Importance of Micronutrients

The Law of the Minimum takes on added importance when fertilizer prices — especially of nitrogen (N) and phosphate (P2O5) products — are high. This may tempt some growers to reduce or even eliminate applications of micronutrient or secondary nutrient fertilizers that provide balanced potassium (K), magnesium (Mg) and sulfur (S). But von Liebig’s “Law” tells us clearly that if a soil is deficient in, say, Mg, yields will be depressed regardless of how much N-P-K product you apply.

GROWTH (YIELD) RESPONSE: PRINCIPLES
Plant growth is dependent upon available water, solar radiation, C, H, 0,and at least 13 mineral elements (Marschner, 1986). Six of these (N, K, Ca, Mg,P, and S) are macronutrients. They normally occur in plants at concentrations greater than 1 g kg– 1 (30 mmol kg- 1 ) level (Table 6-1). The remaining seven micronutrients or trace elements (B, Cl, Cu, Fe, Mn, Mo, and Zn) normally occur in plants at concentrations less than 3 mg kg- 1 . Trace amounts of other elements(e.g., Co, Na, Ni, and Si) may be beneficial for plants (Marschner, 1986). For example, Si is commonly found in many cool-season grasses as an important structural component of cell walls, trichomes, and rasplike leaf margins. Siliconin grasses provides protection against various herbivory (McNaughton et al.,1985). Nickel has been identified as required for plants in ultra-small amounts (Brown et al., 1990).

For an element to be essential for the growth of plants. it should meet the following three criteria (Marschner, 1986):
1. The organism cannot complete its life cycle without the element
2. The function of the element must not be replaceable by another element.
3. The function of the element must be direct. For example, it must be part of an essential plant constituent such as an enzyme or be required for metabolism.

Law of the Minimum
The amount of plant growth in a given environment depends on quantity and balance of growth-determining factors, with the least optimum factor limiting growth. This concept is the Principle of Limiting Factors or Liebig’s Law of the Minimum and is extremely useful. However, two additional aspects must be considered. First, Liebig’s Law of the Minimum applies to conditions where inflows and outflows of energy, minerals, and other factors are balanced (steadystate condition); e.g., where forage growth is limited by N, a sudden increase in available N may remove N as the limiting factor. During the transitional period to a new production level, the next limiting factor or factors may be difficult to identify until a new steady-state condition is established. Second, factors interact to modify effects of individual factors. Thus, when solar radiation, temperature and soil water are nonlimiting, fertilizer requirements are higher than when such factors are limiting. Pasture-, range-, or forage-land productivity is controlled primarily by temperature, water (rainfall), soil fertility, and defoliation (grazing) management.

All green plants require the same essential elements for growth. Various forage plants differ in their abilities to extract nutrients from the soil in required amounts because of differences in their responses to the range of soil and climatic conditions. Forage yield multiplied by nutrient concentration equals nutrient uptake. Yield is usually the most important factor in nutrient removal by forage crops. Nutrient removal is an important part of plant-nutrient requirement (fertilizer requirement).

Fertilizer and Plant Nutrient Requirements
Fertilizer requirement is the amount of a nutrient needed (beyond that supplied by the soil) to increase plant growth to a desired or optimum level (external nutrient requirement). The amount of soil nutrient available to the plant may be determined by various chemical or biological tests. Chemical extractants and procedures vary in different geographic regions. Soil test extractants and procedures are calibrated to provide reliable indication of the particular nutrient status of groups of soils. The reader is referred to Brown (1987) for detailed discussion of soil testing to evaluate external plant nutrient status. Specific soil test values for various levels of nutrient sufficiency should be obtained from organizations having responsibility for that geographic region. Internal nutrient requirements are those concentrations needed in plant tissue for a given yield level. Yield levels often are expressed as a percentage of maximum, for that environment (Table 6-3).

Crop yield and quality responses, as a function of nutrient input or plant concentration, may be separated into four zones: deficiency, or inadequate to complete a life cycle; critical nutrient range (CNR), where near maximum yields are obtained with minimum amounts of nutrients; adequacy, where no further changes in yield occur; and yield depression, where yield decreases occur with increasing nutrient concentrations (Fig. 6-1). Yield depression may be caused by nutrient toxicity, imbalance, or antagonism leading to deficiency of another nutrient. The reader is referred to Black (1993) for a comprehensive discussion of soil fertility evaluation and control, Relationships between concentrations in plant tissue and yield increases from adding fertilizer also are described succinctly in Beeson and Matrone (1976).

Research that covers the full range of response is important for biological, economic, and environmental reasons. Plant and soil analyses help prevent yield and stand losses while identifying optimum fertilization practices allowing the achievement of full production potential of the soil, crop, and environment. Such analyses help diagnose nutrient excesses, conserve nonrenewable resources, and prevent negative impacts on the environment caused by over-fertilization. Animal performance and health also are important criteria in evaluating fertilizer requirement. Yields of animal products are controlled by the effective utilization of increased forage yield and forage quality produced by the fertilizer.

Cation-Anion Balance
Biological systems such as plant cells, tissues, and soil systems operate under the principle of electrical neutrality. That is, the total sum of anion equivalents in plant tissue is equal to the sum of cation equivalents. Since the uptake of ions like NO3 or K+ is rapid, while the uptake of ions like Ca 2+ or SO,?- is slow, cations and anions are removed from the soil in unequal amounts. These cationanion imbalances are compensated within plant tissue by the degradation, or accumulation of organic acids, particularly malate (Mengel & Kirkby, 1987).

Ionic balance is maintained within the soil by H+, or OH- (HCO3) accumulation (Mengel & Kirkby, 1987). This aspect of mineral nutrition can significantly affect mineral composition and organic acid composition. For example, the application of K2SO4 fertilizer can result in more rapid uptake of K than SOi-, creating an imbalance compensated within the plant by the production of organic anion equivalents (malic acid). On the other hand, KC1 fertilizer may not result in this cation-anion imbalance because K and Cl uptake rates are similar. These cation-anion relationships in grass may affect growth rates, mineral concentrations, and concentrations of organic acids in the plant. These can affect Mg uptake by the plant and bioavailability of herbage Mg to the grazing animal. Reduced bioavailability of Mg may cause grass tetany (Ilypomagnesemia) in grazing animals. Among the noninfectious diseases, economic losses from this disorder are probably second only to bloat in ruminants. Gn.mes et al. (1985) found that high rates of N and K fertilization or high rates of broiler litter fertilization more than doubled malate concentrations in ‘Kentucky 31’ tall fescue, thus increasing the grass tetany potential of the grass. Grass tetany is discussed further under antiquality components.

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