Guam Soil Test Summary, 1984-1993

Peter P. Motavalli, Jose A. Cruz, and Rosenilda Y. Marasigan

1996 Soil and Plant Testing Laboratory

Agricultural Experiment Station College of Agriculture and Life Sciences

University of Guam • Mangilao, Guam 96923 U.S.A.

Jeff D.T. Barcinas, Interim Dean

GUAM SOIL TEST SUMMARY, 1984-1993

Peter P. Motavalll, Jose A. Cruz, and Rosenllda Y. Maraslgan

Soil and Plant Testing Laboratocy Agricultural Experiment Station

College of Agriculture and Life Sciences University of Guam

Mangilao, Guam 96923 U.S.A.

Introduction

The Soil· and Plant Testing Laboratory of the University of Guam's College of Agricul­

ture and Life Sciences has been offering soil analytical services to the Guam commu­

nity for approximately 20 years. The current objectives of this laboratory are:

1. 1b provide appropriate soil, plant and environmental analyses and fertilizer and lime recommendations within Guam and the region of Micronesia.

2. 1b develop information for the public on proper sampling methods, the role of plant nutrients in agriculture, and the need for sampling and appropriate management prac­tices to improve nutrient efficiency and reduce harmful effects to the environment.

3. 1b provide the public with unbiased information on the nutrient content and potential effectiveness of soil amendments available on Guam.

4. To assist in the analysis of quarantined materials and in advising the Guam Depart­ment of Agriculture whether such materials should be allowed to enter into Guam.

5. To keep records of the results of soil and plant analyses for research purposes. These results will be periodically summarized and made available to the public.

6. 1b provide guidance to private and public soil, plant and environmental testing labora­tories as to approporiate soil and plant testing methods and recommendations.

Establishing suitable soil testing procedures involves several steps including (1) se­

lecting an extractant and analytical method for a given nutrient; (2) correlating the

amount extracted with the amount taken up by the plant and; (3) calibrating the test

value with plant yield or any other important growth characteristic (Corey, 1987).

Fertilizer recommendations are based on the calibration information and fertilizer

response curves. Proper sampling methods, sample preparation and analytical tech­

niques are also critical for the success of any soil testing program. Soil test results

should be accurate, have a high degree of precision and be returned to the client in a

timely manner. Development and improvement of these soil testing components on

Guam are an on-going process and will depend on the maintenance of active research

and extension programs.

Soil testing promotes the efficient utilization of nutrients for optimal plant growth

and productivity while minimizing environmental pollution. Soil test results indicate

which and how much fertilizer or other soil amendments may be necessary for a given

crop or plant. In this way, a grower can save on the cost of overfertilization and also

decrease the risk of environmental pollution.

This publication summarizes the soil test results submitted to the Soil and Plant Test­

ing Laboratory from 1984 to 1993. It supplements a previous report which summa­

rized soil test results from 1975 to 1984 (Demeterio et al., 1986). This report also

provides information on the performance of the Laboratory and the use of the Labora­

tory by the general public and the research community.

Performance of the Laboratory

From 1984 to 1993, the Soil and Plant Testing Laboratory has analyzed a total of2926

soil samples. Figure 1 shows the relative number of samples analyzed each year. The

lowest number of samples analyzed was 133 in 1988 and the highest was 444 in 1984.

Fluctuations in the number of samples analyzed partly reflects problems the Labora­

tory has encountered in maintaining public confidence in the performance ofthe Labo­

ratory and in demonstrating the need for soil testing on Guam. The amount of time

the Laboratory takes for completing soil analysis of farmer samples has averaged

approximately 15 days every year, except for 1990 and 1991 when response time was

much higher (Fig. 2). This increase in analysis time during those years was possibly

a result of a decrease in the number of Laboratory personnel. In general, most farm­

ers request the routine analysis offered by the Laboratory, which includes analysis of

soil pH, organic matter, extractable phosphorus (P), exchangeable potassium (K), cal­

cium (Ca), magnesium (Mg) and sodium (Na). Analyses for zinc (Zn), iron (Fe), man­

ganese (Mn) and copper (Cu) were previously also part of the routine soil analysis.

Currently, these elements are analyzed by special request. Special tests for texture

analysis and total nitrogen (N) are also offered by the Laboratory.

Sources of Soil Samples

Over 88% of all soil samples analyzed by the Laboratory from 1984 to 1993 originated

from Guam (Fig. 3). Farmer samples made up 32.5% of total samples. Other sources

of samples included researchers (42.6%), off-island samples (11.5%), golf courses (9.7%),

landscapers and nurseries (2.3%), government agencies (0.8%) and private comp~nies

(0.1 %). Off-island samples came from several locations in Micronesia including Saipan

(25.6%), Pohnpei (21.9%), Kosrae (12.5%) and Yap (11.0%) (Fig. 4). The larger number

of samples coming from these islands may be a result of the activities of extension

personnel located at land-grant universities and of research projects originating from

the University of Guam or from the Soil Conservation Service.

Analysis of the geographic origin of soil samples submitted by farmers on Guam indi­

cates a relatively larger representation of samples from Northern Guam (Fig. 5 and

6A). In Northern Guam, the Dededo district has the largest number of submitted

samples. Yona in the Central region and Talofofo in the South also have a relatively

higher proportion of submitted samples. The geographic origin on Guam of over 27%

of all submitted farmer samples was not indicated on the submission sheet. In con­

trast to samples originating from farmers, samples from golf courses came from a

limited number of districts (Fig. 6B). The geographic distribution of submitted soil

samples may be an indication of greater agricultural activity in a given district and

also possibly a greater lack of awareness of the availability or value of soil testing in

certain districts.

Soil Sample Analyses

Distribution of Farmer Samples: Figures 7-12 show the distribution of Guam farmer

soil test results for soil organic matter, pH, P, K, Ca, Mg, Na, Fe, Zn, Mn and Cu. The

number (n) of samples evaluated, the average (avg) and the standard deviation for

each of the soil tests are also provided. The highest proportion of farmer soil samples

tested 4-6% in soil organic, 7.0-8.0 in soil pH, 0-10 mg/kg P, 40-80 mg/kg K, 3600-4800

mg/kg Ca, 0-280 mg/kg Mg, 0-15 mg/kg Fe, 0-10 mglkg Zn, 75-100 mg/kg Na, 0-50 mg/

kg Mn, and 0-2 mg/kg Cu. These values suggest that a large proportion of farmers'

fields may be experiencing deficiencies in P, K, Zn, and Mn. The large proportion of

farmer soil samples with a relatively high pH also indicates that problems related to

high pH such as low P, Fe, Zn, Mn, and Cu availability may also be common. The

range in soil test values were generally high as indicated by the large standard devia­

tions for each soil test relative to the mean value. This large variability indicates the

wide range of soil test encountered on Guam.

Average Soil 7est Results by Source and By Geographic District on G"f:'am: Table 1 gives the average soil test results by source of sample for samples originating on Guam. These results show few striking differences in average soil test results among the major sources of samples. Although golf courses, landscapers and nurser­ies require high fertilizer maintenance, soil samples originating from this source were not on average higher than other sources, except for slightly higher P levels.

Table 1. Average soil test analysis by source on Guam.

Source

Farmers Golf courses Landscapers Research Schools Government Agencies

Organic pH Matter

% 6.8 6.4 6.8 4.2 6.7 7.8 7.2 7.0 6.6 4.2 6.0 6.2

Soil test results for farmer samples by geographic district indicate some trends by

region (Table 2). Soils submitted from northern districts generally had higher pH,

organic matter and P content and lower K, Mg, Fe and Cu than soils submitted from

the southern districts. Unusually low or high soil test recorded for districts such as

Umatac may be a result of the relatively small number of samples received from these districts.

Table 2. Average soil test analysis for farmers by District on Guam.

Chemical Analysis

Organic District pH Matter p K Ca Mg Na Zn Fe Mn Cu

-%- mg/kg soil North

Yigo 7.0 8.2 26.6 105 4819 406 93 65 52 198 7 Dededo 7.2 8.4 27.3 76 4913 175 100 23 19 233 2 Tamuning 7.1 6.4 32.0 182 5441 448 354 48 34 126 4 Barrigada 6.8 6.6 20.5 144 4582 271 103 40 37 200 7 Mangilao 7.0 6.0 17.7 180 30509 447 109 18 47 225 9 Average 7.0 7.1 24.8 137 10053 349 152 39 38 196 6

Central Mongmong 7.3 5.9 31.0 111 3654 111 AganaHts 6.6 4.7 26.3 118 3776 252 81 2 50 28 5 As an 7.2 5.5 21.0 149 4782 482 95 15 29 164 6 Piti 7.1 12.1 2.2 123 10150 185 Yona 6.0 6.3 6.1 155 3832 500 118 14 74 121 12 Sinajana 6.9 6.2 8.1 206 5569 893 96 18 104 123 6 Chalan Pago 6.8 5.4 5.3 385 7329 1531 273 11 187 121 9 Agana 7.5 5.3 13.3 83 5378 235 85 140 22 213 7 Average 6.9 6.4 14.2 166 5559 649 130 33 78 128 8

South Santa Rita 6.5 4.9 4.1 227 6680 1669 168 22 282 193 21 Agat 6.7 4.3 7.4 378 6399 1081 178 82 233 213 18 Talofofo 6.1 4.8 11.0 184 2870 575 210 13 148 119 11 Inarajan 6.4 4.2 6.4 367 4583 1839 315 7 378 216 13 Merizo 6.2 3.8 7.6 495 5818 1779 206 39 170 140 13 Umatac 7.0 1.4 1.3 1200 3357 3300 4129 4 568 184 6 Average 6.5 3.9 6.3 445 4951 1707 868 28 296 178 14

Not Known 7.0 7.3 35.3 128 6886 550 142 20 47 117 7

Conclusions

1. The Soil and Plant Testing Laboratory has performed an important service to the Guam agricultural community for approximately 20 years. However, the . Laboratory needs to take steps to improve public confidence in the performance of the Laboratory and in increasing public awareness of the need for soil test­mg.

2. The Laboratory provides services to several sectors on Guam and Micronesia including farmers, landscapers, nurseries, golf courses, government agencies, schools, private companies and agricultural researchers. Each sector has spe­cial testing needs which may need to be further studied and included in the services provided by the Laboratory. Development of fertilizer recommenda­tions should also be a top priority.

3. Special efforts need to be made to increase farmer participation in regular soil testing from all regions of Guam, perhaps through special public educational campaigns, extension programs or public media.

4. The relatively high proportion of farmer samples testing deficient in P, K, Zn, and Mn suggests the need for further research on nutrient management op­tions for farmers on Guam. Informational programs, specifically targeted on managing deficiencies in these nutrients for various crops, may also be required.

5. Regional differences in soil test results reflect the general differences in soil properties between the Northern and Southern regions of Guam. However, the large variability in soil test results encountered on Guam indicates that re­gional management recommendations should possibly be avoided. Further re­search is required to develop a better understanding of the chemical and physi­cal properties of Guam's soil resource.

6. Records of soil test results can assist in evaluating the performance of the Soil and Plant Testing Laboratory and in understanding potential soil problems en­countered on Guam. It is recommended that these records continue to be main­tained and that further information should be asked of people who submit soil samples. This information would include questions regarding previous nutri­ent management of the field, crops grown and yield estimates.

References

Corey, R.B. 1987. Soil test procedures: Correlation. p. 15-22. In J.R. Brown (ed.) Soil testing: Sampling, correlation, calibration, and interpretation. Soil Sci. Soc. Am. Spe- . cial Publ. No. 21. Madison, Wisconsin.

Demeterio, J.L., F.J. Young and M.B. Yamanaka. 1986. Guam soil test summary. 1975-84. Agricultural Experiment Station Publication #58. College of Agriculture and Life Sciences, University of Guam, Mangilao, Guam.

Figure 1. Number of soils analyzed by year. 500~------------------------------------~

.I 400 ~ a. E • • -·-a 3oo ~ .. CD .a E :1 200 ~ z

444

378 391

332 . 349

281

214 192

133

100 1984 1985 1986 1987 1988 1989 1990 19911992 1993

Year

Figure 2. Average time of analysis for farmers samples.

40~------------------------------------~

-~30 • ~ -.!! ~ .. 20 c • CD E I= 10

o~~~~~~~~~~~~~~~~~~~~

84 85 86 87 88 89 90 91 92 93 Year

Figure 3. Source of soil samples. Landscapers and nurseries 2.3%

Schools 0.6%,

Golf courses 9. 7°AJ

Government agencies 0.8% Private companies 0.1% ~ ~"""

Research 42.6%

Farmers 32.5%

Figure 4. Source of off-island soil samples.

Pohnpei 21.9%

Rota 6.0°AJ

~~~ Palau 4.3°A, Majuro 3.6%

Marshall lsi. 1 • 1 %

12.5%

American Samoa 1.4o/o .· .. ·. , .. ,,,,,., ,,,,:=======::::::==:::;;:;:='i'\if ,:\=::,::=,::::=;;::,::;::;::::: Other 2. 7%

Yap 11.0°/o

Truk 8.3% Tinian 1.6°/o

Figure 5. Map of Guam showing district and . reg1onal boundaries.

Dededo ··· ..... ······ .....

... • 0 •• ........ ~ . .. ...... .

.... ·· Mongmong· Toto

'---"_.... Barrigada f Agana Heights~---' ...... ········

Asan ~~-·~;·· .... \·~~~:ij~·~. 1.::··ch;ian Pago

Piti \ ..... :·:.:~Prdot ,._......... ............... . ........ . .. ···· ·· ....... :.:..·-~

South

Umatac

Yona

\.~anta Rita · .. ·. ·······... ·~----t .. . ··, ............ :

....... ...- Talofofo ·· ........ ··

·····

..... ~--··

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

······· ......... .

Central

North

-Figure 6. Proportion of soil samples by district and region for A) farmers and B) golf courses.

A.

B.

0 5

Vigo~r~~ DedecJo I!Zo

Tamuning Barrigada Mangilao

Mongmong Agana ~_ghti

Asan Piti Von'a~~~~~

Sin@jana Chalan Pago

Agana

North

Centnll

South

Not known

10

0 5 10 16 20 25 30 36 Percent of soft samples

15 20 25 Percent of soil samples

Golf courses (n = 285)

North

Central

South

Not known

0 10 20 30 40 50 Percent of soil samples Santa Rita

A gat Talofofo ~~~~~~~~~iZ2Si~~~~~SZSZi~~ lnarajan Menzo

Umatac

NotKnown~~~~~22~~~

0 10 20 30 40 Percent of soil samples

30

50

Figure 7. Distribution of A) soil organic matter and B) soil pH in Guam farmer soil samples. ·

30~-----------------------------------------A.

- 25~ -I .... 20-0

n = 712 avg = 6.4% std = 5.5%

0 XXXJI

0-2 2-4 4-6 6-8 8-10 10-12 12-14 14-16 16-18 18-20 >20

Soil organic matter (0/o)

60

B.

--~

n - 716 -50

~ .s40 0

~ avg - 6.8 -- std 0.9 --~ !.. 30 ~ ~ c: ! 20 c:r

~

! u. 10 -

KXXXXXJ

~ lXXX XXXI I KXXXXX

0 3.0-4.0 4.0-5.0 5.0-6.0 6.0-7.0 7.0-8.0 8.0-9.0 > 9.0

Soil pH

Figure 8. Distribution of A) extractable P and B) exchangeable K in Guam farmer soil samples.

70~------~~--------------------------~~

60-

--i 50-.. c

0 40 1-~ - . ~30-c • :s r 20-

10 1-

0

A.

n = 700 avg = 21.2 mg/kg std = 91.9 mg/kg

XXXJII I

0·10 10·20 20-30 30-40 40-50 50-60 60-70 70-80 80.90 90·100 > 100

Extractable P (mg/kg soil)

40~----------------------------------------

35 1-B.

-'i 30 ... s c 25-

n = 722 avg = 182 mg/kg std = 251 mg/kg

5-

0 0-40 40-80 80-120 120-110 110-200 2oo.240 240-280 280-320 320-3H 310-400 >400

Exchangeable K (mg/kg soil)

Figure 9. Distribution of A) exchangeable Ca and B) exchangeable Mg in Guam farmer soil samples.

25

_20 1 s '& 15 fl. -> ~ 10 • ::s cr !

L&. 5

~

" ~

1-

-1-

-

A. n = 725 avg = 6453 mg/kg std = 27924 mg/kg

~

IX1CIXJI

Exchangeable Ca (mg/kg soil)

60--------------------------------------~ B.

_40 r- n = 579 -.I avg = 664 mg/kg s '& 30 - std = 832 mg/kg '# -> ~ 20 r-

• :I cr !

L&. 10 r-

0 ...... ..,20 11Z0.1400 , ... ,.10 , .... ,... 1110-2240 2240-2520 21Z0.2100 > 2100

ICXXX f'XXX1 NN K:XXX ~ ::ll :la

Exchangeable Mg (mg/kg soil)

Figure 10. Distribution of A) extractable Fe and B) extractable Zn in Guam farmer soil samples.

40~----------------------------------------~

35 f-

0

70

60

--.e 50 s 0 ':R.40 0 -~30 c ., ::. c:r 20 !

&&.

10

0

. -

t-

1-

t-

1-

1-

t-

t-

A. n = 499 avg = 102 mg/kg std = 445 mg/kg

0.15 15-30 30-45 ~ 10-71 71-10 IG-101 1CII-120 1»131 13So150 >110

Extractable Fe (mglkg soil)

B .

n = 493 avg = 27 mg/kg std = 89 mg/kg

>

~ I I I I

0-10 10-20 20-30 30-40 40-60 5040 60-70 70-80 80-90 90-100 > 100

Extractable Zn (mg/kg soil)

Figure 11 . Distribution of A) exchangeable Na and B) extractable Mn in Guam farmer soil samples. 40

-A.

35-

! 30 .... s 0 25 r-

::t. ~ 20-> u ; 15 ~ = cr ! 10-u.

n = 705 avg = 169 mg/kg std = 488 mg/kg

I-

5LmB_Hm~~ 0 I KXK~ ~~X~

~ZI .. 10-71 71-100 1~121 121-1110 110-171 171-ZOO ~221 221-210 >210

Exchengeable Na (mg/kg soil)

40~--------------------------------------~ B.

35 ~

0 o.IO 10-100

n = 499 avg = 167 mg/kg std = 208 mg/kg

~~ ~ KXXXJ

Extractable Mn ( mg/kg soil)

Figure 12. Distribution of extractable Cu in Guam farmer soil samples.

40

35 -

- n = 498 -30 -! avg = 8 mg/kg 0 25 1-

std = 12 mg/kg :R ~20 1-

> u i 15 ,_

= c:r ! 10

&&. P-

0 0-2 2-4 4-6 6-8 8-10 10-12 12-14 14-18 16-18 18-20 >20

Extractable Cu (mg/kg soil)