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Rx for Soils and Crops

Date Last Edited:  09/13/2001

A Guide To Soil Testing For Hawaii's Residents

N. V. Hue, R. Uchida, and M. C. Ho

Department of Tropical Plant and Soil Sciences
Agricultural Diagnostic Service Center

College of Tropical Agriculture and Human Resources
University of Hawaii at Manoa

Why have a soil tested?

Having a soil tested is an integral part of good farming/gardening because it will let you know if your soil needs any fertilizer, what kind and how much so that your crop can grow better. In this factsheet, we will show you how to obtain a good soil test. Because laboratory analyses are performed on a small sample of soil from an entire field or garden, the analytical results are useless if the sample submitted does not represent the soil you intend to grow your plants in. A properly collected sample makes test results valid, and will produce correct recommendations that enhance yields, make efficient use of resources, and/or preserve quality of the environment.

How to take a representative soil sample

When sampling home gardens, one composite sample consisting of 5-10 subsamples per 100 sq. ft. collected over the planting area should be taken. For larger areas like pastures or tree orchards, first make a detailed map of your area, then divide your map into smaller uniform soil-test areas of a few (1-5) acres each. Label each area clearly on the map by using a combination of letters/numbers that make sense and thus are easy to remember. Each test area should be uniform with regard to soil type or condition. Fields with different slope, soil color, drainage, apparent texture (for example, heavy, light, or A'a) or cropping history should be sampled separately. A soil-test sample for each area should be a composite of 10-15 subsamples.

Soil subsamples consist of 1-inch thick slices of the soil taken to a specified depth, normally 4 inches for no-till fields or established pasture, lawn and turf and 8 inches for conventionally tilled fields. For tree crops (forests, nuts, tree fruits), where possible, collect a surface sample to a depth of 8 inches and a sub-soil sample from 8-24 inches deep. Each sample to be tested should be a thorough mix of the cores taken randomly, say, in a zigzag pattern as shown in Figure 1.

Figure 1. Sampling soil using a zigzag pattern.

Such a sampling technique minimizes the variability that may be present in your field or garden, and allows you to obtain a reasonably representative soil sample.

A specially designed soil probe is often used for collecting soil subsamples (called cores). However, if you do not own a probe, then use a steel or plastic garden spade or shovel to collect soil cores as follows. Dig a hole to the sampling depth, then cut a 1-inch thick slice from the top to bottom of the hole. Take a 1" wide by 1" thich segment of the slice, and place it in a clean mixing bucket, preferably made of plastic.

Plan to collect soil samples two to three months before planting so you will get your test results in plenty of time to plan your liming and fertilization. Depending on workload, the turnaround time at the lab can be up to two-three weeks. Soil in fields or gardens should be tested at least once every two years.

Submitting samples and providing relevant information

After collecting the soil samples, take them to the county extension office in your area or send them directly to the Agricultural Diagnostic Service Center (ADSC) in the College of Tropical Agriculture and Human Resources, University of Hawaii (University of Hawaii, Agricultural Diagnostic Service Center, 1910 East-West Road, Rm 134, Honolulu, HI 96822). To get the most accurate recommendations from your soil test, be sure to fill out a soil information sheet, which is available from your county extension office or the ADSC. A blank soil information sheet is shown in Figure 2.

Figure 2. Soil information sheet used by the ADSC, Univ. of Hawaii.

Agricultural Diagnostic Service Center Department of Agronomy and Soil Science
College of Tropical Agriculture & Human Resources University of Hawaii at Manoa
1910 East-West Rd., Honolulu, HI 968221910 East-West Rd., Honolulu, HI 96822
Phone: 956-6706/7980 Fax: 956-2592 Phone: 956-7530 Fax: 956-6539
email: agrss@hawaii.edu


Name: Address: City:
State: Zip: Phone: Fax:
Client ID: Sample Type: Sample Location:
Agent: Address: City:
State: Zip: Phone: Fax:
Job Control No.: Sample ID: *Serial No.: *Date (m/d/y):
Soil or Mix: Size of area: sq. ft.: Acres:
Soil Depth (inches): Map unit: Series: Apparent density: heavy, light, or Aa
Elevation: Annual rainfall (inches): Drainage Slope(%): Irrigation:
Lime used: Yr.of application: Rate: Type of Lime:
Fertilizer used: Analysis: Rate: Method:
Manure type: Rate: Compost: Rate of compost:
Has rock phosphate been applied? Plant grown: Plant to be grown:
Can you till in fertilizer 4-6 inches if necessary? Bottom land: Rolling:
Describe the problem & observations:

Send copies to:



Objectives for this sample:

Diagnosis only

Complete information will provide you with the best possible recommendation. Fertilizer and lime requirements vary with soils and crops; therefore, the soil's apparent density, namely heavy (most Hawaii's soils), light (volcanic ash-derived soils on the Big Island) or A'a land (irregular pieces of lava), crop to be grown and crop previously grown are among the most important items of information needed to make correct recommendations.

How are soil samples tested?

The ADSC provides all residents of Hawaii with a reasonably priced soil and plant tissue testing service. Routine analyses of soils include acidity/alkalinity (pH), extractable phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg). Soil organic carbon (organic matter), total nitrogen, extractable aluminum (Al), boron (B), and other micronutrients (e.g., zinc, manganese, copper) are measured on request. Detailed descriptions of the analytical procedures are given in the Soil Fertility Manual, 1996, published by the College of Tropical Agriculture and Human Resources, University of Hawaii.

Soil test results and fertilizer recommendations

Within two to three weeks after you submit your samples to the ADSC, you should receive the test results for your soil along with fertilizer recommendations for your garden or field. The results should include at least pH, and levels of P, K, Ca and Mg in ppm (or mg per kg). A brief interpretation of these levels is also provided as either: very low, low, sufficient, high, very high, or extremely high. An example of the form for the analytical results and interpret-ations is shown in the upper half of Figure 3.


Analytical Results / Interpretations

*Soil Lab No: Interpretation Plant Lab No: Interpretation
Soil pH: Nitrogen (%N):
Phosphorus (ppm) MT or OL: Phosphorus (%P):
Potassium (ppm K) Potassium (% K):
Calcium (ppm Ca) Calcium (% Ca):
Magnesium (ppm Mg) Magnesium (% Mg)
Salinity (EC, mmhos/cm) Sulfur (% S)
Zinc (ppm Zn) Iron (ppm Fe)
Manganese (ppm Mn) Manganese (ppm Mn)
Copper (ppm Cu) Zinc (ppm Zn)
Boron (ppm B) Copper (ppm Cu)
Total N (%) Boron (ppm B)
Org. C (%) Molybdenum (ppm Mo)
NH4-N (ppm) Aluminum (ppm Al)
NO3-N (ppm) Nitrate (ppm NO3-N)

Fertilizer Recommendation

Total nutrient requirement (lbs/A): Nitrogen(N): Phosphorus(P): Potassium (K):
Fertilizer/Material Options
Fertilizer Amount
Est. Cost
lbs/1000sq.ft./Crop lbs/1000sq. ft./mo. $/1000sq. ft.
Fertilizer selection
Other Fertilizers

Note: The interpretations are based on the Fact Sheet No. 3 "Adequate Nutrient Levels in Soils and Plants in Hawaii".

Did the recommendation help? To help improve future recommendations, please answer the following questions, photocopy this form and return it to above address (ADSC).

1. Did you need to modify the

recommendations? If so, how?

2. Did your plants improve? Please give unit area yield before and after the recommendation was applied.

Although these results and interpretations are essential to making fertilizer recommendations, they may not be very helpful to you. That's why we also provide fertilizer recommendations for your garden or field as shown in the lower half of Figure 3. These include amounts of lime (in either lbs/1000 sq. ft. or tons/acre), and its estimated cost, fertilizer type (for example, 21-0-0, 21-0-32, 10-30-10), and its amount and cost.

At the bottom of the form, we ask you to give us some feedback in terms of how your crop performed as a result of our fertilizer recommendations. Such information would allow us to fine-tune our recommendations so that we can better serve you in the future.

We hope that with this information you can make your garden or field more productive as well as help protect the quality of our environment.

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