Key findings:

Nano NPK fertilizers significantly enhance wheat growth and productivity compared to conventional NPK fertilizers. The reviewed studies indicate that nano fertilizers improve nutrient efficiency, promote better growth, and increase grain yield, highlighting their potential for boosting wheat production in various environmental conditions.

What is known and what is new?

Wheat is a crucial cereal crop in Iraq and worldwide, with fertilizers playing a vital role in enhancing its growth and grain production. Traditional NPK fertilizers have been widely used to improve wheat productivity, particularly with high-yielding and fertilizer-responsive cultivars. This literature review highlights the emerging use of nano NPK fertilizers, which are shown to significantly improve wheat growth and productivity compared to conventional NPK fertilizers. The review provides evidence that nano fertilizers enhance nutrient efficiency, promote better plant growth, and increase grain yield across various environmental conditions, offering a promising new approach to boost wheat production.

What is the implication, and what should change now?

The findings suggest that adopting nano NPK fertilizers can significantly enhance wheat growth and productivity, providing a more efficient and effective alternative to conventional fertilizers. This can lead to improved agricultural outputs and greater food security, particularly in regions like Iraq where wheat is a strategic crop. Agricultural practices should shift towards the use of nano NPK fertilizers to maximize wheat production. Farmers and agricultural professionals should be educated about the benefits and application methods of nano fertilizers.

Cereals crops are the greatly crops in the world, that half the harvested area was planted with cereals in the world in year 2018 [1]. Wheat is most important cereals cultivated in the world and Iraq because of its strategic role in achieving food security (Al-Saidan et al., 2019 and Noaema et al., 2020) [2,3], it is the important staple food for the Iraqi people which meets most of the protein requirement of the people. Wheat is the first most important cereal crops in Iraq because its cultivation holds the first position among the cereal crops in the area and production (Al-Abody et al., 2021) [4], also the wheat production has been increased in recent time. Wheat grain yields production in Iraq assessed by 6.238 million tons in year 2020 with increase 43.6 % compared to production in year 2019. Area was cultivated with wheat crop assessed 2.144 million hectares increased by 35.4 % compared to year 2019, average grain yield per hectares assessed 2.94 tons in year 2020 basis on the harvested area [5].

In general, Iraq's soil is characterized low in their content of organic matter, nitrogen, and lack in phosphorus availability and micronutrients. As a result of the low level of organic matter in Iraq's soil the content of total nitrogen is very low; therefor it must be provided soil or plant by nitrogen element to achieve high productivity. Available phosphorus for absorption by plant roots in Iraqi soil is also very low so there is a need to add phosphorus fertilizer to access the required productivity. There was a common belief that Iraq's soil was like the rest soils of the dry areas is rich in potassium, but researches results confirmed that this belief is not an absolute truth and that potassium adequacy in the soil depends on pattern and density of adopted agriculture, dense agriculture and accredited high productivity varieties requires the addition of potash fertilizers for required productivity [6]. One of the reasons for lower grain yield of wheat in Iraq is due to the fertilizers application, particularly balanced fertilizer application which is very important for higher production of wheat crop [7].

Despite the availability in mineral chelated fertilizer sources for macro and micro nutrients and availability of different application methods buy soil or foliar application or both ways but the effectiveness of this fertilizers dose not exceed 5% of the added amounts (Ali and Al-Juthery, 2017) [8] which repeated the addition of fertilizer for more than once (AL-Saray, 2019) [9]; Therefore the usage of nano fertilizer in fertilization program is an effective alternative to the conventional fertilizers, where many advantages are achieved due to their use in lowest amounts and higher stability under different conditions [10]. Conventional chemical fertilizers are not only expensive for the producer, but may be harmful to humans and the environment; this has led to the search for environmentally friendly fertilizers or smart fertilizer, mainly those with high nutrient use efficiency, and nano fertilizers are rising as a promising alternative [11]. 

About the 50% of increase in crops yield can be an escort to the fertilizers plugins alone on condition the availability of other growth factors in optimum form (Ali, 2012) [6]; therefore modern technologies must be adopted to ensure increase the crops yield per unit area such as usage of the nano technology for optimal use of chemical fertilizer technology in Iraqi agriculture, especially with strategic crops like wheat crop. The aim of the present review study is highlights the key role of nano NPK fertilizer in improve growth and productivity of wheat under different environmental conditions.

Nano fertilizers in agricultural production

Nano fertilizers are the ones that are the diameters of their granules less than 100 nm which are in form macro or micro elements or loaded with nanoparticles which has proved its effective compared to conventional fertilizers [12]. Nano fertilizers are considers one of the most important nano technology products in agricultural production field (Kandil and Marie, 2017) [13], it is more efficient than conventional fertilizers for it increases in fertilizers use efficiency, reduce fertilizers losses and reduce environmental pollution [14]. Nano fertilizers are manufactured from conventional fertilizers whether organic, inorganic, rocky materials or extracted from different plant parts by different methods such as physical, chemical, mechanical, biological and hybrid methods with the help of nanotechnology used to improve soil fertility and agricultural products productivity and quality [15].

Fertilizers play a pivotal role in 35 to 40% agricultural production, to enhance nutrients use efficiency and overcome the chronic problems the nano fertilizers might be a best alternative, where the nano fertilizers are synthesized in order to regulate the release of nutrients depending on the requirements of the crops [16]. To minimize nutrients losses in fertilization and to increase the crops yield through the exploitation of new applications with the help of nanotechnology and nano materials (Sddiqui et al., 2015) [17], where that nano fertilizer are consider one of the most important nanotechnologies applications, comparing to conventional fertilizers requirement and cost are economically cheaper and are required in lesser amount (Manjunatha et al., 2016) [18] and then reduce the costs of agricultural production. 

Role of nano fertilizers in plant nutrition 

Nano nutrients are available nutrients in nano scale which helps in the more absorption and use of nutrients such as Nitrogen, Phosphorus and Potassium, these nano nutrients are developed which can be utilized for crop improvement and provides balanced levels of nutrients required for the better growth and productivity of crops [19]. Nano fertilizer plays a vital role in plant nutrition whether used as foliar or soil application, it helps to increase photosynthesis process activity, crop capacity to afford different stress conditions, crop resistance to diseases and increase effective materials in plant [20].

The macro elements contribute more than 95% of plant biomass (Barati, 2010) [21], Nitrogen, Phosphorus and Potassium are consider one of the most essential nutrients needed by plants in large amounts, where enters in many physiological function in plant, and that its use in a large amounts may increase vegetative growth, but decrease the yield in addition to increased environmental pollution [22]. Due to the plant needed for these nutrients (NPK) in large amounts relatively to complete its life cycle, therefore they most are provided in appropriate amounts to get the best productivity in terms of quantity and quality [3].  Rameshaiah et al. (2015) [24] reported that using of nano fertilizers increases the nutrients use efficiency by three times and it also provides stress tolerating ability.

Features of nano fertilizers

Usage of nano fertilizers is consider an effective substitute for chemical conventional fertilizers, where many features are achieved such as used in a few amounts, higher stability under different conditions, with small size therefore do not need large apace, quick absorption, help protect the environment and human health, increases farmers profitability to reduce fertilization and spray expenses, helps overcome soil and water pollution problems, and seeds soaking in nano fertilizers leads to improve their germination and increase the seedling vigor, if it is used by foliar application the plant will benefit from it faster [20,10]. Moreover its have a high surface space due to small particle size, it has a high ability to dissolve, easy to penetrate of the plant surface whether it is used was by soil or foliar application, improves nutrients uptake and use efficiency [15]. 

Also it is reducing environmental pollution and low economic cost which can be a successful alternative for conventional chemical fertilizers [12]. Guru et al. (2015) [25] stated that use of nano fertilizer mainly delay the release of the nutrients and extends the fertilizer effect period. On the other hand El-Sayed et al. (2020) [26] confirmed that nano fertilizers are more advantageous to the conventional fertilizers because they can triple the effectiveness of the nutrients; reduce the requirement of chemical conventional fertilizers, make the crops drought and disease resistant and less hazardous to the environment and they can easily get absorbed by plants due to their high surface area to volume ratio.

Nano fertilizers are considers a new generation of the synthetic fertilizers which contain readily available nutrients in nano scale range, and they are preferred largely due to their efficiency and environment friendly nature compared to chemical conventional fertilizers, where that 40 to 60% of the total world production depends on the application of fertilizers [11]. 

NPK fertilizer

Compound fertilizer is the fertilizer that contains two or three macronutrients N, P and K like NPK fertilizer [6]. About 40 to 70% of N and 80 to 90% of P and 50 to 70% of K of applied chemical conventional fertilizers are lost to the environment and cannot be absorbed by plants, which causes large economic and resource losses and also the environmental pollution [27]. Applying Nitrogen fertilizer to the soil is exposed to lose via leaching, denitrifcation process and volatilization which lead to loss of applied Nitrogen up to 50-70% and Nitrogen use efficiency do not exceed 30-35%, while Nitrogen use efficiency reached to 82% from Nitrogen nano fertilizer compared to conventional fertilizer (Urea) which reached to 42%, this means that increase of 40% in Nitrogen use efficiency by using nano fertilizers, also less than 1% of total Phosphorus in soils is availability for uptake by plants roots and the rest of the Phosphorus is not availability for plants (Fixed), and the fixed Phosphors can be controlled through nano-matrix (Subramanian and Thirunavukkarasu, 2017) [28], whereas Khan and Rizvi (2017) [29] mentioned that fertilizer use efficiency was 30-60% for N, 10-20% for P and 30-50% for K from conventional fertilizers and the rest of these fertilizers are lose to the environment which causes an economic loss in resources and the soil and water are exposed to pollution, so a nano fertilizers application can reduce the amounts of conventional fertilizers as a result of increased use efficiency for applied fertilizers. NPK fertilizer application is vital for the development of crop production, and the importance of NPK fertilizers lies in their role to supply the necessary nutrients for plant growth [30]. 

Role of nano NPK fertilization on wheat growth and productivity improvement

In general, the nutrient management should include the "Four R" of fertilizer use: apply the right nutrient, at the right rate, at the right time and in the right place for the selected crop [31]. Nethravathi et al. (2015) [32] concluded from their studies that cereal plants growth rate was more with using of nano NPK fertilizer than using of conventional NPK fertilizer and control treatment.

Abdel-Aziz et al. (2016) [33] in their studies were examined the effects of nano chitosan NPK fertilizer as foliar application on growth and productivity of wheat in sandy soil, and in clay and clay-sandy soil (Abdel-Aziz et al., 2018 a) [34], their studies included seven treatments of foliar spray with concentrations: (control (non-fertilized), normal NPK 10%, normal NPK 25%, normal NPK 100%, nano NPK 10%, nano NPK 25% and  nano NPK 100% ), fertilization of wheat plants was carried out three weeks after sowing date at three times and three weeks intervals. The results showed that the life cycle of the nano NPK fertilized wheat plants was shorter with the ratio 23.5% (130 days) than control and normal NPK fertilized wheat plants (170 days) from sowing date to harvesting stage, moreover the nano NPK fertilizer treatment with low concentration 10% significantly superiority over all other treatments and gave a highest values in growth and yield traits which included (with an increasing percentage in sandy soil only): root length, shoot length (11.06%), plant height (15.43%), leaf area, fresh and dry weight, spike length (37.5%), main spike weight (124.64%), No. of spikelets . main spike^-1 (26.98%), 100 grain weight (83.03%), No. of grains . main spike^-1 (26.69%), grain yield . plant^-1 (101.85%), straw yield . plant^-1 (23.26%) and crop yield . plant^-1 (89.37%), while the nano NPK fertilizer treatment with concentration 25% significantly gave a highest values in harvest index (63.82%). The increase in wheat growth traits may be due to the sprayed nano NPK fertilizer with low concentration 10% got absorbed through the stomata of wheat leaves and be translocate in the plant, as for the increase in yield traits the possible reason are that nano NPK fertilizer promotes the plant to absorb the water from soil and nutrients, then the photosynthesis is improved, where nano NPK fertilizer is considered the biological pump for the plants to absorb water and nutrients, the activity of water after nano NPK fertilizer application was increased and N, P and K elements were absorbed by the plants a long with the absorbed water, thus the productivity was also increased. They concluded that the low concentration 10% of nano NPK fertilizer show up the best growth and productivity results for wheat crop than of all nano and normal NPK fertilizer used in addition to control treatment in all soil types in their studies (sandy, clay and clay-sand).

As a complement to Abdel-Aziz et al. (2016) [33] and Abdel-Aziz et al. (2018 a) [34] studies Abdel-Aziz et al. (2018 b) [35] found that the least concentration 10% of nano NPK fertilizer foliar application significantly produced the highest total carbohydrate content as well as P and K contents in wheat grains cultivated in clay-sandy soil compared to all concentration of nano and normal NPK fertilizer application and control treatment as well as than cultivated wheat in another soils types (sandy and clay soil). The application of low concentration 10% of nano NPK fertilizer was more effective than other treatments (25 and 100% concentration), that was because below 100 concentration of nano NPK fertilizer perhaps made NPK more efficient and dissolved in water more effectively thus increased their uptake and activities resulting in improvement of wheat grain quality. The study suggested that NPK fertilizer in the nano scale forms is absorbed by wheat plants to a large extent unlike bulk NPK fertilizer, these particles proved effective in enhancing plant growth, development and yield. A lower dose of foliar nano NPK fertilizer application is proved to be significantly productivity. Nano NPK fertilizer could be used to improve the chemical composition of wheat grains (accumulation of carbohydrates), and the least concentration 10% of nano NPK fertilizer application was the best to obtain better wheat grains.

Al-Juthery et al. (2018) [36] revealed that the nano NPK (25% - 25% - 35%) fertilization as foliar application on wheat crop in a field experiment with concentration 100 ml (33.3+33.3+33.3) sprayed at start of the flag leaf stage (120 days after sowing date) and 150 ml (50+50+50) sprayed after 134 days from sowing date by application 400 L.ha^-1 of mixture was improved growth and productivity of wheat traits which included plant height, leaves content of (chlorophyll, N, P and K), spike length, 1000 grain weight, grain and biological yield, harvest index and grain protein percentage compared to control treatment (non-fertilization). The nano NPK fertilizer treatment gave 5.64 ton .ha^-1 grain yield VS 4.06 ton .ha^-1 for control treatment. The increase in previous growth traits due to the role of these nutrients in stimulating plant growth, and these essential elements are required for optimum growth of the plant to complete its life cycle (Ali, 2012) [6], then reflected on improve productivity of wheat. The results in this study revealed that there was a positive effect of combined nano (N+P+K) nutrients supply on growth and productivity of wheat in Iraq conditions compared with control treatment.

Gomaa et al. (2018) [37] investigated in effect of three foliar application levels of nano NPK fertilizer (100, 75 and 50 %) from the recommended rates on growth and productivity of wheat in Egypt. nano NPK fertilizer was sprayed tow times at rate of 5 kg.ha^-1, 75% from the recommended rates of nano NPK fertilizer was significantly gave a highest values in plant height, No. of spikes.m^-2, No. of grains.spike^-1, and 1000 grain weight, while 100% recommended rates of nano NPK fertilizer was significantly gave a highest values in grain and straw yield, harvest index and grain protein percentage compared to 50% from the recommended rates of nano NPK fertilizer which resulted the  lowest values for all above mentioned traits.

Yasser et al. (2018) [38] evaluated effect of using of Fe-NPK nano fertilizer (N=1.8%, P=1.4% and K=1.3%) on wheat crop grain yield and some nutrients compared to recommendation soil dose of conventional NPK fertilizer (as control treatment), four Fe-NPK nano fertilizer levels (0.1, 0.3, 0.6 and 0.9 g.L^-1) was used as foliar application every three weeks, and control treatment was received recommendation conventional NPK fertilizer as soil application (100 kg N.fadden^-1, 200 kg.fadden^-1 super phosphate 15.5% P2O5 and 50 kg K.fadden^-1), the results showed that with increasing Fe-NPK nano fertilizer concentrations grain yield and grain content of N, P, K and Fe increased, where 0.9 g.L^-1 significantly gave a highest values in mentioned traits for plants cultivated in sandy loam and sandy soil compared to control treatment which recorded a lowest values in all above mentioned traits. Nano fertilizer provides more surface area and increase metabolic reactions and nano fertilizers usage may increase photosynthesis process with increasing of dry matter and crop yields [15]. Application of nano Fe-NPK fertilizer enhanced the availability of nutrients to growing plants which increase photosynthesis rate, chlorophyll formation and dry matter production. They concluded from this study that supplemental of Fe-NPK nano fertilizer as foliar application can reduce the amount of conventional NPK soil fertilizer dose and this beneficial effect on the cost of agricultural production.

Burhan (2018) [39] studied effect of foliar feeding with liquid conventional and nano NPK fertilizer (20-9-10%) on wheat growth and productivity by using of seven fertilizer treatments: soil fertilization with conventional NPK according to recommendations as control treatment where included 400 kg urea.ha^-1 (46% N) – 200 kg.ha^-1 triple super phosphate (46% P₂O₅) – 100 kg.ha^-1 K₂So₄.ha^-1 (41.5% K), three concentrations of foliar application with nano NPK fertilizer by concentration 15% (750-90-600 mg.L^-1, 500-60-400 mg.L^-1 and 250-30-200 mg.L^-1) and three concentrations of nano NPK fertilizer by concentration 7.5% plus 50% of recommended conventional NPK fertilizer (750-90-600 mg.L^-1 of nano NPK + 50% of conventional NPK, 500-60-400 mg.L^-1 of nano NPK + 50% of conventional NPK and 250-30-200 mg.L^-1 of nano NPK + 50% of conventional NPK). The results indicated that foliar fertilization with nano NPK by concentration (750-90-600 mg.L^-1) significantly superior over all treatments for wheat growth and yield index which included No. of tillers.m^-2, flag leaf (area, dry weight, and content of total chlorophyll, N, P and K), the duration of survival of a green flag leaf, No. of spikes.m^-2, No. of grains.spike^-1, grain yield (5.93 ton.ha^-1), grain protein content (13.59%) and grains flour gluten content (19.68%), while foliar application of nano NPK fertilizer with concentration (250-30-200 mg.L^-1) significantly gave a highest plant height, and the concentration (500-60-400 mg.L^-1) of nano NPK fertilizer significantly gave a highest specific weight of flag leaf, whereas the treatments of (750-90-600 mg.L^-1 of nano NPK + 50% of conventional NPK) and (500-60-400 mg.L^-1 of nano NPK + 50% of conventional NPK) significantly gave a highest grains carbohydrate content.

Burhan and AL-Hassan (2019) [40] confirmed that percentage of increased in wheat grains yield was (48.99%), grains protein content (27.24%), gluten ratio in flour (58.45%), flag leaf area (38.69%), flag leaf dry weight (41.89%), flag leaf total chlorophyll content (18.33%), leaves nitrogen content (19.37%), leaves phosphorus content (44.11%), and leaves potassium content (12.03%) when the wheat crop was fertilized with nano NPK (20-9-10%) foliar fertilization by concentration (750-90-600 mg.L^-1) compared with the conventional NPK soil application (184 N – 39.56 P – 41.50 K kg.ha^-1). The diameter rate (the rate of granules size) for liquid nano NPK fertilizer (20-9-10%) was determined by Mohammed (2020) by using of atomic force microscope (AFM), where it's reached to (30.50 – 60.75 – 25.01 nm) for nano N-P-K, respectively as shown in Fig. 1,2 and 3. 

Fig.1: shows the size and distribution of nano N fertilizer molecules. From Mohammed (2020)

Fig.2: shows the size and distribution of nano P fertilizer molecules. From Mohammed (2020)

Fig.3: shows the size and distribution of nano K fertilizer molecules. From Mohammed (2020)

Al-Obaidi (2021) [41] examined effect of wheat grains soak in balanced nano NPK fertilizer (20-20-20) for 12 hours on growth, productivity and quality in tow field experiments under rain fed and supplementary irrigation, wheat grains soak in (water only (as control treatment), 2000 and 4000 ppm nano NPK fertilizer) was examined. Grains soak in nano NPK fertilizer with concentration 2000 or 4000 ppm significantly increased laboratory germination ratio which reached to 100% compared to control treatment which was 97%, the grains soak in 4000 ppm nano NPK fertilizer caused significantly increase in root length and root dry weight, moreover the wheat plants were resulting from grains soak in nano NPK fertilizer with concentration 2000 or 4000 ppm significantly gave a highest average in plant height, No. of tillers.m^-2, flag leaf area, flag leaf chlorophyll content index, spike length, No. of grains.spike^-1, 1000 grain weight, grain yield.m^-2, biological yield.m^-2, harvest index, straw yield.m^-2, grain protein percentage, grain gluten percentage and grain specific weight compared to control treatment which gave a lowest average in all above mentioned traits. Nano NPK fertilizer was played a vital role in providing essential nutrients and facilitated it for use in seedling growth as well as acceleration of its growth, wheat grains soak in nano NPK fertilizer before sowing had the biggest impact and was the most pronounce in improving plant growth and increase yield and its components starting with its early role in an increase flag leaf area which produced a significant increase in yield components down to increase grain yield. It is likely that the effect of the grains soak in nano NPK fertilizer extends to flag leaf emergence stage which will complete their role in the supply of developing spikes with nutrition materials and then an increase in grain yield for for crop. It was suggested from this study follow the soak of wheat grains in concentration 2000 ppm of nano NPK fertilizer for 12 hours before sowing to achieve highest grains yield with best quantity.

Mohmmed (2021) [42] used three levels (0, 2 and 4 g.L^-1) of balanced nano NPK fertilizer (20-20-20) as foliar application on wheat plants at tow growth stages tillering and flowering, foliar application of 4 g.L^-1 nano NPK fertilizer was caused a significant earliness in No. of days to 50% spike appearance and to physiological maturity, also significantly gave a highest average in plant height, flag leaf area, flag leaf chlorophyll content, No. of tillers.m^-2, spike length, spike weight, No. of spikes.m^-2, No. of speklite.spike^-1, No. of grains.spike^-1, 1000 grain weight, grain yield, biological yield and grain protein percentage compared to zero and 2 g.L^-1 nano NPK fertilizer application. The study confirmed that foliar application of nano NPK fertilizer as supplementary fertilization was a good factor to improve wheat growth and productivity under Kirkuk Governorate environmental conditions, Iraq. 

On the basis of the previous a literature review and studies on usage of nano NPK fertilizer it was confirmed that nano NPK fertilizer is more efficient than conventional NPK fertilizer and has potential to improve wheat crop growth and productivity under different environmental conditions, so usage of nano NPK fertilizer lead to improving growth, yield and grain quality of wheat whether using it by grains soak or foliar application. Also many of previous studies demonstrated that supplemental of conventional NPK fertilizer as foliar application can reduce the amount of conventional NPK fertilizer rate as soil application and this reduces costs of agricultural production.

Conflict of Interest: The authors declare that they have no conflict of interest.

Funding: No funding sources 

Ethical approval: The study was approved by the Institutional Ethics Committee of Universitas Negeri Makassar

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