**5. Optimization of agronomic practice**

Agronomic practices can change soil physicochemical properties and biological characteristics. As a result, a number of agronomic practices have been proposed to enhance nutrient availability under diverse climatic conditions [55, 56]. The rhizosphere (root-soil interface) is the most important area for plant–soil-microorganism interactions and is the hub for controlling nutrient transformation and plant uptake [7]. This modification is paramount to increase nutrient availability and to minimize losses in surface runoff. Possible management strategies options for improving NUE through optimizing agronomic practice or rhizosphere modification [57] are the following:

#### **5.1 Application of nutrient stewardship concept**

The 4R Nutrient Stewardship framework promotes the application of nutrients using the right source (or product) at the right rate, right time, and right place. The framework was established to help convey how fertilizer application can be managed to ensure alignment with economic, social, and environmental goals [58]. Nutrient Stewardship defines the right source, rate, time, and place for fertilizer application as those producing the economic, social, and environmental outcomes desired by all stakeholders of the plant ecosystem (**Figure 3**). This 4R techniques applies (1) right rate—supplying growing crops with the right amount of nutrients for healthy growth and development based on experimentation under various environmental conditions; (2) right time—matching nutrient availability to with the timing of plant peak nutrient uptake and demand; (3) right placement adding nutrients to the soil at a place where crops can easily access them related to volume of roots.; (4) right source applying the correct fertilizer and organic resources that provide growing crops with all nutrients required for good growth and maturity [58]. The 4R concept was established to help convey how fertilizer application can be managed to ensure alignment with economic, environmental, and social goals [22, 59].

Soil testing remains one of the most powerful tools available for determining the nutrient supplying capacity of the soil, but to be useful for making appropriate fertilizer recommendations good calibration data is also necessary [2]. As P is less mobile, less soluble, and highly prone to soil fixation; effectiveness of applied P depends on the properties of soil being fertilized, fertilizer itself, and time and method of its application [60]. To enhance phosphorus use efficiency (PUE) of applied P fertilizer, *Toward the Recent Advances in Nutrient Use Efficiency (NUE): Strategies to Improve… DOI: http://dx.doi.org/10.5772/intechopen.102595*

**Figure 3.** *The 4R nutrient stewardship concept (adopted from [22, 59]).*

time and method of its application are critically important, because different P application methods differ in PUE [61]. In highly sandy soils, P may need to be managed like N, by splitting applications and applying small amounts at sowing and topdressing later in the crop growth cycle [62]. Studies of Jing et al. [63] suggested that localized supply of superphosphate combined with ammonium-N (NH4 + -N) significantly stimulated root proliferation, especially of fine roots, and thus improved maize growth in a calcareous soil. Further studies indicated that localized supply of P and NH4 + -N at both seeding and later growth stages increased maize yield by 8–10%, P uptake by 39–48%, and localized increases in root density and length of 50% [64]. Rehim et al. [65] also reported that the fixation of broadcasted P is much greater than the fertilizer applied in bands because of less contact with P fixing ingredients. At higher P application, the adsorption of P increased because the plants readily utilize only 8–33% of applied P in the first growing season and remaining portion remained fixed that consequently resulted in higher Olsen P. So, at higher P application rates, plants used smaller proportion of fertilizer P that resulted in low PUE [61].

In principle, N deficiency increases root growth, resulting in longer axial roots (primary roots, seminal roots, and nodal roots), and this helps maize roots to explore a larger soil volume and thus increases the spatial N availability [66]; however, longterm N deficiency stunts root growth due to insufficient N. But also, root elongation can be inhibited if the N supply is too high. Excessive application of N-P fertilizers may lead to high concentrations of soluble nutrients in the root zone, which can also restrict root growth and rhizosphere efficiency [67], even small amounts of P lost can be a cause of the adverse effects of eutrophication of surface waters. Therefore, judicious application of fertilizer best management practices (BMP) [22] that includes the right rate [68], right time [69], right source, right place, and balanced fertilization (4RB) is the best management practice for achieving optimum nutrient efficiency [2, 22].

### **5.2 Cereal-legume intercropping**

Cereal-legume intercropping is a crop production system utilized to improve productivity and sustainability under diverse environmental conditions. It can also improve nutrient use efficiency and crop productivity [7]. Intermingling of maize and faba-bean roots increased N acquisition by both crop species by about 20% compared

with complete or partial separation of the root systems. Further studies indicate that N2 fixation can be improved by yield maximization in the intercropping system. The improved productivity observed in this production system has been associated with increased levels of available phosphorus (P) in the root rhizosphere. Hinsinger et al. [70] reported more stable yield, superior land resource utilization or conservation, and enhanced pest or weed control [71–73]. Furthermore, cereal-legume intercropping can also enhance the phosphatase enzyme activity and available P in the soil due to rhizosphere acidification by the legumes in the cropping system [74].

The possible mechanism that increases PUE in intercropping is the increased rhizosphere soil acid phosphatase (RS-APase) activity observed in intercropping due to the fact that large amounts of acid phosphatase are known to be released from their roots into the root rhizosphere. The (RS-APase) activity was significantly higher (26–46%) in the intercropping and occurred concomitant with a significant increase in available phosphorus (RS-Pavailable) in the rhizosphere on podzols in cool climate boreal ecosystem [75]. Another mechanism could be secreting H<sup>+</sup> into the soil that acidifiies the rhizosphere [57, 76] and improves dissolution of phosphorus and then enhances P-availability [70]. Additional possible mechanism that improves of plant growth and P uptake in mixed planting was due to root interspecific complementation or facilitation. The complementarity between root morphological and physiological traits of neighboring plants underpins the interactive facilitation, which was the main underlying mechanism improving nutrient-use efficiency, particularly of P, in mixed cropping system [77, 78]. The complementary niches of maize and faba bean significantly reduce interspecific nutrient competition and thus improve nutrient-use efficiency [79]. The presence of maize increased the secretion of carboxylates from alfalfa roots, suggesting that the root interactions between maize and alfalfa are crucial for improving P-use efficiency and productivity in intercropping [80]. Subsequently, Sun et al. [76] reported that decreasing rhizosphere pH and increasing organic anion exudation played key roles in soil P mobilization of maize and alfalfa, with little contribution of acid phosphatase.

#### **5.3 Effective microbial inoculation**

The mycorrhizal symbiosis particularly, arbuscular mycorrhizal fungi (AMF), is arguably the most important symbiosis on earth [81]. AMF colonize the roots of many agriculturally important food and bioenergy crops form (approximately 80–90% of all known land plant species) [81] and could serve as "biofertilizers and bioprotectors" in environmentally sustainable agriculture [82]. In AMF associations, two pathways for plant P uptake exist: the direct pathway (P uptake by roots) and the AM fungal pathway [83]. AMF facilitates the uptake and transfer of mineral nutrients, such as phosphorus, nitrogen, sulfur, potassium, calcium, copper, and zinc, from the soil to their host plants by means of the extraradical mycelium extending from colonized roots into the soil [84]. The contribution of AMF to P uptake reaches up to 77% under low P supply compared with only 49% under high P supply [85]. Furthermore, the commercial inoculum Mycobiol, consisting of Glomus spp., *Entrophospora colombiana, and Acaulospora mellea*, enhanced P acquisition and plant growth in a pot experiment [86]. González and Walter [87] observed that *Glomus aggregatum* increased P uptake and biomass production of inoculated plants compared.

Various mechanisms have been suggested for the increase in the plant uptake of P. These include: exploration of larger soil volume; faster movement of P into mycorrhizal hyphae; and solubilization of soil phosphorus [88]. Exploration of larger soil

*Toward the Recent Advances in Nutrient Use Efficiency (NUE): Strategies to Improve… DOI: http://dx.doi.org/10.5772/intechopen.102595*

volume by mycorrhizal plants is achieved by decreasing the distance that P ions must diffuse to plant roots and by increasing the surface area for absorption. Faster movement of P into mycorrhizal hyphae is achieved by increasing the affinity for P ions and by decreasing the threshold concentration required for absorption of P [88]. Solubilization of soil P is achieved by rhizospheric modifications through the release of organic acids, phosphatase enzymes, and some specialized metabolites such as siderophores [55].

The composition and amount of root exudates affect the composition of microbes in the rhizosphere and the structure of the rhizosphere microbiome, affecting plant growth and nutrient uptake [81]. For precision rhizosphere management, plantmicrobe interactions must be finely tuned to improve P use efficiency by crops [57]. **Figure 4** illustrates the main structural differences between AM (more for P absorption) and ectomycorrhizal (more for N and few for P absorption) associations of angiosperms or gymnosperms [81].

Among the soil bacterial communities, ectorhizospheric strains from Pseudomonas and Bacilli and endosymbiotic rhizobia have been described as effective phosphate solubilizers [90]. Phosphate-solubilizing bacteria (PSB) are also capable of making P available to plants from both inorganic sources and organic ones and increasing P-fertilizer-use efficiency by different mechanisms [91]. They are rhizobacteria that convert insoluble phosphates into soluble forms through acidification, chelation, exchange reactions, and the production of organic acids [92]. Therefore, combined application of AMF and P solubilizers [93] and N fixers are the best inoculants. AM fungi together with PSMs could be much more effective in supplementing soil P. Understanding AM-plant symbiosis, developing AM fungi that could be cultured in vitro, and developing P-solubilizing AM will help realize their potential as phosphate biofertilizer [94].

#### **Figure 4.**

*Phosphorus acquisition efficiency related traits of wheat and barley roots affected by arbuscular mycorrhizal symbiosis in comparison to a non-colonized counterpart (adopted from [89]). (A) Representation of P depletion zone around the rhizosphere; (B) access to smaller soil pores by AM fungal hyphae; and (C) modulation of plant P transporters following colonization.*

#### **5.4 Regulating soil pH**

Soil pH is one of the most important chemical properties influencing nutrient solubility and hence availability to plants. Large amount of P applied as fertilizer enters in to the immobile pools through precipitation reaction (fixation) with highly reactive Al3+ and Fe3+ in acidic and Ca2+ in calcareous or normal soils [94]. Acidic, highly weathered, iron (Fe)-rich soils rapidly bind phosphates at mineral surfaces, limiting access to plant roots. Furthermore, applied Pi (inorganic P) is quickly fixed into insoluble inorganic or organic forms due to its high reactivity and microbial action [95].

Soil pH markedly limits plant growth and P chemistry in soils through its effect on P adsorption and through interactions that affect precipitation of P into solid forms in soil [62]. Consequently, about 80–90% soil P becomes unavailable depending on soil composition and pH [96], 50–70% of the total applied conventional fertilizers are lost to the environment. This level of loss in agricultural nutrients not only leads to the loss of valuable resources but also causes the severe reduction of yield [97]. The pH of a calcareous soil is reduced by the presence of gypsum (CaSO42H2O) due to the concentration of Ca2+, which would be expected to decrease the sorption of P, if followed by leaching to removed much of the soluble Na<sup>+</sup> and Ca2+ [98]. Thus, adjusting soil pH and base saturation are methods to reduce the amount of P that is bound by Al, Fe, and Ca, further reducing the effects of Al toxicity to plants, which can inhibit uptake, and use of P by the plant (**Figure 5**) [23, 99].

Lime acidic soil is widely used in agriculture to create and maintain a soil pH optimal for plant growth in acid soils. Lime reduces toxic effects of hydrogen, aluminum, and manganese, improves soil biological activities, cation exchange capacity (CEC), P, Ca, and Mg availability and soil structure, promotes N2 fixation, stimulates nitrification, and decreases availability of K, Mn, Zn, Fe, boron (B), and Cu [11]. An increase in soil pH, as a result of liming, was due to an increase in hydroxide ions, which increases microbial activity and communities, hence, increasing decomposition of soil organic matter and release of Fe and Al [100]. The decrease in Al-P and Fe-P could be due to their precipitation as insoluble Al(OH)3 and Fe(OH)3 after increased addition of liming material [101]. In addition, Al and Fe oxides become more negatively charged with an increase in pH contributing to an increase in available P [102].

Liming, gypsum application, or mixing of both is an effective practice to improve pH, improve Ca content, and control Al toxicity. Lime has very low mobility in soil, and when surface applied, it does not reduce the acidity of subsurface soil horizons. Contrary to lime, gypsum (CaSO4) has a greater downward movement, and when applied to the surface, it can still impact and reduce the acidity of the subsoil [4]. The pH of a

**Figure 5.** *Soil P availability as affected by soil pH (adopted from Havlin et al. 1999).*

calcareous soil is reduced by the presence of gypsum (CaSO42H2O) due to the concentration of Ca2+, which would be expected to decrease the sorption of P, if followed by leaching to removed much of the soluble Na+ and Ca2+. The uptake of nutrients by plants, content of nutrients in plants and in soil were substantially positively influenced by both the wood ash, especially by FGD gypsum [103]. Gypsum application can ameliorate saline-sodic soil, thereby increasing crop yield and NUE [104].

## **5.5 Application of advanced techniques**

Apart from traditional methods, new techniques have been developed such as sitespecific/real-time nitrogen management, slow release/controlled release fertilizer (SR/ CRF), site-specific precision nutrient management, and urease/nitrification inhibitor. Those techniques play an important role in decreasing fertilizer loss and increasing NUE [105]. The remote sensing is quicker than the previous two methods, and it obtains continuous data rather than spot data, which is more advantageous. It is becoming the major means of obtaining data for precision farming. GIS (geographic information system) establishes the field management information system by processing, analyzing, and trimming the data of soil and crops [105]. Another approach to synchronize release of N from fertilizers with crop need is the use of N stabilizers and controlled release fertilizers. Nitrogen stabilizers (e.g., nitrapyrin, DCD [dicyandiamide], NBPT [n-butyl-thiophosphoric triamide]) inhibit nitrification or urease activity, thereby slowing the conversion of the fertilizer to nitrate. The most promising for widespread agricultural use are polymer-coated products, which can be designed to release nutrients in a controlled manner.

Agronomic management strategies such as precision P fertilization, polymer coated P-fertilizers, and recycling of P from domestic, agricultural, and industrial wastes can be helpful in improving P use at farm level [106]. Modern concepts for tactical N management should involve a combination of anticipatory (before planting) and responsive (during the growing season) decisions [9]. On soils with moderate P and K levels and little fixation, management must focus on balancing inputs and outputs at field and farm scales to maximize profit, avoid excessive accumulation, and minimize risk of P losses. Improving the internal, on-farm and field recycling is the most important K management issue worldwide. As for N, the primary determinants for REP and REK are the size of the crop sink, soil supply, soil characteristics, and fertilizer rate.

Control release fertilizers with polymer coatings are commonly applied to crops to increase efficiency of nutrients [96]. One way of improving the P availability to crop plant is by coating diammonium phosphate (DAP) with polymer that allows a steady but controlled discharge of phosphorus from the granules for crop plant uptake and improved P recovery percentage. Thus, by the use of polymer, availability of P to plant increased because it has high cation exchange capacity, which holds the divalent calcium (Ca+2) and trivalent cations iron and aluminum (Fe+3 and Al+3) and stop P fixation with these cations. Moreover, polymer absorbs water efficiently and holds more water and keeps P in available form that enhanced the plant growth and yieldcontributing factors [97]. This is because polymer-coated diammonium phosphate (DAP) absorbs water many times of its original weight, which increases the availability of phosphorus for longer period of time [107] and creates a diffusion shell around the grain of DAP and directly reduces the fixation and precipitation by reducing the availability of calcium and magnesium (Ca+2/Mg+2) cations [108]. As the result of this mechanism, availability of phosphorus to plants increases and leads to more P uptake, and this uptake indirectly influences the other nutrient absorption by crop plants.

#### **5.6 Use integrated soil nutrient management practice**

Considering the wide variety of soil types, cropping patterns, and farmers' resources, several management practices are adopted to reduce the magnitude of soil fertility degradation. Integrated Plant Nutrient Management System (IPNMS) is defined as the package of practices for the manipulation of the plant growth environment to supply essential nutrients to a crop in an adequate amount and proportion for optimum production without degrading the natural resources [3]. Many authors have reported that combining organic and inorganic P can improve and sustain crop yields in low fertility soils [109–111]. Best management practices (BMPs) such as use of fertilizer and amendment (lime), proper crop rotations, increases in organic matter content, and control of erosion, insects, diseases, and weeds can significantly improve crop yields and optimize nutrient use efficiency [11]. Integrated use of organic manures and fertilizers not only improves efficiency of crops but also significantly increases the availability of P [112, 113].

Organic amendment improves the structure and fertility of the soil by adding nutrients and organic matter and consequently promotes soil microbial biomass and activity. Blockage of P sorption sites by organic acids, as well as complexation of exchangeable Al and Fe in the soil, is potential cause of this mobilization [114]. Organic materials can reduce P fixation by masking the fixation sites on the soil colloids and by forming organic complexes or chelates with Al, Fe, and Mn ions, thereby improving P uptake efficiency of crop plants. Decomposition of organic matter produces organic anions that interact with soil to reduce P sorption via (1) complexation/competition for soil P binding sites such as Fe and Al oxyhydroxides or (2) increased soil PH. Organic materials also increase agronomic efficiency by improving availability of P by promoting soil aggregation, increased soil PH, microbial biomass, and parameters controlling soil-P-sorption [115]. The integration of biochar FYM, poultry manure, and inorganic P sources increases in PUE under both wheat and maize crops, and there is a concomitant increase in crop yields compared with the unamended soil [112, 113]. This increase in PUE with biochar addition could also be the result of the additional nutrients made available by biochar [112]. Similarly, FYM applications increase soil P bioavailability more than applications of triple supper phosphate that enhance P Uptake Efficiency. FYM is also a source of other nutrients used by crops via mineralization, which promotes root development and root area interception and thus increases nutrient uptake including P uptake [116].

Rotating a legume with a cereal can enhance P acquisition by cereals through indirect feedback interactions [117]. A legume crop modifies the rhizosphere through biological and chemical processes, thereby increasing P uptake by the following cereal crop. As reported by [77], legumes are able to mobilize P that is not initially available to cereal species, thereby improving the availability of P for the following crop. The biological processes include the promotion of symbiotic mutualists such as nitrogenfixing rhizobacteria and mycorrhizal fungi, while the chemical processes are acidification of the rhizosphere and secretion of organic anions [79].
