cryptogamic soil crust on the population dynamics

of Arabis fecunda (Brassicaceae). American Midland Naturalist 128:53-60.

30. Loope, W.L., and G.F. Gifford. 1972. Influence of a soil microfloral crust on select properties of

soils under Pinyon-Juniper in southeastern Utah. Journal of Soil and Water Conservation

27:164-167.

31. Marble, J.R., and K.T. Harper. 1989. Effect of timing of grazing on soil-surface cryptogamic

communities in a Great Basin low-shrub desert: a preliminary report. Great Basin Naturalist

49(1):104-107.

32. Mayland, H.F., and T.H. McIntosh. 1966. Availability of biologically fixed atmosphere nitro

gen-15 to higher plants. Nature 209:421-422.

33. Mucher, H.J., C.J. Chartres, D.J. Tongway, and R.S.B. Greene. 1988. Micromorphology and

significance of the surface crusts of soils in rangelands near Cobar, Australia. Geoderma 42:227-

244.

34. Pentecost, A. 1985. Relationships between light, temperature and photosynthesis in a temperate

Microcoleus (cyanobacterium) mat. Microbios 43:141-148.

35. Rychert, R.C., and J. Skujins. 1974. Nitrogen fixation by blue-green algae-lichen crusts in the

Great Basin Desert. Soil Science Society of America Proceedings 38:768-771.

36. Schulten, J.A. 1985. Soil aggregation by cryptogams of a sand prairie. American Journal of

Botany 72(1):1657-1661.
15

37. Shields, L.M., C. Mitchell, and F.Drouet. 1957. Alga- and lichen-stabilized surface crusts as soil

nitrogen sources. American Journal of Botany 44:489-498.

38. St. Clair, L.L., and J.R. Johansen. 1993. Introduction to the symposia on soil crust communities.

Great Basin Naturalist 53(1):1-4.

39. West, N.E. 1990. Structure and Function of soil microphytic crusts in wildland ecosystems of

arid and semiarid regions. Advances in Ecological Research 20:179-223.

40. Williams, J.D., J.P. Dobrowolski, and N.E. West. 1995a. Microphytic crust influence on

interrill erosion and infiltration capacity. Transactions of the American Society of Agricultural

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41. Williams, J.D., J.P. Dobrowolski, N.E. West, and D.A. Gillette. 1995b. Microphytic crust

influence on wind erosion. Transactions of the American Society of Agricultural Engineers

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42. Worley, I.A. 1973. The “black crust” phenomenon in upper Glacier Bay, Alaska. Northwest

Science 47(1):20-29.
Soil quality is the

capacity of the soil to

function. The symbol for soil quality

represents all natural

resources, their

dependence on soil,

and human dependence on the health

of these resources.