**1. Introduction**

For sustainable food production, it is an absolute requirement that nutrients removed with the harvest of crops are replaced to prevent nutrient depletion and soil degradation. Achievement and maintenance of high nutrient use efficiency (NUE)

together with high crop productivity have become a major challenge in both developed and developing countries with an increasing growing population, depletion of natural resources, and deteriorating environmental conditions. This is occurring at the same time as society becomes ever more concerned about resource management practices and the environment, especially when it comes to nutrient management [1]. Fertilizer nutrients applied that are not taken up by the crop are also vulnerable to losses from leaching, erosion, and denitrification or volatilization in the case of N, or they could be temporarily immobilized in soil organic matter to be released at a later time, all of which impact apparent use efficiency [2].

Improving nutrient use efficiency (NUE) in plants is vital to enhance the yield and quality of crops, reduce nutrient input cost and improve soil, water, and air quality [3]. Higher NUE by plants could reduce fertilizer input costs, decrease the rate of nutrient losses, and enhance crop yields. Improving crop nutrient use efficiency ideally requires an understanding of the whole system, from the macro (agroecosystem) to the molecular level [4]. Nutrient uptake and their internal utilization efficiencies are the two central cores for improving crop NUE [5]. This can be achieved through optimizing agronomic strategies (soil-rhizosphere management) and breeding nutrient-efficient cultivars. Plant genetics and physiological mechanisms and their interaction with best agronomic practice are also a tool that can be used to increase efficiency of cropping systems [3]. Thus, it needs involvement of integrated nutrient management strategies that take into consideration improved fertilizer along with soil and crop management practices are necessary [6]. Sustainable nutrient management must be both efficient and effective to deliver anticipated economic, social, and environmental benefits.

Plants experience nutrient deficiency when soil nutrient availability is either an inherently low amount or low mobility of nutrients in the soil, or poor solubility of certain chemical forms of soil nutrients [7]. Of the various nutrients essential for plants, nitrogen (N), phosphorus (P), and potassium (K) are required in the largest quantities, and their deficiency severely limits crop yield [8]. The dynamic nature of N and P in soil-plant systems creates a unique and challenging environment with nitrate and phosphate contamination of surface and/or groundwater, which can be attributed in large part to low efficiency in plant nutrient uptake. The main challenge for improving P and K use efficiency at the farm level is to apply the existing knowledge in a practical manner [9]. Hence, the best management practice for N, P, and K must consider the specific characteristics of crops, cropping systems, environments, and soils is application of 4R nutrient stewardship. Therefore, this chapter tries to summarize the concept of NUE and recent strategies for enhancing use efficiency of N, P, and K. These approaches consider economic, social, and environmental dimensions essential to sustainable agricultural systems and afford a suitable context for specific NUE indicators.
