Soybean is the world’s most important source of protein and accounts for nearly 70% of the world protein meal consumption. This has led to the production of soybean in a wide range of environmental conditions across a huge geographic expanse. Soybean has a distinct advantage over non-leguminous crops through its ability to acquire N via symbiotic N-fixation. However, other nutrients are critically important to optimize soybean production, both through direct effects on growth and development as well as through their influences on soybean biological N fixation. In this review we highlight the fertility requirements and considerations for soybean production and examine the relationships between soybean mineral nutrition and biotic factors such as selected disease, insects, and nematodes.
Introduction
Soybean has been grown in Asia for thousands of years; however it has only been in the last 70 years that soybean has risen to be the world’s most important source of protein and vegetable oil. In 2006, soybean accounted for 68 % of the world’s protein meal consumption and 29 % of the world’s vegetable oil consumption. Soybean has clearly become a staple
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crop of the modern world. Additionally, there are many other uses for soybean ranging from ink to glue and increasingly as a biofuel (biodiesel). Worldwide hectarage of soybean is roughly estimated at 94 million ha. More than 70 million ha are in large-scale commercial production in the Western Hemisphere from about latitude 56○ N in Canada to about latitude 30○ N in the southern USA and then from just south of the equator in northern Brazil to about latitude 38○ S in Argentina. Together the nations of the Western Hemisphere accounted for greater than 86% of the total worldwide soybean production (top producers: United States 38%, Brazil 25%, Argentina 19%, Canada and Paraguay each at 2%).
World-wide soybean is grown in a tremendous range of environments including different soil types, cultural practices, temperature norms, rainfall and irrigation, and varied disease and insect pressures. All of these factors and many more can singly and in varied combinations affect soybean fertility management. Soybean production in the tremendous range of environments encountered is made possible by the availability of adapted cultivars and tailored management practices. Covering all possibilities in one chapter would be a monumental if not impossible task. However, there are common principles, practices and considerations that are universal (or nearly so) and are highlighted in this chapter.
Soybean fertility and nutrient management has been well studied and documented.
Reviews in the first two editions (1973 and 1987) of the American Society of Agronomy monograph “Soybeans: Improvements, Production, and Uses”, delve into great detail about soybean mineral nutrition (deMooy et al., 1973b; Mengel et al., 1987) and are an excellent starting point for a detailed understanding of soybean fertility. Today, most soybean producing states in the USA have soybean fertility recommendations readily available online.
Most are excellent resources applicable to local conditions and problems. In many cases local extension agents, state soybean specialists and/or consultants can provide specific recommendations. However, to make accurate site-specific recommendations, a soil test is imperative.
Plants require elemental nutrients in various amounts although the amounts vary greatly with species, genotype, soil, and environmental factors (Table 1). Essential elements are generally defined as either a macronutrient or micronutrient based on the relative amount required by the plant (Table 1). Although, micronutrients are required in very small amounts, lack of any one nutrient can adversely impact production. The processes and rates of natural accumulation of nutrients in the soil (mineralization, organic matter decomposition, etc.) are complex and highly dependent on soil type and local conditions. For grain crops, as seed is produced and removed from the field, nutrients are lost every season. This loss must be replaced naturally or with fertilizers in order to maintain production.
Table 2 shows an estimate of nutrient removal from a 50 bu ac-1 (3359.5 kg ha-1) soybean crop. In most cases, the removal of at least some nutrients is faster than replenishment by natural processes. In such situations nutrient reserves are exhausted and soils become deficient. Obviously, the higher the yield environment, the greater the demand for all nutrients. This can be especially important for the macronutrients nitrogen (N), phosphorus (P) and potassium (K) as they are needed in the greatest amounts. However, yields can be limited or even severely restricted via deficiencies in any of the other nutrients.
Understanding the yield potential of an environment coupled with the nutrients required to achieve maximum yield is necessary in maintaining a fertility program. There is no economic
advantage in fertilizing for a 3000 kg ha-1 or greater yield if other environmental/production factors restrict yield to a 1500 kg ha-1 potential (i.e. low rainfall and no irrigation).
Conversely, fields consistently producing higher yields, require special attention to fertility due to the higher nutrient removal rates year after year. In some cases, over-abundance of mineral elements can be toxic and result in lower yields (e.g. Mn and Al toxicity).
Table 1. Nutrient concentrations considered adequate in plants and the range of concentrations found in crop plants. (Derived from Tables 3.3 and 3.4 of Epstein and
Bloom, 2005).
Nutrient Symbol Dry Matter Concentration†‡ Range of Concentrations†¶
µmol g-1 mg kg-1 mg kg-1
Micronutirents
Nickel Ni 0.001 0.05 0.05-5
Molybdenum Mo 0.001 0.1 0.1-10
Cobalt†† Co 0.002 0.1 0.05-10
Copper Cu 0.1 6 2-50
Zinc Zn 0.3 20 10-250
Sodium‡‡ Na 0.4 10 10-80,000
Manganese Mn 1 50 10-600
Boron B 2 20 0.2-800
Iron Fe 2 100 20-600
Chlorine Cl 3 100 10-80,000
Macronutirents
Silicon‡‡ Si 30 1,000 1,000-100,000
Sulfur S 30 1,000 1,000-15,000
Phosphorus P 60 2,000 1,500-5,000
Magnesium Mg 80 2,000 500-10,000
Calcium Ca 125 5,000 1,000-60,000
Potassium K 250 10,000 8,000-80,000
Nitrogen N 1000 15,000 5,000-60,000
Oxygen O 30,000 450,000
Carbon C 40,000 450,000
Hydrogen H 60,000 60,000
† Values are based shoot material of mainly crop plants.
‡ Values represent the threshold concentration over which limitations are not likely experienced.
¶ Range of values commonly found in crop plants on a dry weight basis. However, values vary greatly with species, genotype, soil, and environmental factors.
††Considered essential in biological nitrogen fixation.
‡‡ Currently not considered an essential nutrient.
Managing soil fertility is critical to optimize soybean production and to maximize economic return. In this chapter we will focus on soybean specific information and provide an update on current soybean fertility management and research. We will examine the major nutrients in turn and then consider selected micronutrients. Next we will discuss fertility issues related to disease, insect and nematode pressure. Lastly, we will conclude by highlighting fertility issues for future research.
Table 2. Recently reported estimated nutrient content of soybean seed based
on a 50 bu ac-1 (3359.5 kg ha-1) seed yield†.
Nutrient
Zublena (1991)
Franzen and Gerwing
(1997)
Wiebold and Scharf (2000)
Rehm (2001
)
Murrell (2005)
Averag e
Seed Concentratio
n --- kg ha-1 --- g kg-1 Nitrogen
(N)
210.6 210.6 235.2 235.2 212.8 220.9 65.7
Potassium (K2O)
82.9 73.9 70.6 84.0 72.8 76.8 22.8
Phosphorus (P2O5)
45.9 49.3 22.4 50.4 47.0 43.0 12.8
Sulfur (S)
25.8 5.6 11.2 11.2 13.4 4.0
Calcium (Ca)
21.3 10.1 11.2 11.2 11.8 13.1 3.8
Magnesium (Mg)
11.2 10.1 12.3 12.9 10.1 11.3 3.3
Copper (Cu)
0.056 0.056 0.017
Manganese (Mn)
0.067 0.067 0.020
Zinc (Zn)
0.056 0.056 0.017
† Sources may have summarized previously reported research.