Decreasing Dependence on Man-Made Fertilizers for Crop Production in Tropical Limestone Soils

Final Report for FW02-017

Project Type: Farmer/Rancher
Funds awarded in 2002: $5,200.00
Projected End Date: 12/31/2004
Region: Western
State: Guam
Principal Investigator:
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Project Information


This project provided information on sweet corn growth and yield in Guam’s shallow limestone soils and the impact of sweet corn production on soil nutrient levels.

The first experiment compared the growth of sweet corn with inorganic fertilizer versus inorganic fertilizer plus sunn hemp and chicken manure added to the soil. The sunn hemp green manure crop was tilled into the soil before planting and the chicken manure was added before planting at rates of 1, 1.5, 2 and 3 tons per acre. No significant differences were found in plant height or stem width. However, the incorporation of sunn hemp and chicken manure had a positive effect on the soil, elevating organic matter to 4.4% from 1.4%. The increased soil organic content was attributed to the addition of the sunn hemp, not the chicken manure as the organic matter values for 1, 1.5, 2 and 3 tons of chicken manure were 4.2%, 4.6%, 6.25% and 2.5%, respectively. Results of a soil test at the end of the experiment showed that the chicken manure plus sunn hemp did not increase N, P or K levels in the soil. The average phosphorus level increased 68% across all treatments to 56 mg/Kg from 38, indicating the farmer was applying excessive inorganic fertilizer.

In the second experiment, normal levels of inorganic fertilizer were amended with 1 and 2 tons of chicken manure per acre. Average stem width and plant height were higher in the 2-ton treatment. Total yield in the 1-ton plot was 7,580 pounds per acre compared with 5,820 pounds in the 2-ton plot. However, the 1-ton treatment was harvested twice. The 2-ton treatment was harvested once.

The project had two objectives: 1) provide producers with a cost-effective means of improving the health of the farm’s soil and 2) provide agricultural researchers with information regarding the benefits of adding organic matter to Guam’s shallow limestone soils.

The project failed to provide evidence that productivity of Guam’s limestone soils can be improved by replacing manmade fertilizer with enriched chicken manure and sunn hemp incorporation. However, the failure may have resulted from a compromise of the original plot design and uniform application of inorganic fertilizer across the farm instead of as indicated by soil test. Despite the modification, useful results were obtained.

The experiments were conducted in a 3.5-acre field in the northern village of Dededo. The soil was 6 to 12 inches deep, limestone, Guam Cobbly Clay with a pH range between 7.1 and 7.8. Each treatment consisted of eight 50-foot rows of a 90-foot by 90-foot treatment plot. Four rows were adjacent to each other, and the other four were randomly scattered. Project coordinator Ernie Wusstig followed his standard farm practices, including Diazinon for ant control and herbicides for weed control, but failed to keep reliable records on fertilizer application.

The main benefit of the project was the knowledge gained about growing sweet corn on Guam’s rocky limestone soils. The amount of fertilizer currently used needed to be accurately recorded and scaled back to determine minimum fertilizer levels for maximum production.

Project coordinator Ernie Wusstig is the only farmer on Guam with a substantial acreage in sweet corn. It is anticipated that Wusstig will see the value in keeping accurate fertilizer and yield data as a means of evaluating farming practices that may be beneficial.

Wusstig is an active member of the Northern Soil Conservation District, which often holds meetings at his farm. He is a strong advocate for the use of green and animal manures, and this project has helped reinforce his belief and persuade other growers to do the same.

In the future, it is recommended that growers be given a short course in statistical design so they can appreciate the use of experiment replications, accurate recordkeeping and the need to adhere to experimental design.

The findings of the project were deemed not adequate as a stand-alone publication; other information needs to be obtained and added to what was found.


Participation Summary

Research Outcomes

No research outcomes
Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.