Among the 17 essential plant nutrients, nitrogen (N) plays the most important role in augmenting agricultural production, potential environmental risks, and impacting human and animal health. Nitrogen, which is required in the greatest quantity of all mineral nutrients absorbed by plant roots, is an essential component of protein. While the beneficial effects of fertilizer N application on crop production are well documented, concern on the long-term role of fertilizer N in maintaining soil productivity, crop quality (including elemental composition and balance, protein, oil, fatty acids), and environmental safety is being expressed more frequently in recent years. The long-term strategy of N use in agriculture likely will involve increased reliance on fertilizer N, biological N fixation (BNF) by leguminous crops, and wastes (including farm, urban, and industrial wastes) and their efficient management. Recovery of applied N by crops in field experiments has been found commonly ranging from 25 to 34% for rice (Oryza sativa L.) and 40 to 60% for other crops, with the global average value of about 50% (Mosier, 2002). Unutilized N may remain in the soil in various forms and/or get lost through several processes, including NH3 volatilization, denitrification, and nitrate leaching, which have been discussed in detail in several reviews (Aulakh et al., 1992; Carpenter et al., 1998; Goulding, 2004; Guillard et al., 1995; Malhi et al., 1991, 2002a; Singh et al., 1995; Zhang et al., 1995). For enhancing applied nitrogen use efficiency by crops (NUE), several management techniques have been developed for different soils, crops, seasons, and regions. Examples of these include (a) integrated and judicious use of chemical fertilizers and organic manures, (b) synchronizing N supply with crop need by split applications during crop growth, (c) reducing loss of applied N with nest or band placement or large urea granules in soil, and (d) use of slow-release fertilizers and nitrification inhibitors (Aulakh, 1994; Keeney, 1982; Malhi and Nyborg, 1985, 1988; Nyborg and Malhi, 1992). The amounts of different nutrients absorbed by a crop from soil may vary 10,000-fold, from 200 kg of N ha-1 to less than 20 g of Mo ha-1, and yet rarely do these nutrients work in isolation. As agriculture becomes more intensive, the extent and severity of nutrient deficiencies and the practical significance of nutrient interactions increase. Interactions among nutrients occur when the supply of one nutrient affects the absorption, distribution, or function of another nutrient (Robson and Pitman, 1983). In crop production, nutrient interactions assume added significance by affecting crop productivity and returns from investments made by farmers in fertilizers. Interaction between two or more nutrients can be positive (synergistic), negative (antagonistic), or even absent (reflected as additive effect). When the effect of one factor (e.g., nutrient) is influenced by the effect of another factor, the two factors are said to interact. When the crop yield reaches an early plateau, it may be due to the limiting supplies of another nutrient illustrating the operation of Liebig's "law of the minimum" (Cooke and Gething, 1978). When that nutrient is supplied, yield will continue to increase until another factor becomes limiting. When solar radiation, temperature, and soil water availability are nonlimiting, plant nutrient requirements will be higher; for which Wallace (1990) proposed the "law of the maximum" in contrast to the "law of minimum." The law of maximum states that when the need is fully satisfied for every factor involved in the process, the rate of the process can be at its maximum potential, which is greater than the sum of its parts because of a sequentially additive interaction (Wallace, 1990). Production of field crops has already entered into the stage of multiple nutrient deficiencies management. For instance, in south Asia, a farmer in the rice-wheat belt needs to manage four to six nutrients to safeguard and sustain an annual harvest of 10 tons grain ha-1, whereas a tea planter in India targeting for a yield of over 4.5 tons tea ha-1 must worry about seven to eight nutrients. Thus, when the combined effect of two factors is more than their additive effects, the interaction is positive. When their combined effect is less than their additive effects, the interaction is negative. Therefore, just because factor A + factor B plot produces a higher yield than either A-treated or B-treated plot does not mean that there is a positive A × B interaction. A brief review on the interaction effects of N with P, K, S, and Cu has been given previously (Aulakh and Malhi, 2004). As N function in plant growth and nutrition is closely connected to C, the C/N ratio controls N availability and potentially affects interactions through the processes of organic residue decomposition and soil organic matter (SOM) formation in soil, biomass production (photosynthesis minus respiration) in plant, and energy flow in and through all levels of the ecosystem (Wilkinson et al., 2000). An understanding of the nature of different interactions, factors affecting them, and the ways and means of managing these for useful purposes is vital for developing, advocating, and practicing a balanced and efficient crop nutrient management strategy. Identification and exploitation of positive interactions hold the key for increasing returns in terms of crop yield, produce quality, and nutrient use efficiency from applied N. Knowledge of the negative interactions is equally valuable because the test of "precision crop nutrition" lies in the ability to minimize the losses from antagonistic effects. Nutrient interactions have a role to play in determining the course and outcome of two major issues of interest in fertilizer management, namely balanced fertilizer input and efficient fertilizer use. This article reviews and analyzes the available information to (a) examine the impact of interactions of applied N with other nutrients, nutrient cycles, and water on NUE, (b) consider the influence of NUE on the utilization of other nutrients and water, and on C sequestration and storage in soil, (c) compute relative consumption of major fertilizer nutrients (N, P, and K) for assessing their over, under, or balanced use in different regions, and (d) pinpoint the gaps in knowledge for future research needs to optimize the use of N and other nutrients for sustainable crop production and reducing environmental risks. The fertilizer use efficiency indicators considered in this chapter are (i) agronomic efficiency calculated as production of grain over control per unit of applied fertilizer nutrient (Novoa and Loomis, 1981), (ii) improvement in nutrient uptake as measured by apparent recovery efficiency, namely uptake of fertilizer nutrient by plant over control per unit of applied fertilizer nutrient (Dilz, 1988), and (iii) improvement in crop quality parameters such as protein, oil, and fatty acid content. Similarly, water use efficiency (WUE) computed as (i) crop yield m-2 field area m-1 water used and (ii) crop yield m-2 field area m-1 water transpiration or evapotranspiration is considered here. As the impacts can be on a local, regional, or global scale, examples have been cited for different levels. Emphasis is placed on the cereal crops that occupy more than 50% of the harvested area of crop land and contribute more than 75% to annual world food production, but other field crops, such as oilseeds and pulses, vegetables, horticultural crops, and perennials are also discussed. Keeping the practical utility of research in view, a preference has been accorded to results obtained from field experiments over greenhouse and pot culture studies. © 2005 Elsevier Inc. All rights reserved.
