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CHAPTER 8 PROPERTIES OF DESERT SOILS R. E. CAMERON Terrestrial soils have evolved in a dynamic situation under the influence of at least five major interacting factors. These factors include time (geo- logical), topography, soil parent mater.uis, climate (especially moisture and temperature combinations) and organisms (especially vegetation) [Jenny, 1941]. In regions of favorable climate, soil development is optimal with typical soil profiles and horizons. In desert areas, climate is especially arid and the dearth of organisms is noticeable. Organisms have little or no influence in the formation of true desert soils. Therefore, in very arid areas there is little, if any, classical soil development and no soil profiles of distinguishable horizons are present. Physical and chemical processes have been the primary agents in the development of desert soils, and in barren areas the biological system in soils is entirely restricted to desert micro- organisms [Cameron, 1962; 1963; Killian and Feher, 1939]. Virgin desert soils can be characterized on the basis of typical soil prop- erties. These properties include or indicate limiting factors for life to a greater or lesser extent and determine the kinds of dependent and inter- acting biota in a particular soil ecosystem [Cameron, 1962; 1963]. For a given soil ecosystem the following properties can be considered: (a) mois- ture, bound and available, (b) surface and subsurface temperature, maxi- mum, minimum and diurnal, (c) quantity and quality of solar radiation received and emitted at the soil surface, (d) gas exchange and gas compo- 164

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Properties of Desert Soils 165 sition, particularly the exchange and concentrations of carbon dioxide and oxygen, (e) essential elements, toxic elements, available forms and soluble salts, especially the biogenic salts of carbon, nitrogen, phosphorus, and sulfur, (f) organic matter, organic carbon and nitrogen and living or- ganisms, (g) cation exchange capacity, (h) buffer capacity, (i) soil pH, (j) soil Eh, (k) porosity, (1) texture, (m) structure, (n) bulk density, (o) mineralogy, especially clay, (p) color. Most of these properties have been determined for typical soils of economic value in agricultural areas. It cannot be assumed that measurements on desert soils will yield the same information, or that measurements obtained for typical well-developed terrestrial soils should be relied upon for purposes of testing and evaluating extraterrestrial life detection systems. Very little detailed information on the characteristics of desert soils is available, but some recent investigations provide information on the nature, abundance and distribution of microorganisms within a desert soil eco- system and on the characteristics of their immediate environment. Studies of several hundred North American desert soils have yielded results that may illuminate the problems of sampling and detection of life. Unless otherwise stated, the information presented here is based primarily on investigations of soils occurring in the Great Basin, Mohave and Colo- rado Deserts. Additional information is derived from soil investigations of volcanic deserts in the Kau Desert of Hawaii, the Valley of 10,000 Smokes Desert in Alaska and high altitude areas in the relatively arid White Mountains of California. Where no references are cited, unpublished data are quoted that contain details of methods and results of desert soil investi- gations [Cameron et OCR for page 164
166 RECOGNITION OF LIFE AND SOME TERRESTRIAL PRECEDENTS Figure 1. z Relationship of silt and clay content to soil moisture status at hygroscopic coefficient (98 per cent RH). PROFIL PROFIL PROFIL PROFIL PROFIL WATER RETAINED BY SOIL. PWCW1 HYGROSCOPIC COEFFICIENT (98 pff cwrt RHJ mum soil water suction required for activity of desert microorganisms in various soil ecosystems has not been determined. Soil temperatures vary considerably depending on several factors, for example, amount of solar radiation received and emitted at the soil surface, soil color, mineralogy, moisture content, amount and distribution of plant cover, porosity, texture, structure, degree of slope and exposure. Light colored, barren desert soils generally show rapid and wide heat flux changes at the surface. Soil surface temperatures have been found to be 40 to 80°F (about 25 to 45°C) higher than the air temperature three feet above the soil surface, even at elevations of 12,000 to 14,000 feet. However, at a depth of three feet below the soil surface the temperature may be relatively constant for much more than a 24-hour period. Cold soils decrease microbial activity. Freeze-thaw cycles generally stimulate activity. Gas exchange and variations in concentrations of gases in the soil atmos- phere are especially important for soil microorganisms but have not yet been adequately investigated in desert soils. On the basis of carbon dioxide evolution in Sahara soils, Killian and Feher [1939] found that the desert microflora are quite active. For typical soils, carbon dioxide concentrations can increase from 5 to more than 500 times that of the atmospheric concentration, depending on such factors as depth of soil, porosity, moisture content, solar radiation effects, time of day, season of the year and abun- dance and activity of microorganisms. Oxygen concentrations commonly decrease with soil depth. In soil investigations at high elevations, oxygen concentration was found to show diurnal variations, but sometimes it increased with depth as compared to the value at the surface. Salt content is quite variable in desert soils, ranging from nil to total salt. These salts are commonly sodium, calcium, and magnesium carbonates,

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Properties of Desert Soils 167 chlorides and sulfates. Nitrates, nitrites and phosphates may also be present. Typical electrical conductivity values for desert soils are plotted in Figure 2. Weight loss upon ignition of desert soils lacking in humus is greatly dependent upon the salt content as shown by a comparison of Figure 2 with Figure 3. In general, an increase in salt content decreases the abun- dance of common groups of desert soil microflora when the salt content approaches 5 per cent. Organic matter content of desert soils is considerably below those of WKJFtE BftC \ \ -SS5 -sio TOO 250 I 3. SOU. • MjO EXTRACT. EC x 10s mhoi/cm flT 25'C Figure 2. Variation of soil electrical conductivity with soil depth. Figure 3. Variation of total loss on igni- tion at 700°C* with soil depth. BASED ON OVENORY SOIL (I05-C15 «C FOR 24 hr) PROFILE PROFILE PROFILE PROFtE PROFILE PROFILE TOTAL LOSS ON IGNITION AT 700* C. perceni

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168 RECOGNITION OF LIFE AND SOME TERRESTRIAL PRECEDENTS LA PROFILE A PROFILE B PROFILE C PROFILE D PROFILE E PROFILE F Figure 4. Distribution of organic matter with soil depth. I 06 0.8 1.0 I ORGANIC MATTER, per on! typical agricultural soils, approaching values of 1.0 to 0.05 per cent or less [Cameron and Blank, 1963; Radwan, 1956]. Reported values for soil organic matter, nitrogen and carbon are greatly dependent upon the method of analysis [Cameron and Blank, 1963]. Unless some surface litter is present, organic matter in desert soils may show little deviation with depth of soil, and sometimes increases with depth, as shown in Figure 4. Organic carbon and nitrogen values in desert soils frequently, but not always, have narrow ratios of less than 10:1, approaching 3:1 or 2:1 [Killian and Feher, 1939; Cameron and Blank, 1963], values similar to those of protein or microbial cells. The greatest numbers of desert microorganisms are not always found in the greatest accumulations of organic matter. The cation exchange capacity in desert soils is dependent on the same phenomena as typical well-developed soils. Factors influencing cation ex- change include soil texture, the degree of aggregation of soil particles, the kinds and amounts of clays, salts and organic matter. The cation exchange capacity of desert soils low in clay and organic matter is also commonly low. Cation exchange capacity curves for desert soils are shown in Figure 5. With available moisture present, a high cation exchange capacity of balanced elements is favorable for microorganisms. Buffer capacity, pH, Eh, salts and cation exchange capacity are related phenomena. Some desert soils have an appreciable buffer capacity and greatly resist a change in pH upon addition of acids or bases. Values for pH may or may not vary with depth of desert soils as shown in Figure 6.

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Properties of Desert Soils 169 Many, but not all, desert soils show a pH value above neutral, but this depends on a number of factors such as porosity, kinds and concentrations of exchangeable cations and the moisture content. Values of pH 8.5, or greater, almost invariably indicate an exchangeable sodium percentage of PROFILE C PROFILE 0 PROFILE E PROFILE F Figure 5. Variation of cation exchange pacify with soil depth. ca- CATION EXCHANGE CAPACITY, MEO/IOO GM Figure 6. Variation of pH saturated soil paste with soil depth. PROFILE A PROFILE B PROFILE C PROFILE D PROFILE E PROFILE F SATURATED SOIL PASTE. pH

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170 RECOGNITION OF LIFE AND SOME TERRESTRIAL PRECEDENTS g| 1 1 + 1 M s's— + + + 5Si + 1 + + Is + + .e M M H| q .. m ..' o- i * ._. 0> fcx C *-* o .2 1 E If- 22o o f5 *o \o Tt *o w *> c/5 11 Sx •o -o « :§=! + | + , M .1 S'g.S M) 1- ~* 5 > 111 1 .0 O s 3 3 ^ ^* ^ g *j O Z o> o ~ -f- -)_ -)- -(_ o .2 B C 0 1 a s .c uu o 2 >»M~ — 1 § a — -O 'u P O 'S la <«lx m — —: t: • w on 3 a —§ I I E 1 .— «i £ V A V A M

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Properties of Desert Soils 171 15 or more and the presence of alkaline earth carbonates [Richards, 1954], which is unfavorable for many microorganisms. Measurements of soil pH, whether in the field or laboratory, show dilution effects. Usually the more dilute the soil, the higher the pH value. This dilution effect helps to explain in situ soil pH values of 5 or less for relatively dry desert soils which, when wet, show a pH of 8 or more. However, at high elevations in the White Mountains, relatively moist soils showed pH values of 7 or greater com- patible with the observed indigenous microflora, but when the soils were tested in the laboratory at saturation percentages, pH values were between 7 and 5. Values of pH 4 or less have been obtained on comparatively raw desert volcanic soils from Hawaii and Alaska. Eh values are much more erratic than pH values for desert soils, but commonly have high positive values and show dilution effects. Some of the phncipal components of desert soil redox systems are iron, manganese and clay, while minor com- ponents are humus and organic acids. Many desert soil microorganisms can evidently tolerate a wide pH and Eh range, while others require a more narrow, specific range. Porosity, texture, structure, and bulk density are interdependent soil physical factors. For desert soils, porosity measurements have ranged between 30 to more than 65 per cent, with the lowest values for compact sands and highest values for aggregated clays and raw pumice soils. Bulk density figures have commonly ranged between 0.70 to 1.00 for raw pumice soils and clays to more than 1.65 for sands. As indicated previously, desert soils show little typical soil structure, although there are sometimes accumu- lations of salts and clay and distinctive hardpans due to evaporation and leaching, Figures 12 and 13. Desert soils tend to show single-grained structure, and aggregation of particles is generally poor. Particle size distri- bution for three different desert soil profiles is shown in Figures 7, 8 and 9. It is worth noting that for a completely unfractionated soil sample there is no evidence that the varieties or distribution of desert soil microflora depend on particle size distribution (cf., Table 1). Mineralogy and soil color are closely related and greatly dependent upon such factors as climate, degree of weathering, nature of the soil parent materials, and oxidation and hydration state of soil particles. Desert soils show wide variations in mineralogy, although quartz and plagioclase are common minerals. The glassy phase of certain volcanic soils is apparently detrimental to general microbial development. Colors of desert soils range through gradations and shades of red, yellow, pale-brown or gray, again showing dependence upon climate, minerals present, moisture and the oxidation and hydration state of iron compounds. Red soils are more prevalent in hot deserts while gray soils are more evident in cold deserts. The kinds of microorganisms, their abundance and distribution in desert soils are of special importance with regard to sampling and life detection.

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172 RECOGNITION OF LIFE AND SOME TERRESTRIAL PRECEDENTS siLj_a CLAY- — SILT CLAY 100 200 900 400 500 «Xi No OF MCnOORGAMSMSIIO9) "(AEROBES 8 ANAEROBES 8 FUNGI ft ALGAH Figure 7. Distribution of microorganisms* with panicle size distribution for Profile "A". Figure 8. Distribution of microorganisms* with particle size distribution for Profile "B". SILT o 200 «» OCR for page 164
Properties of Desert Soils 173 .M.T» CLAY SILT -—cuw Figure 9. Distribution of microorganisms* with particle size distribution for Profile "C". io 120 140 160 IBO ZOO 22O 240 No OF MICROORGANISMS IK)9) .lAEROBES AND ANAEROBES AND FUNGI AND ALGAE) of the soil environment on the microflora. However, information can be obtained as to the numbers of kinds of microorganisms that actually do exist in any sample of soil environment. Figures 10 and 11, for example, show the distribution of aerobes and anaerobes with depth of soil in two desert soil ecosystems. An examination of bacterial isolates from these and other desert soils has shown the predominance of soil diptheroids and filamentous motile spore-forming bacilli [Bollen]. Coccoid bacteria are seldom found in virgin desert soils. It has been found that populations of different bacteria in desert soils are affected differently when the environmental conditions are varied. Desert soil bacteria develop at different rates when the available moisture and organic matter are increased or decreased, the concentrations and rates of gases change, and temperatures are varied. An immediate determination of the numbers of kinds of microorganisms at the sampling site does not usually duplicate results obtained at a later time in the laboratory. Tables 2, 3, and 4 indicate summaries of microflora for 34 processed air-dry desert soils from six sample sites. Distribution of desert soil microflora is not always directly correlated with organic matter content of soils as shown by a comparison of Figures 4, 10, and 11. Available moisture is evidently the most crucial factor in most desert soils for stimulating metabolism, repro- duction and growth of microorganisms. Desert soil algae, for example, have been observed to become active within 5 to 15 minutes after rain- storms or following the wetting of dry soil crusts in the laboratory. After wetting desert soil crusts that have been kept continuously desiccated for

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174 RECOGNITION OF LIFE AND SOME TERRESTRIAL PRECEDENTS PROFILE-A - PBOF1LE-B - PROFILE-C - PROF1LE-0 PROFUE-E - PROFILE-F - Figure 10. Distribution of anaerobes with soil depth. KXi I5O 200 290 MO ANAEROBES X I03/GM SOIL Figure 11. Distribution of aerobes: Bacteria + actinomycetes with soil depth. TOO 39OO • AEHOBE5 BACTtfU » ACTMOMVCETES X 5 to 10 years, new abundant growth of oscillatorioid blue-green algae is macroscopically evident in less than 12 hours. This phenomenon may be analogous to color changes observed on the Martian surface following the recession and assumed melting of polar icecaps. Of further interest for life detection experimentation is the variability of

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176 RECOGNITION OF LIFE AND SOME TERRESTRIAL PRECEDENTS || C r~ « o «n •-« oo t~ « 00 O VI ^ U-l M m >n r** CN vo oo ^ oo n oo ri O r-J — < O m Tt 1 •o c a •-i « fn >n l^* NO «M a 0 1 £2 cndooS^^So\ ^ O oo t~ oo O t-; Tf t~ V£3 O in r-^ fvI ON •- 1 J§ < x — « n — < — — .1 n n n n n n n O O O O O O © « « eo eo e o » eo eo eo » w O O O O O O o o o o O C rs w o 3 '5 ac/5 X X X X X X X X X X X X ^ X X X X X X j, C/5 &? r- >n O o , ON r- ^ ^ r~ O O 1 m f**J oo ^- oj o\ '"^ rj o f> oo O o , rt TJ- r- oo O — ~- 01 t^ ' i/I ci « n O O o\ oo O O O — ° O °- O 0 o « \o O o O X X •-l O \O «-^ 1-| Sum of Microflora X KWgSoil \o o ri t^" c^ oo ^^ 10 ^^ ^^ ] O ON oo r~ i r<) Tj- OCR for page 164
Properties of Desert Soils 177 r4 r4 oo oo in OCR for page 164
178 RECOGNITION OF LIFE AND SOME TERRESTRIAL PRECEDENTS TABLE 3. Summary of Distribution of Desert Soil Microflora with Depth of Soil for Six Soil Profiles Depth of SoU Standard Deviation Standard Error Minimum Maximum Average Aerobes X l0VgSoil 0-1 in. 21 3900 945 1380 521 1-6 in. 42 2700 572 1050 429 1 ft. 29 587 250 215 87.9 2 ft. 19 440 147 155 58.5 3 ft. 8.3 225 79.5 86 35.1 4 ft.' 55 62 58.5 4.9 3.5 Anaerobes X l0Vg Soil 0-1 in. 1.9 397 98.4 145 54.7 1-6 in. 0.1 136 39.9 52.6 21.5 1 ft. 0.1 95 34.4 40.0 16.3 2 ft. 0.3 150 31.8 53.1 20.0 3 ft. 0.1 47 18.2 18.5 7.5 4 ft. 0.1 11.4 5.8 8.0 5.7 Facultative Anaerobes X 103/g Soil' 0-1 in. 1 100 17.4 36.7 13.9 1-6 in. 1 100 23.5 37.6 15.4 1 ft. 1 100 23.5 37.6 15.4 2 ft. 1 100 20.3 35.4 13.4 3 ft 1 100 22 38.4 15.7 4 ft. 1 100 50.5 70.0 49.5 Fungi per g Soil 0-1 in. 95 19,750 3695 7,167 2709 1-6 in. 17 7,833 2852 3,558 1453 1ft. 3 600 344 220 90 2 ft. 0 7,566 1229 279 1057 3 ft. 0 28,500 5052 11,494 4692 4 ft. 133 1,800 966 1,178 833 Algae X 108/g Soil* 0-1 in. 0.1 100 30.4 47.6 18.0 1-6 in. 0.1 10 3.7 4.9 2.0 1 ft. 0 1 0.34 5.01 2.0 2 ft. 0 0.1 2.9 .005 .002 3 ft. 0 0 0 0 0 4 ft. 0 0 0 0 0 ' Only two soils represented b Growth at highest dilution

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Properties of Desert Soils 179 I2 .o CJ f') *O ^^ VI II m 2 \C> « ^- O ON ^^ T-J- ^ UH ^ O ' OCR for page 164
180 RECOGNITION OF LIFE AND SOME TERRESTRIAL PRECEDENTS o H 8 o .-a O C/5 3 e j= eo «-. .. e i4-« O 1) .S3 (5 I U H CO H « Is o '' o - CO + 1 1 + +I 1 + +I I O VI >/•> O O 0 0 >n o — m O ^ \o OO 00 OO OO OO + 1 + 1 1 O n O o "- I 1+ | + VO O rn r-; t~; \e> pj -^ co .. i4-t 1 b ' """ CO 00 0\ O — i OCR for page 164
Properties of Desert Soils 181 Figure 14. Loose organic matter accumulation on the leeward side of a blowing sand dune, Yuma Desert, Calif. soil below the one-foot level, with corresponding changes in the soil microflora. The variability of microorganisms in a given surface area of a soil of similar properties at a single time of sampling is indicated in Table 6. These values are shown for 12 separate 10 gram samples of soil obtained within a plot 24 feet on each side. There is comparatively little varia- bility, except in the case of the soil algae. Sampling from the same area over a period of several successive years and at different seasons has shown a number of changes in the relative abundance of the microflora, and certain species either disappear or become more numerous. Specialized microorganisms are sometimes the most predominant micro- flora in some desert soils rather than common groups. The Valley of 10,000 Smokes Desert, for example, is an area in which there are com- paratively few common soil microflora. Numbers of these common micro- organisms in the surface one inch of soil are shown in Table 7. Sample site locations for three of these soils are indicated in the following photos: (a) soil 115, Figure 15, (b) soil 118, Figure 14, and (c) soil 119, Figure 16. Water was not a limiting factor in this case, although sulfur fumes and a unique mineralogy with unusual ratios or absence of certain elements were evident. Soil algae were usually the most abundant group of soil microflora present on barren "soil" within the valley, whereas microaero- philic bacteria were the most predominant group present in the periphery of the valley, near vegetation and in soil of greater development. Microenvironments in soil ecosystems are extremely important: One microenvironment evident in some desert soils consists of accumulations of organic matter and populations of microorganisms in surface soil crusts, Figures 17, 18, and 19. These crusts contain greater accumulations of organic matter and greater numbers of heterogeneous populations of soil

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182 RECOGNITION OF LIFE AND SOME TERRESTRIAL PRECEDENTS E o ;« Du '§•5 opC/5 o .e S 3 •« § X) > .2 c M I O & •=§ .& 115" S « = Sen * « &» r§is o o z ^s ~ O\ t> o ^= + + I + + I g . -^ o CQ ^^ 2 t + + + + I I + + I + + + OCR for page 164
Properties of Desert Soils 183 TABLE 7. Microbiological Determinations for Valley of 10,000 Smokes Soils Microorganisms Per Gram of Soil Soil No. Aerobcs Facultative Anaerobes Anaerobes Fungi Algae 115 4700 1,000 12,500 1900 100,000 118 190 1,000 0 0 10,000 119 0 10 5 0 0 122 5200 10,000 15 15 100 Figure 15. Close-up of wind-swept pebbly pumice pavement, Valley of 10,000 Smokes, Alaska. Figure 16. Close-up of loose mixture of un- sorted, cohesionless, unstable pum- ice and ash on steep bank of eroded fumerole, Valley of 10,000 Smokes, Alaska.

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184 RECOGNITION OF LIFE AND SOME TERRESTRIAL PRECEDENTS Figure 17. Loose gravelly surface (white gran- ules) with irregular, stable, cohesive patches of algal-lichen soil crusts (dark areas), Colorado Desert, Thermal, Calif. Figure 18. Thin, fragile, dusty, eroded surface | soil crusts between dead, wind- I blasted vegetation and encroaching sand dune, Yuma Desert, Calif. . Figure 19. Thin, coherent desert soil crusts over sand, Figure 13, contain ac- cumulations of organic matter and microorganisms.

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Properties of Desert Soils 185 Figure 20. Rocky desert pavement, Upland Desert. Arizona microorganisms than the surrounding soil. This accumulation is especially noticeable under the immediate surface of the crusts when it is separated from the underlying soil [Fletcher and Martin, 1948]. Moisture is also conserved in these crusts for a greater time period than in the surround- ing area of surface soil, as shown in the moist dark areas of Figure 17. Translucent rocks, sometimes forming desert pavement, Figures 20 and 17, modify solar radiation and function as unique microenvironments in the desert. Portions of these stones extending beneath the soil surface to depths of one or two inches are covered by distinctive macroscopic ad- herent accumulations. This accumulation also serves as a moisture-con- serving mechanism of organic matter and microorganisms [Drouet, 1959; Vogel, 1955]. Certain other microenvironments in desert soils can also easily enhance or limit the numbers of kinds of microorganisms distributed Figure 21. Hard, cohesive clay surface en- crusted with evaporated salt, Mecca Hills, Colorado Desert, Calif.

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186 RECOGNITION OF LIFE AND SOME TERRESTRIAL PRECEDENTS Figure 22. Hard, dense salt banks and flats surrounded by rugged mountains, Death Valley National Monument, Calif. in the soil ecosystem. Hard salt crusts occurring at the air-soil interface can be detrimental, Figure 21. However, in some situations, Figure 22, the opposite is true. In the latter case, a humid greenhouse effect has been provided on a salt bank. Underneath the thin, upraised, translucent salt crusts, soil algae were found to be macroscopically conspicuous. REFERENCES Bollen, W. B. Microorganism Study on Culture and Identification of Desert Soil Bacteria. JPL Contract #950783, Oregon State University. Cameron, R. E. (1962), Soil Studies—Microflora of Desert Regions. JPL Space Programs Summary No. 37-15, pp. 12-20. Cameron, R. E. (1963), The Role of Soil Science in Space Exploration. Space Science Reviews 2:297-312. Cameron, R. E. and G. B. Blank (1963), Soil Organic Matter. JPL Tech. Rep. No. 32-443. Cameron, R. E., F. A. Morelli, and G. B. Blank (1965), Desert Soil Charac- teristics. I. Preliminary'Studies. In preparation. Drouet, F. (1959), Distribution of Algae on the A. E. C. Nevada Test Site, 1958. In: A Botanical Study of Nuclear Effects at the Nevada Test Site, pp. 97- 101. New Mexico Highlands Univ., Las Vegas, N. M. Fletcher, J. E. and W. P. Martin (1948), Some Effects of Algae and Molds in the Rain Crusts of Desert Soils. Ecology 29:95-100. Jenny, H. (1941), Factors of Soil Formation. McGraw-Hill Book Co., New York. Killian, C. and D. Feher (1939), Recherches sur la microbiologie des sols desertiques. Encycl. Biol. 21:1-127. Radwan, M. K. (1956), A Field and Laboratory Study of the Soils of the Tahreer Province of Egypt. Unpublished M. S. Thesis, Univ. of Ariz., Tucson. Richards, L. A., ed. (1954), Diagnosis and Improvement of Saline and Alkali Soils, Govt. Printing Office, Washington, D. C. Vogel, S. (1955), Niedere "Fensterpflanzen" in der siidafrikanischen Wiiste. Beitrage zur Biologie der Pflanzen 57:46-135.