Now more than ever, we are recognizing the importance of an adequate supply of plant nutrients to ensure efficient crop production. Growers are continually striving to overcome nutrient deficiencies and adopt improved management practices, in order to increase their yields and profits. Great progress in fertilizer technology and the use of plant nutrients has been made in recent decades. A wider understanding of plant and soil chemistry has led to improved fertilization and farming practices that have improved crop yields worldwide (Tisdale et al. 1985).
However, over-application of chemical fertilizers may reduce farm profits, create a risk of soil degradation, and cause environmental pollution (Tisdale et al. 1985). Synthetic fertilizers are easy to apply, and we often do not know how to match fertilizer applications to the amount of nutrients needed by certain crops. These have added to the problem.
Among the problems inherent to tropical soils, soil acidity is the most serious (Hue 1992). Acid tropical soils are characterized by a low pH, excessive aluminum and a deficiency of calcium. Tropical soils are often unproductive because they have strong phosphate fixation that renders phosphorous unavailable to plants. Soils that show strong phosphate fixation (adsorption to oxides and clay minerals) often require extremely high applications of phosphate fertilizer in order to alleviate the effect of phosphate fixation.
Soil acidity and mineral deficiencies can be corrected by lime and fertilizers. Unfortunately, lime and fertilizers are not always easy options for small-scale and resource-poor farmers (Hue 1992). However, Hue (1992) reported that green manures and composted organic materials provide plant nutrients, alleviate aluminum toxicity, and render phosphorus more available to crops. This increased availability of phosphorus is probably caused by the reaction of organic matter-derived molecules with soil minerals (Hue 1992).
In our soil program at the University of Guam, USA, we are investigating the use of organic material as an alternative to chemical fertilizers. More specifically, we are studying the effect of organic matter and inorganic soil constituents in improving soil productivity and maintaining environmental quality. Our goal is to develop management strategies which use available resources to improve crop production while conserving resources and preserving environmental quality. The project discussed in this Bulletin is designed to improve soil fertility status by using composted organic wastes, and assessing how the nitrogen and other essential nutrients contribute to long-term soil fertility and crop productivity without the application of chemical fertilizers.
Issues in Soil and Plant Testing in the Pacific Region
Suitable soil and plant testing is the key to environmentally safe production practices that both avoid excessive use of fertilizer and detect any nutrient deficiencies which may reduce the yield. Establishing suitable soil testing procedures involves appropriate steps in the field, such as proper sampling techniques, and laboratory procedures such as matching the best analytical method for each nutrient (Motavalli 1997).
There are many challenges that need to be faced before an effective soil and plant testing and analysis program can be put in place for producers in the Pacific Islands. Some of these challenges include the following:
- Growers and extension agents are not convinced of the need or value of soil and plant testing.
- Quarantine regulations for soil samples and plant tissue can hinder growers who wish to send samples to a regional soil and plant testing laboratory.
- Sometimes soil and plant analysis take so long that the results are not useful for growers.
- Communication and cooperation among institutions responsible for agricultural extension and those doing research are limited.
- Frequent typhoons interrupt the capability of laboratories to maintain a dependable and timely soil and plant analysis service.
- Growers and extension agents are not convinced of the value of organic matter. Hence evaluation of organic matter content is not integrated into evaluations of soil fertility.
However, despite these difficulties, there are several major advantages in maintaining a soil and plant testing laboratory.
- Excessive use of chemical fertilizers can be avoided, thereby reducing environmental pollution.
- Farmers can be warned if they are applying insufficient fertilizer
- Timely testing helps in management decisions on when and how much fertilizer to apply for optimizum plant growth.
- The effects of alternative management practices that use organic fertilizer can be observed over time.
- Profits can be increased by reducing unnecessary fertilizer applications.
Unique Characteristics of Soils of the Pacific Region and Their Management
In a hot, humid tropical environment, weathering of soils is rapid. Thus, large areas of Ultisols and Oxisols occur in Pacific regions (Motavalli 1997). The inherently poor chemical properties of Ultisols and Oxisols pose problems for agriculture. The fertility of these soils is often limited by their high iron and aluminum contents, their low-activity clays, and their low organic matter content. These limitations are often reflected in the following properties:
- High phosphate-fixing capacity.
- Low pH and high exchangeable A1.
- Low CEC and low base saturation, further complicated by the zero or net positive charges brought about by variable-charge Fe and Al Oxide/Hydroxide colloids.
In humid tropical climates, the loss of basic cations and the related acidity and aluminum toxicity require special management techniques. High rainfall and temperatures also promote the rapid decomposition of organic matter. This may also release H* ions that acidify the soil and increase exchangeable Al to toxic levels, thus limiting the growth of roots in the subsoil.
In this regard, deep soil sampling is the key to detecting excessive Al ions in the soil. Often Al ions exist as free agents at or below the root zone. A plant may not be affected until the roots reach below the root zone where the free Al ions enter the plant root. At this point plants may die as the result of Al toxicity. There have been cases where crop plants died from Al toxicity and yet no Al was detected from soil testing. This was mainly because the samples were taken at or near the soil surface (Golabi, personal observation).
Soil-test results are worthless, and may even be misleading, unless the samples tested accurately represent the area and the depth to be tested (Cook and Ellis 1987). Shallow soil samples taken on or near the soil surface may not be able to pick up the excessive Al ions that may be present below the plow layer. It is therefore recommended that people taking a soil sample should use a 30 cm long sampling augur, or dig a V-shaped wedge in the soil down to the bottom of the plow layer. The depth of the plow layer is usually 15-20 cm. Thus, 10 to 15 samples from 15-20 cm should be taken throughout the field. These samples should then all be combined to make a composite sample, and an appropriate amount of this soil used for testing.
The soils in the Pacific Islands need improved farming practices that increase the levels of exchangeable calcium (Ca) and magnesium (Mg), and reduce the level of Al saturation in the subsoil. This is the only way to reduce plant damage resulting from Al toxicity, and also improve root proliferation and the exploitation of available soil water and nutrients. If productivity is to be maintained, an agricultural system able to preserve a satisfactory physical condition in the soil must also be developed. A general problem with soils in almost all tropical regions, including the soils of Guam, is the deterioration of the soil's physical condition. This degradation can take many forms, and has a variety of consequences including low fertility status due to poor soil quality (Lal and Pierce 1991).
One of the most important nutrients limiting yields in the tropics is nitrogen (N). The concentration of native available N in tropical soils fluctuates considerably in response to seasonal changes (Sanches 1976). Fertilizer recommendations and the timetable for cropping need to take into account this seasonal availability of N. Nyamangara et al. (2003) reported that nitrogen uptake from organic waste material (animal manure) was greater in the second cropping season than in the first. This implies that the N from the organic fertilizer became more available (through mineralization) for plant uptake in the second year (Nyamangara et al. 2003).
In addition to the nutrient value, the application of aerobically composted organic waste (manure) from the smallholder farming areas of Zimbabwe to soil did not pose any environmental problems from nitrate leaching, presumably because of the high degree of stabilization that occurs during the decomposition of the organic wastes (Nyamangara et al. 2003). The report indicated that the application of organic wastes (composted manure) enhanced the use efficiency of mineral N fertilizer by crops when the two were applied in combination.
Productivity and Organic Matter Content of Soils in the Pacific Islands
In the hot, humid environment of the tropical Pacific Islands, soil has a very low level of organic matter due to rapid decomposition. In addition to its slow release of nutrients, organic matter is largely responsible for aggregation, soil moisture holding capacity and other improved physical properties of the soil. Thus, increasing the soil organic matter content must be the first step in any farming practice in the Pacific region. Fuller (1951) has even stated that the continued productivity of soil depends largely upon the replenishment and maintenance of the soil organic constituents. Applying organic matter is the only way of making some soils economically productive (Cook and Ellis 1987).
A living soil is one which contains active microflora. The by-products of organism activity, mucilaginous in nature, stabilize soil aggregates and thus greatly improve moisture and air relationships. The carbon dioxide given off by the organisms and the production of certain organic acids increase the capacity of the soil solution to dissolve minerals and release the potassium, phosphorus, calcium, magnesium and other minerals needed by higher plants (Cook and Ellis 1987).
The phenomenon of cation exchange has been said to rank next to photosynthesis in importance to agriculture (Cook and Ellis 1987). Well-decomposed organic matter has a very high cation-exchange capacity that adds to the buffer capacity of the soil. Organic matter has the ability to hold leaching substances other than catoins. Hence, a good supply of soil organic matter makes it safe to apply rather large applications of fertilizer at planting time and thus avoid the need for a second application (Cook and Ellis 1987).
Organic matter serves as a very important source of plant nutrients. Micronutrients may be satisfactorily supplied by decomposing organic matter. This is especially true during the production of crops that have specific micronutrient needs. For instance, it may be necessary to supply boron in the fertilizer for alfalfa on a boron-deficient soil, because the need of alfalfa for this element is quite high. A corn crop, on the other hand, does not have special boron needs and is easily injured by the direct application of borax. Therefore, corn planted after an alfalfa crop can take enough boron from the decomposing alfalfa residues or from the nutrient-rich composted organic material (Cook and Ellis 1987). Other micronutrients may also be furnished by decomposing organic matter (Cook and Ellis 1987).
Compost: Source of Organic Matter for Soils of the Pacific Region
Among the practices recommended for improving the soil quality and soil fertility in tropical regions is the application of composted organic wastes. These slowly release significant amounts of nitrogen and phosphorus (Muse 1993, Zibilske 1987, Eghball 2001). In addition to supplying plant nutrients, organic compost has been shown to increase the level of soil organic matter, enhance root development, improve the germination rate of seeds, and increase the water-holding capacity of soil (Muse 1993, Zibilske 1987). Applied organic materials promote biological activity in the soil, as well as a favorable nutrient exchange capacity, water balance, organic matter content and soil structure (Muse 1993, Zibilske 1987).
Frequently, the regular use of organic material (compost) is essential for the sustainable cropping of upland soils with inherent low natural fertility (Schoningh and Wichmann 1990). The management of soil organic matter is the key to sustainable agriculture in such areas. As far as possible, a farmer should "feed" the soil organisms for maximum activity, which means frequent additions of easily decomposed organic matter (Cook and Ellis 1987).
Compost does several things to benefit the soil that chemical fertilizer cannot do. First, it adds organic matter, which improves the way in which water interacts with the soil. In sandy soils, compost acts as a sponge to help retain water in the soil that would otherwise drain down below the reach of plant roots. In this way, compost helps to protect the plant against drought.
In clay soils, compost helps to add porosity to the soil. This helps the soil to drain more easily, so that it does not stay waterlogged, and does not dry out into a bricklike substance.
Compost also inoculates the soil with vast numbers of beneficial microbes (bacteria, fungi etc.) and the habitat that the microbes need to live. These microbes are able to extract nutrients from the mineral part of the soil and eventually pass the nutrients on to plants (Johnson 1996).
Furthermore, properly processed compost reduces the incidence of soilborne diseases without any use of chemical control (Rynk et al. 1992) Minnich and Hunt 1979). The disease suppressing quality of compost is just beginning to be widely recognized. Farm fields treated with compost are also less prone to erosion. In short, high-quality compost will generally do more for soil fertility and soil quality than chemical fertilizer.
The use of composted organic waste as a fertilizer and soil amendment not only brings economic benefits to small-scale farmers, but also reduces pollution because of reduced nutrient run-off, and N leaching (Nyamangara 2003). Most subsistence and small-scale farmers will be able to adopt composting technology if they are introduced to it by extension programs or field demonstrations. Once they have accepted the technology, they will be much more effective in convincing their fellow farmers to accept the technology than government employees could be.
In our case study at the research station of the University of Guam, we used primary and secondary solid waste from households, hotels and restaurants, tree trimmings from the roadsides, chicken, hog and horse manure from local farmers and ranchers, and wood chips from typhoon debris for compost. We applied the composted organic matter, not only as a soil amendment, but also as a source of fertilizer, to enhance the nutrient status of the low-fertility soils of southern Guam. The results of the initial data indicated that under the climatic conditions of Guam, land application of organic compost enhanced soil quality and increased soil fertility and crop yield.
The following Tables represent the average values per treatment of the results conducted so far. Considerable improvements in soil quality indices such as bulk density, organic matter content, and nutrient distribution occurred after the application of compost ( Table 1(516)).
There was also a gradual increase in crop yield as the compost application rate was increased from 0 mt/ha acre (control) to 20 mt/ha ( Table 2(526)).
The data indicates that the application of compost significantly enhanced soil fertility. If farmers continue to apply compost before planting, the soil quality should be further enhanced and the yields should continue to rise over time.
Incorporating Compost Applicatoins into a Soil Diagnostic Program
The use of plant and soil analysis can easily be incorporated into a farming system using organic fertilizer. Such a system can be characterized as low-input (in terms of chemical applications). Developing it into a sustainable farming system that combines science with best management practices requires research and extension projects to monitor the fertility of the soils to which compost is applied as the main source of fertilizer. This in turn will rely on the use of demonstration plots, and cooperation with tropical growers.
Compost can also be applied to forest lands to improve tree establishment and growth. Rainbow and Wilson (2002) showed that after the application of green compost, the mortality of tree seedlings in the first year was remarkably low (1%) despite the absence of irrigation on a dry, exposed site. One year after the application of green compost and the establishment of tree seedlings, analysis of the topsoil showed a marked improvement in key properties. These exposed surface soils could at last be fairly described as topsoil.
Currently, there is no accurate method which farmers can use to monitor the nutrients available in composted material. Trial and error is often used to determine the fertility status of the soil. As for the application rate, extensive research is needed to establish accurate application rates to maintain adequate amounts of essential nutrients for different crop species.
Jackson et al. (2003), incorporated compost into their soil fertility program, and reported that the use of compost increased soil quality in the irrigated, intensive production of lettuce and broccoli in the Salinas Valley, California. The application of compost increased soil microbial biomass, increased total soil carbon and nitrogen, reduced surface bulk density and decreased the potential for groundwater pollution that would otherwise result when was nitrate leached below the root zone after the application of chemical fertilizer. Compost application not only increased the yield, but occasionally resulted in fewer weeds and a lower incidence of lettuce corky root disease (Jackson et al. 2003). Also, the use of cover crops, and compost combined produced high vegetable yields and acceptable net economic returns over a 2-year period (Jackson et al. 2003).
Advantages and Disadvantages of Compost Applications
In using composted organic material as part of an effective soil management program, the advantages, disadvantages, and feasibility of compost need to be considered. Composting has benefits and drawbacks, and producers need to decide if it is a good option for their particular farm.
Composting improves the handling characteristics of manure and other organic waste, by reducing the volume and weight. The other advantges of composting include the destruction of weed seeds, pathogens, and fly larva if the compost reaches a sufficiently high temperature (above 70oC).
Livestock manure and other organic wastes may have a high water content, and may need to be dried. Alternatively, bulking agents with a high carbon dontent can be included for proper composting. Other points to consider are whether farmers have spare land to use for a compost heap, the costs and returns, and any environmental problems (e.g. bad odors).
Composting can be good option for small-scale farms in the Pacific islands, and will help them become more sustainable. The use of plant and soil analysis can easily be incorporated into an agricultural system that uses composed organic material as the main source of soil nutrients. However, small-scale farmers need to be informed about the economic incentives using for from composted organic material as a soil amendment. Soil and plant analysis laboratories at the University of Guam, include the testing and analysis of organic waste in their diagnostic programs, in order to promote low-input agriculture systems that use composted organic material as the main source of soil fertility. Some of the unfavorable soil properties, such as low phosphate fixating capacity or aluminum toxicity, that are common in Guam and the other islands of the Pacific, may be corrected by the application of organic materials.
Preliminary findings from our research clearly indicate that productivity can be improved by the proper use of organic materials. The environment also benefits through the reuse of organic wastes that otherwise would be discarded in landfill or burned.
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Index of Images
Table 1 Effect of Different Rates of Applied Compost on Soil Characteristics
Table 2 Effect of Different Rates of Compost Application on Corn Yield
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