Micronutrients are required in extremely small quantities (Table 1). Nonetheless, when the nutrient concentration drops below a certain optimal level, plant growth and/or development can be negatively impacted. In fact, micronutrient deficiencies can have dramatic effects on crop plants and severely reduce yield. Conversely, high concentrations of micronutrients can be toxic to plants. Micronutrients are essential constituents of enzymes and other metabolic entities (Cu, Fe, Mn, Mo, Ni, and Zn), can activate or control enzymatic activity (Cl, Cu, Fe, Mn, and Zn), serve as a cellular osmotica or counter-ion (Cl-) and in the case of B, be structurally associated with the cell wall (Epstein and Bloom, 2005). As summarized in Table 3, micronutrients are involved in a wide range of critical biochemical and physiological plant processes.
Specific micronutrient deficiencies can occur either because of inadequate quantities in the soil or unavailability due to soil properties (very sandy soils, low organic matter soils, out-of-range pH, excess lime or salts, poor drainage, and/or soil compaction). In some cases, micronutrient deficiencies on problem soils can be overcome by planting tolerant soybean cultivars. Using tolerant cultivars is usually the best option, for example, Goos and Johnson (2000) tested three methods of reducing Fe-deficiency chlorosis in soybean and concuded the most practical control measure was cultivar selection. Nonetheless, foliar applications of some micronutrients (notably Fe) may be required to overcome immediate problems.
Deficiency symptoms are well characterized in production literature and on state extension web sites, most often accompanied by excellent photographic documentation (for examples see Hoeft et al., 2000; Sawyer, 2004; Potash and Phosphate Institute, 2007). In the USA, Fe and Mn deficiencies are the most common soybean micronutrient deficiencies (Adams et al., 2000a). As with S, in some cases soil analysis is not sufficient for generating fertilizer recommendations and tissue analysis must be conducted. Some success in using non-distructive fluorescence and reflectance measurements has been demonstrated for Mn, Zn, Fe, and Cu (Adams et al., 2000a and b) although traditional plant tissue analysis remains the standard.
Table 3. Primary biochemical/physiological functions of micronutrients.
Nutrient Primary Functional Area Key Enzymes/Complexes
Boron (B) Cross-link two molecules of a cell-wall polysaccharide (pectin) Other roles unclear
Cell wall structure
Chlorine (Cl) Enzyme cofactor Oxygen evolution Cellular osmoticum
Photosystem II
Activate tonoplast proton pumps Guard cells of stomata
Cobalt (Co) Biological nitrogen fixation specific to bacteriods
cobalamin
Copper (Cu) Enzyme cofactor Protein component Electron transport Detoxify oxidants
Photosystem I
Mitochondrial electron transport Superoxide dismutase
Iron (Fe) Enzyme cofactor Protein component Detoxify oxidants Electron transport Nitrogen metabolism Sulfur metabolism
Mitochondrial electron transport Photosystem I
Cytochrome Nitrogenase
Manganese (Mn)
Enzyme cofactor Protein component (two) Oxygen evolution Detoxify oxidants
Photosystem II Superoxide dismutase Allantoate amidohydrolase
Molybdenum (Mo)
Nitrogen metabolism Protein component
Nitrate reductase Nitrogenase
Xanthine oxidase/dehydrogenase Nickel (Ni) Nitrogen metabolism
Protein component (one enzyme)
Urease Hydrogenase Sodium (Na) Essential in some C4 Photosynthesis
(NAD-malic enzyme type) Cellular osmoticum
role unclear
Zinc (Zn) Required at the active site of many enzymes
Enzyme cofactor
Protein component (many)
DNA transcription (Zinc fingers)
>80 Zn containing proteins
It is not only deficiencies that can affect micronutrient status in soybean. For example, high Mn concentrations can interfere with the uptake of Fe and induce Fe-chlorosis symptoms (Roomizadeh and Karimian, 1996). Additionally, if micronutrient applications are recommended based on soil tests, the irrigation water should be tested before applying micronutrients as adequate levels may be applied in the irrigation water (Penas and Ferguson, 2000). Often soybean is grown in rotation with corn and in many situations a well-fertilized corn crop provides all the nutrients needed for soybean production and this may be especially true for micronutrients. Care must be taken with application of micronutrients because what is optimal for one crop may be toxic to another crop. For example, Penas and Ferguson (2000) state that the optimum level of B for sugar beets may depress the yield of oats.
Certainly, with many micronutrients there may be a narrow range that defines deficiency and toxicity. Additionally, in heavily leached, acidic tropical soils, metals such as Al3+ and Mn2+
can be problematic for soybean production.
Influence of Nutrient Status and Fertilization on Pests and Diseases
More than 100 soybean pathogens of varying importance to particular production regions have been identified (Hartman et al., 1999). While information available for some of these diseases is extensive, comparatively little is known with regard to their relationships with soybean mineral nutrition. Studies examining the influence of fertilization and mineral nutrition on plant growth and yield by far outnumber those exploring the effects on pests and diseases. However, it is clear that the nutritional status of a plant influences its interaction with other organisms. Mineral nutrients not only influence the chemical composition of a plant but also affect its morphology and anatomy and in turn raise or reduce the sensitivity to pest and/or pathogen attack (Marschner, 1995). In general, adequate nutrition resulting in vigorous plants reduces the severity of the impact of pest or disease attack. However, the dynamics of plant-pest and plant-disease interactions are often very specific and need to be examined on a case by case basis.
The impact of a fertilizer application on plant-pathogen/pest interactions may be due to its influence on the overall plant nutritional status, the function of individual mineral nutrients in plant metabolism, through a direct effect on the pathogen/pest, or may be a function of indirect effects such as through the impact on soil microbial dynamics. The outcome will also depend on the timing of application relative to plant and pathogen developmental stages as well as the stage of disease development. In addition, the method of application with regard to placement (e.g. broadcast, band, foliar etc.) will modulate its impact on disease or pest development. A number of these aspects will be highlighted in the following paragraphs. The information given in this section is not intended to be comprehensive but rather to illustrate the importance of mineral nutrients with regard to plant-pathogen/pest interactions. At the same time, it is clear how little information is available on the effects of soybean nutritional status and fertilization on pest and disease dynamics, particularly in regard to the mechanisms underlying the documented observations.
Phytophthora Rot (Phytophtora sojae (Mart.) Sacc. f. sp. glycines)
Phytophthora root and stem rot is a serious problem in many soybean-producing regions around the world. For instance, for the period of 1996 through 1998, Phytophthora was the second most important biotic stress factor limiting soybean yield in the northern soybean producing states of the USA (Wrather et al., 2001a and b). Worldwide annual losses to soybean caused by P. sojae have been estimated at $1-2 billion annually (Tyler, 2007).
Disease resistance genes, at least 14 of which have been identified, are important targets for breeders and are effective disease management tools (Grau et al., 2004; Tyler, 2007).
Cultivars differing in tolerance to the disease in a race-independent manner also exist. The losses caused by P. sojae are influenced by numerous factors including genotype susceptibility and environmental factors such as soils that are prone to water logging.
Management practices including tillage, improved drainage, crop rotation and timing of planting are important components of an integrated approach to control the disease and should go hand in hand with the selection of resistant or tolerant cultivars (Schmitthenner, 1999).
The relationship of soybean mineral nutrition and Phythophthora rot is not well understood. However, Dirks et al. (1980) reported a positive relationship of disease severity with increased level of fertility. They observed a linear relationship of the number of plants killed by Phytophthora rot and the amount of N-P-K fertilizer applied. While the experimental design did not allow the separation of the influence of the individual mineral elements, based on other reports, the authors speculated that N may have played a critical role. However, in a 3-yr field study, Pacumbaba et al. (1997) found that plants fertilized with K (muriate of potash) had a greater incidence of Phytophthora rot than those fertilized with P (superphosphate), and fertilization with N, P, and K (20-20-20) resulted in the lowest disease incidence. Similarly, Schmitthenner (1999) indicated that application of large amounts of KCl prior to planting may increase damage. Xu and Morris (1998) demonstrated that application of Ca influences the developmental progress of P. sojae under controlled conditions and suggested that Ca fertilization under field conditions may reduce disease incidence by hampering zoospore development and release. Sugimoto et al. (2005) reported a reduction in Phytophthora rot as a result of Ca application. In their in vitro study, these authors applied increasing concentrations of CaCl2 and Ca(NO3)2 to the agar medium and monitored the disease severity on two soybean cultivars. Disease incidence decreased in both cultivars in response to both Ca sources, but Ca(NO3)2 was more effective. The two cultivars were found to differ with regard to calcium uptake and disease reduction was related to increased tissue Ca concentrations. In addition, Sugimoto et al. (2005) observed that high concentrations of Ca reduced the release of zoospores from P. sojae isolates grown on agar and suggested that disease reduction could be a combination of the effect of Ca on the plant tissue and a direct effect on pathogen growth.
Sudden Death Syndrome (Fusarium solani (Mart.) Sacc. f. sp. glycines)
Sudden death syndrome (SDS) is caused by a slow growing soil-borne fungus that can cause significant yield losses (Hartman et al., 1995; Rupe and Hartman, 1999; Scherm and Yang, 1996). In fact, yield reductions in the USA due to SDS were estimated to be approximately 1.3 million tons during the three-year period of 1996-1998 (Wrather et al., 2001b).
In 1993, Rupe et al. reported on a study that examined the relationship of numerous plant nutrients and the severity of SDS in Arkansas. Results from five different multiple regression equations indicated that increased levels of exchangeable soil Na, Ca, and Mg, were associated significantly with increased disease severity in two or three of the five models.
However, increased P and soluble salt levels were consistently associated with increased SDS severity across all five multiple regression equations. Based on these results, they concluded that SDS appeared to be favored by increased soil fertility. Scherm et al. (1998) conducted a field survey to investigate the relationship between SDS occurrence and various soil characteristics in Iowa. They reported that increased available K was positively correlated with increased disease severity in four out of nine commercial soybean fields and negatively correlated in one out of the nine fields. Further analyses indicated that the relationship of available K and disease severity was influenced both in magnitude and sign by the overall soil K concentration. As the overall soil K concentration increased, the magnitude of the positive correlation coefficient decreased. Thus, they concluded that nutrient management would only be of limited utility to reduce SDS in Iowa. Information to date appears to consistently indicate that favorable production environments enhance SDS and that the presence of soybean cyst nematodes results in more rapid disease progress and greater severity (McLean and Lawrence, 1993a and b; Melgar et al., 1994; Roy et al., 1989; Roy et al., 1997; Rupe et al., 1993; Rupe et al., 2000; Scherm et al., 1998).
Field investigations into the effect of elevated Cl levels did not influence SDS severity in three out of four cultivars (Rupe et al., 2000). However, in an SDS susceptible cultivar that limits Cl translocation to the shoot (Hartz 6686), elevated levels of soil Cl increased SDS severity. Because the soil K concentrations were at recommended levels, the authors attributed the observed effect to the presence of high concentrations of Cl and not K, even though the treatments were established using KCl. Interestingly, Sanogo and Yang (2001) found that the application of KCl to potted soybean plants reduced SDS severity by 36%
while K2SO4 and KNO3 increased disease severity compared to the control. In addition, regardless of the source, P application increased the severity of SDS in that study. Results reported by Howard et al. (1992) were consistent with those of Sanogo and Yang (2001) in that they also observed reduced SDS severity and increased SDS severity in response to KCl and K2SO4 application, respectively.