Using Organic Fertilizers

REN-SHIH CHONG, 2005-12-01

8.1 Introduction

Sustainability of agriculture has become a major global concern since the 1980s. Soil organic matter is very important in the functions of soil inasmuch as it is a good indicator of soil quality because it mediates many of the chemical, physical, and biological processes controlling the capacity of a soil to perform successfully. A comparison of cultivated and uncultivated soils has demonstrated a reduction in soil organic matter with cultivation (Mann 1986). Soil organic matter properties (e.g., C:N ratio and macroorganic matter) have been proposed as diagnostic criteria for soil health and performance. However, the importance of organic matter to crop production receives less emphasis, and its proper use in soil management is sometimes neglected or even forgotten. Moreover, understanding nutrient supply or agricultural systems is essential for maintaining long-term productivity.

Yields of crops grown in organic and conventional production systems can be the same (Drinkwater et al. 1995; Stamatoados et al. 1999). However, agriculture or agroindustries produce high quantities of organic wastes that are typically rich in nutrients, which can well be used in agriculture to conserve nutrients as well as reduce waste discharge and the use of chemical fertilizers.

8.2 Beneficial Effects of Organic Fertilizer

Incorporating moderate amounts of animal manure and other organic materials into the field is an established agricultural practice generally recognized to have beneficial effects on the soil's physical, chemical, and microbiological properties. For example, the use of properly composted organic soil amendments has been associated with desirable soil properties. These properties include greater plant water-holding and cation exchange capacity, lower bulk density of soils, and inducer of beneficial microorganisms (Lin et al. 1973; Parr et al. 1986; Chao et al. 1996).

One of the reasons for the unsustainability of cultivated soils is the decline in soil organic matter content. Adequate amount of soil organic matter also greatly reduces the difficulties of good crop production (Figs. 1-4) (Allison 1978). Therefore, restoring and maintaining a high soil organic matter content is the principal strategy for attaining economic progress and improving environmental quality. Increases in soil biomass, biological abundance, and diversity are directly related to increased levels of organic matter and good management practices, which, in turn, positively influence soil structure, nutrient cycling and availability, buffering capacity, and pest and disease control in cultivation systems.

There is also a close relationship between the nutrient status of soils and the organic matter content. Researches have shown that under long-term treatments, adding farmyard manure has raised soil fertility and yields to levels greater than those under synthetic fertilizer treatments. In addition to directly supplying nutrients from the mineralization of organic matter, the mechanisms of higher availability of nutrients with soil of higher organic matter contents are multiple. Parsa and Wallace (1979) showed that both dog manure and sewage sludge at lower rates were very effective in correcting the Fe deficiency of sorghum in calcareous soil by significantly increasing the dry matter yield and the uptake of Fe, Zn, Cu, and Mn. Benefits of compost amendments to soil also include pH stabilization and faster water infiltration rate due to enhanced soil aggregation (Stamatoados et al. 1999). Soils applied with compost initially had a lower soil pH than those applied with synthetic fertilizers, but over time soil pH increased to higher levels in soils with compost than those with synthetic fertilizers (Figs. 5 & 6) (Bulluck et al. 2002). The levels of mycorrhizal colonization were greater under organic treatments than under the conventional. Organic matter increased the available phosphorus in the soil through the organic anion, preventing P fixation and replacing the P bound to the soil (Swenson et al. 1949; Nagarajak et al. 1970; Kafkafi et al. 1998).

It has been shown that microbial activity and biomass are higher in fields with organic amendments than fields with conventional fertilizers (Drinkwater et al. 1995). Soils with compost application have higher propagule densities of Trichoderma species than soils amended with synthetic fertilizers regardless of their production system history (Bulluck et al. 2002). The supply of organic manure allows the direct uptake by plants of specific chemicals needed for the development of their immune system. Therefore, the application of organic manure also makes a direct contribution to the anti-phytopathogenic potential of soils (Fig. 7). This is particularly important in the case of the fungal damping-off diseases such as Rhizotinin, Fusarium, and Pythium (Lampkin 1990). The most important mechanism is the antagonism of soil microorganisms toward each other, which may take the form of producing toxins and antibiotics, competing for nutrients and energy, and/or parasitism (Lampkin 1990).

The buildup of soil organic matter and maintenance of a protective surface cover under organic and minimum tillage systems favor a reduction in soil loss and its associated problems.

8.3 Adverse Effects of Organic Fertilizer

Some of the organic matter can be added to soil without any risk while others can produce toxic and depressant effects on plants and the microbial community. Among the possible negative effects of compost application to cropland is the potential environmental release of toxic heavy metals into the environment, as mentioned in earlier chapters, and the transfer of these elements from the soil into the food chain (Petrzzelli et al. 1989). Therefore, a prime concern in the use of organic matter is the presence of heavy metals in the amendments. Industrial and municipal wastes used as soil amendments in agricultural lands may result in undesirable levels of heavy metals. More than 10% of fruit and vegetable samples like carrots, potatoes, silverbeet, and safflower monitored in Victoria, Australia, in 1992-1993 contained cadmium residues in excess of maximum permitted concentrations (Conacher and Conacher 1998).

Kirchmann and Tengsved (1991) showed that compounds such as phenol, cresol, and dibutyl phthalate were present in significant concentrations in pig slurry and high levels of phenol, cresol, nonylphenol, and di_2-ethylhexyl phthalae (DEPH) were found in sewage sludge.

Nitrate pollution of surface water and groundwater, especially potable water, is an increasingly serious environmental problem. Although soluble N fertilizers are considered major culprits, compost is a significant contributor to this problem. In the long term, any waste disposal system may not be sustainable because of the harmful accumulation of ions in the soil and/or groundwater.

8.4 Fundamentals of Organic Fertilizer Application

Just like chemical fertilizers, adequate organic fertilization programs supply the amount of plant nutrients needed to maximize crop production and net return. Essentially, fertilization management makes certain that soil fertility is not a limiting factor in crop production. The major factors that affect the selection of the kind, rate, and placement of organic fertilizers are fertilizer characteristics, crop characteristics, soil characteristics and management, fertilizer placement, and carryover effects.

Manure application in excess of crop needs can cause a significant buildup of P, N, other ions, and salts in the soil. Dormar and Chang (1995) showed that cattle feedlot manure application for 20 years resulted in a significant increase in soil P levels (from

9 mg/kg to 1,200 mg/kg). The research of Hao et al. (2003) showed that the high application rate of manure resulted in considerable nitrate N accumulation, reaching 80-100 mg/kg.

An ideal agricultural soil can contain as much as 5% organic matter by weight. However, an increase in soil organic matter content increases the risks of nutrient losses to the environment. Organic matter losses can be as great as 1.5% annually (Raymond 1986). Darwish et al. (1995) showed that at least 95% of total applied annual manure over 15 years was degraded. However, soil organic content can still be increased and maintained by the application of compost. Insufficient knowledge of specific fertilizer values and inadequate application rates can result in under or over use (Conacher and Conacher 1998). The ultimate purpose of applying organic fertilizer is to establish the suitable soil organic matter content. High initial applications to build up the organic pool and cut back in subsequent years would be appropriate.

8.5 Characteristics of Organic Fertilizer

The requirement for an organic manure to supply inorganic N synchronously with crop demand is in conflict with the requirement for a soil conditioner to provide persistent, stable organic matter. It is important to quantify the effect of compost on 1) the nutritional and 2) the conditioning value of organic materials, to enable one function to be maximized over the other, or a balance to be found between the two functions, according to the requirements of the situation (Robertson and Morgan 1995).

Composting is a biochemical process converting various components in organic wastes into relatively stable humus-like substances that can be used as a soil amendment or organic fertilizer (Jeong and Kim 2001). It helps improve the physical and chemical properties of the waste and reduce its phytotoxicity (Marchain et al. 1991). Composting is also considered one of the most suitable ways of disposing of unpleasant wastes and of increasing the amount of organic matter that can be used to restore and preserve the environment (Stentiford 1987). The finished compost was rated as "stable" with minimum impact on soil C and N dynamics. A good compost should be tolerated readily by growing crops and should not interfere with root growth and development in the way which fresh manure can do. Composted organic materials, therefore, can act as slow-release sources of plant-available N. Therefore, mature compost is the first choice (Fig. 8).

Nutrient contents can vary widely according to manure type (Titiloye et al. 1985) or compost materials (Tables 1-4). Organic matters added to soils contain a wide range of C compounds that vary in rates of decomposition. The biological breakdown of the added organic matter depends on the rate of degradation on each of the C-containing materials present in the sample (Reddy et al. 1980). Ajwa and Tabataba (1994) showed that the amount of CO2-C releases increased rapidly initially, but the pattern differed among the organic materials used. Gilbertson et al. (1979) showed that the annual mineralization rate of organic N in animal manure was positively correlated with the N content of waste. Variation in environmental factors, however, may cause a change in the decomposition rates of organic materials in soils. Of these factors, moisture content, temperature, soil pH, aeration and soil structure and texture, agricultural practices (e.g., cultivation), substrate specificity, and available minerals have been reported to be most important (Broadbent et al. 1964; Kowalenko et al. 1978; Clark and Gilmour 1983).

Most of the N found in a composting mixture is organic, principally as part of the structure of proteins and simple peptides. The proportion of added organic matter that is mineralized after compost application ranks from several up to a hundred percent, depending on experimental conditions and compost types. Hadas and Pornoy (1994) reported that the mineralization constant for composted manure was commonly 5% to 10% per year. Bitzer and Sims (1988) found that an average of 66% of the organic N in poultry manure was mineralized in the first year. Cabrera et al. (1994) confirmed this rapid mineralization from poultry manure, estimating that 35% to 50% of organic N could be mineralized within 14 days of incorporation into soil. Griffin et al. (2000) reported that the amount of N in manure mineralized in a cropping season varied with the different manures: cattle manure, 25%; dairy manure, 35%; poultry manure, 60%; and swine manure, 50%. Traditionally, manure has been applied to farmlands to increase soil fertility on the basis of crop N requirement. Organic matter applied, therefore, should be calculated based on its mineralization rate. For example, the application of cattle manure is 20,000 kg/ha at a rate of 100 kg N/ha. Compost is a source of fertilizer N in varying degrees. Thus, understanding the factors that control mineralization will make compost more valuable for agricultural and horticultural uses (Sikora and Szmide 2001).

However, owing to the limited soil in a pot culture, the recommended application rate of manure is 3-5% of the soil by weight, depending on the N content of the manure.

Chicken manure is one of the most economically efficient types of manure. This efficiency is due to its high pH, low organic C, high inorganic N, and low C:N ratio compared with the other types of manure. In practice, in order to increase soil fertility on the basis of crop N requirement, organic materials of low C:N ratio such as green manure, seed cake, or poultry manure are the better choice. In this purpose, the optimum C:N ratio for finished compost is about 15:1. On the other hand, when the significant organic C accumulation is more important, the organic manure of higher C:N ratio such as bark compost, compost with a higher proportion of woody materials, or cattle manure would be better.

8.6 Crop Characteristics and Growth Stage

The amounts of nutrients, especially N, required by different crops are different. The amounts of nutrients absorbed by a crop also differ depending on its growth stages, owing to differing biomass accumulation. A synchrony between crop demand and nutrient mineralization is desirable. Therefore, in programming fertilization with organic materials for different crops, the N-uptake dynamic is equally or more important than the total N uptake. This is to say that the timing of nutrient release from manure is important and needs to be known for manure of different quality. For annual crops, manure with sufficient nutrients supplied in the short growth stage to meet the crop requirement is essential. Because N is required during the whole growth period of vegetables, manure with high mineralization rate is superior to that of low mineralization rate.

On the contrary, for perennial crops, the application of manure is quite different from that of annual crops. Generally, the organic matter content of fruit-cultivated soils is low. Cultivated soils with less than 2% organic matter content are considered unfavorable for sustainable agricultural production. A corrective dressing (applied before planting) in fruit production systems is the application of soil organic amendments in quantities that provide a sufficient reserve of organic matter for several years at the recommended application rates of 50-100 t/ha (Pinamonti and Siche 2001). On the other hand, maintenance dressing at the recommended application rates of 40-60 t/ha every two to three years keeps an adequate vegetative production balance. Manure with low mineralization rates is a better choice for both dressings for perennial crops.

8.7 Soil Characteristics and Management

The properties of the soils significantly affect the mineralization rate of organic N. Under similar climatic conditions, the organic C accumulation varies possibly due to different soil types and the quality and quantity of litter accumulation. The important soil factors that influence the mineralization of compost includes pH, salts, presence of toxic quantities of inorganic or organic compounds, moisture, and temperature (Broadbent et al. 1964; Kowalenko et al. 1978; Clark and Gilmour 1983).

The decomposition rates of organic manure are generally slow in clay soil, minimal tillage, sod culture, and organic mulch. The estimated annual dry organic matter decomposition is 3-4 t/ha in cold-temperate regions (Pinamonti and Sichee 2001). On the contrary, fast decomposition of organic manure occurs in sandy and stony soils and in soils with persistent solar radiation. The estimated annual dry organic matter decomposition is 7 t/ha in cold-temperate regions (Pinamonti and Sichee 2001).

8.8 Fertilizer Placement

Generally, the organic manure for annual crops is surface-applied and should be mixed with topsoil by conventional tillage. The organic manure for the maintenance dressing of perennial crops is surface-applied or placed on the under-row area, with or without mixing with topsoil by conventional tillage. The organic manure can also be applied directly into the planting hole near the root system at the time of planting to improve the edaphic environment of the root system. For pot culture, the organic manure is generally completely mixed with the soil so that the root system can develop vigorously in the whole pot soil.

8.9 Carryover Effects

The application of composted manure to provide for crop N requirements may greatly increase the levels of P and other ions in the soil. Long-term or heavy-manure application increases microbial activity and the potential for mineralization of soil organic matter. Consequently, it may result in nitrate accumulation deep into the soil that may induce the transformation of soil Po fractions to Pi fractions. Long-term application of organic manure may also result in an imbalance in base composition. It is important to alternate the application of organic manure with different composition or properties.

8.10 Conclusion

The use of organic manure to fertilize agricultural lands is positive from the perspective of a recycling economy. Application of organic matter to soils directly maintains an adequate level of soil organic matter, a critical component of soil fertility and productivity. Organic manure is considered as slow-release N fertilizer because it releases or mineralizes only a fraction of its total N content during the application season. High initial applications to build up the organic pool and cut back in subsequent years would be appropriate. In supplying the nutrient requirements, the amount of manure applied can be calculated based on the rate of N applied and the rate of organic N mineralization in the application season.

8.11 References

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Index of Images

Figure 1 Vigorous Growth of Brassica Campertris L. SSP. Rapifera Var. Laciniifolia (Kitam.) under Appropriate Application of Compost.
Figure 2 Vigorous Growth of Lactuca Sativa Var. Cispa L. under Appropriate Application of Compost.
Figure 3 Vigorous Growth of Oryza Sativa Var. Taikeng No. 5 under Appropriate Application of Compost
Figure 4 Growth of Tea Plant (Camellia Sinensis L. Var. Ttes No. 12) in an Oxisol with Chemical and Organic Fertilizers.
Figure 5 Growth of Lactuca Sativa Var. Cispa L. in an Oxisol (PH 4.39) with Different Kinds of Compost on the Basis of the Same Amount of N. from Left to Right: Hog Dung Compost; Sawdust-Cattle Dung Compost; Sugarcane Residue-Cattle Dung Compost; Liming with Calcium Carbonate to PH 6.5 and Applied with Chemical N, P, and K Fertilizer; Pea Seedling Residue-Rice Hull Compost; and Control (with Only Chemical Fertilizer).
Figure 6 Growth of Lactuca Sativa L. CV. Tsueyhwa in an Oxisol with Different Rates of Compost and Lime Top: From Left to Right: B) Control Only with Chemical N, P, and K Fertilizers (Final Soil PH 3.85); Si, Sii, and Siii) Rates of Compsot Applied Were 5, 10, and 20 G/KG and with the Final Soil PH 3.99, 4.32, and 4.44, Respectively.
Figure 7 A &Quot;Stable&Quot; Compost Resulted in Healthy Soil; Cucumis Melo L. Var. Chito Was Not Affected by Powdery Mildew (Sphaerotheca Fusca) (Right), As Compared with the Affected Plant (Left) (Courtesy of DR. C.H. Liao).
Figure 8 Incorrect Application of Fresh Crop Residue to Fruit Trees.
Table 1 Selected Chemical Composition of Some Raw Plant Materials for Compost (G/KG).
Table 2 Selected Chemical Compositions of Animal Dung for Compost (G/KG).
Table 3 Selected Chemical Compositions of Some Animal Source Materials for Compost (G/KG).
Table 4 Selected Chemical Compositions of Compost Derived from Different Materials (G/KG).