Some Chemical and Physical Properties of the Agricultural Soils of Guam

J.L. Demeterio, D.D. Ventura, and F.J. Young

Technical Report

AES Publication # 56

Agricultural Experiment Station College of Agriculture and Life Sciences

University of Guam Mangilao, Guam 96923

ABSTRACT

Plow depth samples of the agricultural soils of Guam were

collected from pedons used for Soil Taxonomy purposes. The soils were

grouped according to pH. Routine soil test and total chemical

analysis were performed on the samples. Soils of volcanic origin

tended to be acid in nature. Soils on top of limestone had pH above

7. Soils from volcanic and argillaceous limestone had pH ranging from

slightly acid to basic. Phosphorus levels were low in acid soils.

Soils in general appeared to be fertile although phosphorus maybe

limiting. As organic matter levels decrease due to cultivation, an

increase in nitrogen inputs would be necessary. Current soil

fertility status should be determined using soil testing before

earnestly embarking on agriculture.

Introduction

The chemical and physical make-up of a soil to a large extent

determines the inherent fertility of the soil. Literature on the

soils of Guam (Stensland and Tracey, 1959; Park, 1979; Carroll and

Hathaway, 1963; Cope et. al. 1985 and Young, in press) have been

pedologic in nature. This report endeavors to describe the soils of

Guam in the edaphological sense. Hopefully the data presented herein

could serve as the benchmark in the soil fertility improvement efforts

of the island.

The use of soil testing (Walsh and Beaton, 1973 and Davidescu and

Davidescu, 1982) allows a quick and precise evaluation of the current

fertility status of a soil. Soil test values have been used as the

main basis for fertilizer recommendations and also predict the yield

levels attainable. The value of a soil test in a management scheme is

enhanced by the amount of research done to facilitate soil test

interpretations. Guam subscribes to the use of the principles of soil

testing in soil fertility work.

Materials and Methods

Most bf the samples taken were from pedons in 1983 examined for

the 9th International Forum on Soil Taxonomy and Agrotechnology

Transfer. Other samples (Pulantat, Kagman, Shioya, and Inarajan) were

collected by Mr. Fred Young and Mr. Greg Yamanaka of the Soil

Conservation Service. Sampling depth was ] fmited to depth of the

A-horizon or the plow depth or 0 to 25 tm. ,, Efforts were made to

sample from areas which have not been cultivated. The soils were

air-dried, pulverized, allowed to pass a 2 mm sieve, and stored in

air-tight plastic bottles.

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The following chemical analyses were done; pH, organic matter, P,

K, Ca, Mg, Na, Fe, Mn, Zn, Cu, and texture. Both exchangeable and

total analyses were done. The pH test was conducted on a 1:1

soil/water paste, P by Olsens method, the bases by extraction using

normal pH7 ammonium acetate, the micr·o-elements by extraction using

.2N HCl. Organic matter analysis was done with the Walkley Black

method. Cation exchange capacity measurements were done with the

ammonium acetate (Normal pH7) method. All cations were analyzed using

atomic absorption spectroscopy. For total analysis, the method

suggested by Jackson (1958) using Na2co

3 fusion was used. Color

development in the ascorbic acid procedure for phosphorus was measured

using a Bausch and Lomb Spec 20. Texture was done with the hydrometer

method of Bouyoucous.

Discussion

Soil Series

Seventeen different soil series have been mapped and correla~ed

on Guam (Langan, 1985). Of these, 13 series are used for agricultural

production. The classification of the agricultural soils by Soil

Taxonomy (Soil Survey Staff, 1975) is listed in Table 1. Many of the

series have clay-enriched subsoil horizons; these are classified as

Alfisols or Ultisols. Other soils have no horizon development, either

because they have formed in recently deposited materials or because

they are very shallow. These series are classified as Entisols. One

series (Yigo) has a very highly weathered subsoil with an extremely

low Cation Exchange Capacity (CEC), and is classified as an Oxisol.

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\ ~ I

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The taxonomic classification is not a fertility classification,

and is in fact designed so that fertility management practices will

not change the classification of a soil. Nevertheless, the taxonomic

classification of a soil provides many clues as to the probable

fertility status of the soil and the management practices nec~ssary

for farming the soil. For example, the Akina soil is an Ultisol. The

subsoils of Ultisols are low in bases relative to their CEC, a

condition called low "base saturation". The implication for fertility

management is that liming may be necessary for adequate crop growth.

Akina soils are in the Humult suborder, indicating relatively high

amounts of organic matter, as corroborated in this paper. The

Tropohumults great group denotes a year long growing season, and the

Ustoxic subgroup denotes a seasonal moisture deficit and low CEC

subsoil, all properties related to agricultural management. The

family classification provides information on the subsoil texture,

mineralogy and soil temperature. From a fertility viewpoint, these

features are indications of possible fertilizer losses through

leaching (texture) or fixation (mineralogy), and the degree of

biological decomposition and subsequent nitrogen mineralization

(temperature) in the soil. Detailed soil profile descriptions and the

data used for taxonomic classification can be found in Young (in

press).

Extent of agricultural soils

The soil series is a concept; the actual occurrence of soils on

the landscape is recorded on soil maps as >mapping units. Mapping

units may be dominantly composed of soils that fit the concept of one

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soil series, or they may be composed of 2 or more soil series. For

example, many of the deep, agriculturally important soil series on

Guam occur in mapping unit complexes with shallow soils. A given soil

series may be part of

important, as well as

some mapping units that are agriculturally

some mapping· units that are unsuited for

agriculture. For example, Akina soils that are in mapping units with

steep slopes cannot be farmed, whereas Akina soils on gentle slopes

provide good farmland.

Table 2 presents the hectarage and proportional extent of the

mapping units on Guam that are suited to commercial farming. For the

purposes of this paper, some slope groups have been combined. Mapping

units with slopes in excess of 15% are excluded as being too steep for

most commercial farming. The totals show that over 40% of the island

is at least theoretically suitable for commercial agriculture. Much

of this hectarage is precluded from agricultural development by such

factors as remote or otherwise undesirable location and conflicting

land uses. However, with only about 200 hectares in commercial

production in 1983 (Khamoui, 1984), there appears to be plenty of room

for agricultural expansion.

Guam cobbly clay loam soils dominate the broad northern plateau

and are by far the most extensive agricultural soils on Guam (Table

2). These very shallow soils have severe limitations which farmers

have overcome with the use of drip irrigation. Guam soils also occur

in complex with the deep Saipan and Yigo soils, whtch provide

excellent farmland but are of very limited extent. Pulantat clay,

along with the associated Chacha and Kagman clays, provide tmportant

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agricultural lands on the rolling argillaceous limestone landscape o~

central Guam and the southeast coast. The deep Inarajan soils of the

southern valleys and coastal plains are farmed in the dry season, but

are subject to flooding and high water tables in the rainy season.

Most of the vast southern volcanic uplands are too steep to farm, but

there are small, gently sloping plateaus and valleys that can be

utilized, as well as the extensive Dandan basin. Togcha and Akina

soils are dominant on these landscapes, along with the Atate,

Sasalaguan and seasonally wet Ylig soils. Shioya loamy sand soils are

formed in narrow coastal strandline deposits. Although neither

extensive nor extensively farmed, these soils are productive if

managed properly. Detailed distribution of the mapping units is shown

on the maps in Young (in press).

Grouping soil series by pH

As an aid to learning and understanding the relationships between

the many soil series, pedologists have grouped the soils into three

categories based on geology, landform, and ·parent material (Stensland,

1959; Park, unpublished; Young, in press). These categories are: 1)

soils over limestone, 2) soils on volcanic uplands, and 3) soils on

bottomlands and coastal strands. The limestone soils can be furthur

subdivided into soils on argillaceous limestone and soils over pure

limestone. However, these groupings do not represent a classification

system as they are not mutually exclusive (Young, in Cope et. al.,

1984).

There is a need to group soil series from a fertility standpoint

as well. This paper uses soil pH to group the series into three

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categories: 1) strongly acid; pH < 5. 5, 2) slightly acid to neutral;

pH 5.5 to 7.0, and 3) neutral to alkaline; pH ) 7.0. This grouping

will facilitate laboratory analysis of samples for which the soil

series is not verified. Table 3 relates the pH groupings of the

series to the landscape groupings of .. the pedologists. The strongly

acid soils group and the volcanic soils group are identical, and

comprise the Akina, Atate, Sasalaguan and Togcha series. The higher

pH groupings include all of the pedologists. The strongly acid soils

group and the volcanic soils group are identical, and comprise the

Akina, Atate, Sasalaguan and Togcha series. The higher pH groupings

include all of the limestone and bottomland soils, but no clear

separations between the limestone types is apparent. The pH grouping

distinguisites between bottomland soils that are heavily influenced by

acid volcanic uplands (Ylig series) and series that are influenced by

limestone and coastal sediments (Inarajan and Shioya).

Sampling depth, texture and CEC

The sampling depth, texture, and cation exchange capacity

(milliequivalents per 100 g soil) of representative samples grouped

according to soil reaction is shown on Table 3. The depth is usually

the measured A horizon, however, in cases where a thin A exists, the

plow depth (0-20 em) sample is taken. All soils of Guam are clay

except for the Guam series which is clay loam and Shioya which is

sandy loam. The cation exchange capacity of the soils of Guam ranges

from 4.8 meq (Shioya soils) to 50.6 meq (Ylig). Except for the sandy

Shioya, the CEC levels are considered moderat,e and represent a good

source of plant nutrients.

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Routine soil tests conducted in the laboratory include pH,

organic matter, and extractable P and K (Table 4). These parameters

are essentially the basls for fertilizer recommendations (Cope, et.

al., 1985). Akina, Atate, Sasalaguan, and Togcha are strongly acid

soils. Studies are needed to determine the response of various

vegetables crops to addition of lime on these soils. At present,

watermelon and pineapple are grown without lime on these soils.

Soils with low pH (Akina, Atate, Sasalaguan, Togcha, and Ylig)

have extremely low phosphorus levels (0. 5-1.2 ppm.). Soils with high

levels of Ca (Chacha, Pulantat, Yigo, Guam, Inarajan, Kagman and

Saipan) have P levels ranging from 5. 6 to 20.2 ppm. which in some

cases are adequate. The total P represents the bound and fixed

portions of soil P. Potassium levels below 75 ppm are considered low.

Akina, Pulantat, Guam, Kagman and Shioya may need added K for optimal

yield. Other soils which showed high levels of K include A tate,

Sasalaguan, Togcha, Chacha, Yigo, Saipan, and Ylig.

Extractable Na, Ca, and Mg including total Ca and Mg are shown in

Table 5. These bases are routinely measured but are not used in

determining routine fertilizer recommendations. Na is taken into

consideration when CEC is measured by sum of bases plus Al and H in

acid soils. Na ranges from 73 to 804 ppm. No poor soil structure in

Guam soil has been attributed to sodium. The calcium and magnesium

content of all soils of Guam are considered adequate.

Although the soil pH of Guam, Inarajan, Kagman and Saipan are in

excess of 7, no micro-element deficiencies,;have been observed. Perhaps

the high levels of organic matter (5.4-10.51%) adequately supply the

micro-element needs.

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The micro-elements, Zn, Fe, Mn, and Cu, are shown on Table 6.

There has been no report of micro-element deficiency or toxicity in

Guam. Phosphorus has been known to induce zinc deficiency. Frequent

addition of farm manure and maintaining adequate levels of organic

matter could insure minimal problems with micro-elements. Levels of

extractable Fe and Mn in acid soils are not excessive.

Conclusion

The general fertility of the agricultural soils of Guam are good.

With adequate irrigation year-round agriculture could be possible in

Guam. The nutrient status, except for P, is adequate. Nitrogen which

must be continuously applied could best be satisfied with ammonium

phosphate (18-46-0).

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Table 1. Classification of the Soils of the Territory of Guam (from Langan, 1985)

Series Classification

Akina Clayey, oxidic, isohypertherrnic Ustoxic Tropohumults

Atate Very fine, oxidic, isohyperthermic Oxic Haplustalfs

Chacha Very fine, kaolinitic, isohyperthermic Aquic Oxic Haplustalfs

Guam Clayey, gibbsitic, nonacid, isohyperthermic Lithic Ustorthents

Inarajan Very fine, mixed, nonacid, isohyperthermic Aerie Tropic Fluvaquents

Kagman Very fine, kaolinitic, isohyperthermic Oxic Paleustalfs

Pulantat Clayey, montmorillonitic, isohyperthermic Shallow Udic Haplustalfs

Saipan Fine, oxidic, isohyperthermic Oxic Paleustalfs

Sasalaguan Very fine, montmorillonitic, isohyperthermic Vertic Haplustalfs

Shioya carbonatic, isohyperthermic Typic Ustipsamments

Togcha Very fine, oxidic, isohyperthermic Ultic Paleustalfs

Yigo Clayey, gibbsitic, isohyperthermic Tropeptic Eutrustox

Ylig Fine, mixed, acid, isohyperthermic Aquic Tropofluvents

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Table 2. Hectarage and proportional extent of the soil mapping units with agricultural potential on Guam (from Young, in press)

Series and % Slope hectare % of island

Akina clay, 3-15 777 1.4

Akina-Attate complex, 0-15 383 0.7

Chacha clay, 0-5 331 0.6

Guam cobbly clay loam, 3-15 13,162 24.0

Guam-Saipan complex, 0-7 341 0.6

Guam-Yigo complex, 0-7 493 0.9

Inarajan clay, 0-4 1,595 2.9

Pulantat clay, 0-15 1,447 2.6

Pulantat-Chacha, 0-15 288 0.5

Pulantat-Kagman complex, 0-15 797 1.4

Sasalaguan clay, 7-15 333 0.6

Shioya loamy sand, 0-5 546 1.0

Togcha-Akina clays, 3-15 1,134 2.0

Togcha-Ylig complex, 3-15 475 0.8

Ylig-clay, 0-7 1,029 1.9

Total 23,131 41.7

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Table 3. Landscape position sampling depth, texture, and cation exchange capacity (meq per 100g) of the agricultural soils of Guam grouped according to soil ·reaction

Soil S«-Y"ies Landscape Depth, em % clay % silt % sand Texture CEC

( EH5.5

A kina volcanic 5-30 69 23 8 -clay 21.6 Atate volcanic 0-30 69 19 12 clay 17.8 Sasalaguan volcanic 0-14 73 15 12 clay 40.0 Togcha volcanic 0-12 63 29 8 clay 25.5

:eHS.S-7.0

Chacha argillaceous* 0-14 81 15 4 clay 24.0 Pulantat argillaceous* 0-15 69 20 11 clay 31.1 Yigo pure* 0-17 73 16 u clay 18.3 Ylig bottomland 0-15 59 25 17 clay 50.6

) :eH7 .0

Guam pure* 0-10 33 36 20 clay loam 22.0 Inarajan bottomland 0-15 61 25 14 clay 37.7 Kagman argillaceous* 0-15 70 16 14 clay 20.0 Saipan argillaceous* 0-15 77 18 5 clay 17.7 Shioya strandline 0-15 15 9 76 Sandy loam 4.8

*refers to limestone type

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Table 4. Routine soil test levels and total chemical analysis in ppm of the agricultural soils of Guam grouped according to soil reaction

p K

E.!! % o.m. Extractable Total Extractable Total --

< pH5.50

Akina 4.60 4.60 0.9 535 35 680 A tate 5.45 4.92 0.7 273 150 3972 Sasalaguan 5.25 5.40 1.1 310 240 3027 Togcha 5.05 5.20 1.2 618 335 2406

pH5.50-7.00

Chacha 5.85 7.78 14.9 4502 240 1125 Pulantat 6.73 12.53 16.] 4178 68 2875 Yigo 6.97 7.46 10.0 3969 135 470 Ylig 6.87 9.90 0.5 520 1100 6000

) pH7.0

Guam 7.55 9.53 20.2 3987 50 634 Inarajan 7.65 10.51 5.6 488 140 5375 Kagman 7.60 8.02 7.0 4349 44 2325 Saipan 7.20 5.40 14.6 3863 135 1112 Shioya 8.10 4.98 13.4 1663 40 1827

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Table 5. Extractable and total calcium and magnesium and available sodium in parts per million of the agricultural soil of Guam grouped according to soil reaction

Ca Mg

Extractable Na Extractable Total Extractable Total -- --pH5.5

Akin a 86 200 871 280 2992 Atate 86 1200 2778 340 4722 Sasalaguan 294 2000 3068 4000 15346 Togcha 114 1200 2957 1740 7419

pH5.5-7.0

Chacha 98 4000 6500 400 3150 Pulantat 147 5939 11500 519 3438 Yigo 73 5200 12017 260 1254 Ylig 804 3900 6625 4125 17188

pH7 .0

Guam 75 9200 151599 240 2095 Inarajan 145 11520 25000 750 10812 Kagman 133 6375 15625 106 2875 Saipan 76 4400 8475 140 3019 Shioya 92 4000 343750 265 8469

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Table 6. Extractable and total Zn, Fe, Mn, and Cu in parts per million of the agricultural soils of Guam grouped according to soil reaction

Zn Fe Mn Cu

Extractable Total Extractable Total Extractable Total Extractable Total -- --< pHS.S

Akina n.d.* 76 50 139924 4 408 9 242 Atate 1 189 38 96953 106 5295 13 185 Sasalaguan 3 156 72 75020 13 558 13 186 Togcha 7 140 117 90234 119 1831 21 179

EHS.S-7.0

Chacha 5 170 22 86840 125 2000 14 167 Pulantat 11 120 21 76820 163 2156 5 167 Yigo 8 63 13 148328 750 5486 3 116 Ylig 8 150 351 75985 128 1484 26 250

) EH7.0

Guam n.d. 77 3 59840 1 2205 3 122 Inarajan 7 115 902 8767 456 2140 29 167 Kagman 8 120 26 84335 150 3547 7 167 Saipan 9 212 25 100837 163 4817 22 235 Shioya 1 50 4 6596 13 344 1 111

*n.d. = not detectable

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References

CARROLL, D. and J.C. HATHAWAY. 1963. Mineralogy of Selected Soils from Guam. Geological Survey Professional paper 403-F. U.S. Government Printing Office.

COPE, J.T., J.L. DEMETERIO, and J.A. CRUZ. 1985. Fertilizing Crops on Guam. AES Bulletin ~ in press.

DAVIDESCU, D. and V. DAVIDESCU. 1982. Evaluation of Fertility by Plant and Soil Analysis. Abacus Press, England. 560 p.

JACKSON, M.L. 1958. Soil Chemical Analysis. Prentice-Hall, Inc., Englewood Cliffs, NJ. 498 p. + xiv.

KHAMOUI, Thao (Compiler). 1984. Guam Agricultural and Related Statistics. Agricultural Experiment Station, College of Agriculture and Life Sciences, University of Guam, Mangilao, Guam.

LANGAN, L.N. 1985. Classification and Correlation o'f the Soils of the Territory of Guam. Soil Conservation Service, USDA, West National Technical Center, Portland, Oregon.

PARK, M.E. 1979. Soil Survey of Guam (unpublished). Department of Commerce, Government of Guam. 243 p.

SOIL SURVEY STAFF. 1975. SOIL TAXONOMY -A Basic System of Soil Classification for Making and Interpreting Soil Surveys. Agriculture Handbook No. 436. Soil Conservation Service. U.S. Department of Agriculture. 754 p.

STENSLAND, C.H., and J.I. TRACEY. Mariana Islands, Part II, Soils, 117-164.

1959. Military Geology of Guam Engineering Aspect of Geology and

WALSH, L.M. and J.D. BEATON. 1973. Soil Testing and Plant Analysis. Soil Science Society of America, inc., Madison, Wisconsin. 491 p. + xvii.

YOUNG, F.J. (In press). Soil Survey of the Territory of Guam. U.S. Department of Agriculture - Soil Conservation Service, Mangilao, Guam.

YOUNG, F.J. 1985. The Soils of Guam. In Cope, J.T., Fred Young, and J.L. Demeterio (ed.). Proceedings of the IX International Forum on Soil Taxonomy and Agrotechnology Transfer. Mangilao, Guam, September 3-14, 1984. University of Gua~, Mangilao, Guam.

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ACKNOWLEDGEMENT

The authors wish to thank Mr. Patrick E.Q. Perez for his patience

and diligence in typing the manuscript.

Trade names of products are used to simplify the information. No endorsement of named products is intended.

The Guam Agricultural Experiment Station is an equal opportunity employer. All information gained through its research program is available to anyone without regard to race, color, religion,

sex, age, or national origin.

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