A novel poly(acrylic acid-co-acrylamide)AlZnFe₂O₄/K-H superabsorbent hydrogel nanocomposite was synthesized for agricultural use and SEM analysis of the synthesized nanoparticles was carried out (Figure 1). This analysis showed that the mean diameter of the nanoparticles is 50 nm. The chemical composition of the nanoparticles was determined by Energy Dispersive X-ray analysis (EDX), which showed that nanoparticles contain only Fe, Zn, Al and O with no traces of by-products (Figure 2). A scanning electron micrograph of superabsorbent PHNC is shown in Figure 3.

It is obvious from Figure 5 that the increase in water retention of soil depends on the quantity of the PHNC used and the highest value of moisture retention was achieved with the addition of 0.4 w/w% PHNC in the soil. In close agreement to our present data, Dorraji et al. [22] achieved moisture retention as high as 0.6 w/w% with the use of hydrophilic polymer in sandy and loamy soils.

The sandy loam soil amended with 0.1 to 0.4% of PHNC possessed a good capacity for water retention at field capacity (0.03 MPa pressure) even four weeks after initial watering (Figure 5) [2,6,8]. This remarkable increase in soil water retention might be attributed to the hydrophilic polymer network and introduction of adequate amount of potassium humate in the superabsorbent hydrogel polymeric network which enhanced hydrophilicity of PHNC. However, a gradual decrease in water contents of soil amended with 0.1 to 0.4 w/w% PHNC was observed which finally approached 5 to 8 g/100 g after 30 days. This gradual decrease in water contents can be linked to the water uptake by the plants and some transpiration. The previous studies reported in the literature verify this phenomenon [2,22,23].

The pH and EC are important factors of the soil chemical, physical and biological properties [23]. The pH and EC varied with the application of PHNC (Figure 6 and Figure 7). Soil pH was reduced by 2 to 5% at concentration of 0.1 to 0.4%, compared with the control. Meanwhile, the electrical conductivity (EC) of PHNC amended soil increased about 6 to 57% (after 1st, 2nd and 3rd hydration) as compared to the control at concentrations 0.1 to 0.4%. The decrease in pH of the soil might have enhanced the discharge of soil inorganic salts thereby increasing EC of the soil. Similar effect on the pH and EC of soils due to the chemical structure of the superabsorbent polymer and soil characteristics has previously been appraised by Liu et al. [19] and Bai et al. [24], while studying the characteristics of chitosan-graft-poly (acrylic acid)/sodium humate superabsorbent.

Bulk density has relationship with other properties of the soil such as porosity, moisture, and hydraulic conductivity. The maintenance of adequate bulk density is an important objective in agriculture. The soil bulk density varied with soil moisture (Figure 8) and decreased by 9 to 22% (after 1st, 2nd and 3rd hydration) at PHNC concentrations 0.1 to 0.4 w/w%, respectively, probably due to the swelling of soil with the incorporated superabsorbent polymer embedded with potassium humate. The decrease in soil bulk density to different extent is in agreement with the studies appraised by Liu et al. [19]. The soil porosity increased by 9 to 36% (Figure 9) at hydrogel concentrations 0.1% to 0.4 w/w%.

The addition of 0.1, to 0.4 w/w% of PHNC reduced the hydraulic conductivity 19 to 65% (after 1st, 2nd and 3rd hydration). A remarkable decrease in hydraulic conductivity (Figure 10) was observed with the increase in the concentration of PHNC super absorbent hydrogel nanocomposite. Studies by Bhardwaj et al. [25] and El-Shafei et al. [26] in sandy soils showed almost similar effects as those observed in the recent study. However, they applied polyacrylamides/gel-conditioner in sandy soils and sprinkler irrigation system. The newly synthesized PHNC enhanced the moisture retention of sandy loam soils and plant available water significantly, thereby slowing down the rate of moisture loss, due to which a delay of 6 to 9 days in wilting point was observed. Such a delay in wilting point reduces the water requirement of plants [2,27,28].

The water potential of soil strongly affects seedling emergence and plant establishment [29]. The enhancement in soil moisture retention with superabsorbent hydrogel amendments, improvement in seed germination and seedling growth, have been reported in literature, however there are variations due to the individual superabsorbent hydrogel materials [2]. In the present study seed germination was considerably higher in 0.2 to 0.4% PHNC amended soils as compared to control. The wheat seedling growth was enhanced by the addition of PHNC. Besides, wheat shoot length was significantly higher at 0.3 and 0.4% PHNC compared with other PHNC levels. The addition of 0.3% and 0.4% PHNC in sandy loam soil significantly increased the fresh and dry weights of wheat shoots (Table 1).

The maintenance of proper soil moisture is a prerequisite for soil water retention for horticulture purposes. The moisture contents of soil increased with amendment of PHNC at field capacity of sandy loam soils. Hence a significant increase in plant available soil water (AW) was observed with the addition of PHNC compared with untreated soils (Figure 5). At the same time, the amendment with PHNC decreased the hydraulic conductivity and slowed the rate of soil moisture loss thereby delaying the wilting of seedlings. The onset of permanent wilting point (PWP) was delayed by 6 to 9 days with PHNC concentration 0.1 to 0.4%, respectively. Similar effect on the wilting point of barley and chickpea has been reported by Akhtar et al. [2] in loam soil.

In the present study the effect of PHNC on different attributes of soil water retention of sandy loam soil amended with different concentrations (0.1, 0.2, 0.3 and 0.4 w/w%) of PHNC was estimated at field capacity (0.03 MPa pressure). The results obtained (Figure 6, Figure 7, Figure 8, Figure 9, Figure 10) reveal that the applied PHNC had a positive effect towards improving the soil characteristics, i.e., the values of bulk density and hydraulic conductivity were decreased, whereas total porosity and moisture retention at field capacity were increased with increasing the concentration of PHNC in sandy loam soil. In comparison to our previous study on sandy loam soil amended with SHNC (when moisture was retained for three weeks only) [30], the same soil amended with the newly developed PHNC retained the moisture for more than four weeks. This increase may be attributed to the increase of storage pores in the sandy loam soil which can be regarded as an index of an improved soil structure. The pronounced decrease in hydraulic conductivity of the sandy loam soil may be attributed to the creation of micropores, and the dominance of meso- and micropores. These results are in agreement with those of El-Fayoumy and Ramadan [31] who applied organic soil conditioners surpassed K-humate for improving the soil hydrophysical properties. This was true, since the active -OH and -COOH represent pronounced values and had a profound effect on soil structure as reported by Moustafa et al. [32].

Data illustrated in Figure 6, Figure 7, Figure 8, Figure 9, Figure 10 revealed that potassium humate containing basically humic acid when added as individual treatment or combined with other organic soil conditioners surpassed the other treatments for enhancing the availability of essential plant nutrients (N, P, K, Fe, Mn and Zn). This is true, since humic acid partially is capable to retain nutrients for growing plants, where it can act as complexing agent [33]. Enhanced plant growth with the addition of humic substances in soil is related to increase micronutrient availability especially that of iron and zinc. Soil pH and organic matter content significantly affect the solubility of Fe, Mn, Zn and Cu [34].

Humic acid can incorporate iron into the chelate, maintaining its availability to plants, even in insoluble form [35]. Therefore, these chelating agents, through active groups for micronutrients, are considered as a storehouse with easily mobile or available to uptake by plant roots, and in turn reflect positively on development of yield and its attributes for the studied crops.

It is worth mentioning that the positive effect of organic soil conditioners may be due to these organic soil amendments which enhanced crop production and fertilizer uptake by plants through the improvement of hydrophysical properties and thus increased soil ability to supply plants with their requirements of water and air which, consequently, stimulates root growth and the activities of beneficial microorganisms [36].

The above mentioned results indicated that the organic soil amendments affect directly or indirectly the plants nutrients uptake. This means that the applied organic soil amendments are considered as a storehouse with easily mobile or available nutrients to be taken by plant roots. Consequently, these benefits are reflected positively on development of yield. Also, these findings suggest an important role for K-humate in improving the efficiency of nutrient uptake, and in turn increasing the quantity and quality of wheat. The present results confirmed the findings of Mackowiak [33] and Madlain [37] who reported that the beneficial effect of humic acid on dry matter yields may be attributed to improving the bio-availablity of micronutrients by complexion, which prevent early micronutrient deficiency.

All the chemicals including zinc oxide, aluminum oxide, sodium hydroxide, ferric oxide, acrylic acid, acrylamide, potassium persulphate, potassium metabisulphite and potassium humate were analytical reagent grade purchased from Sigma Chemical Co. (St. Louis, MO, USA). Sandy loam soil (sand 59%, silt 21%, clay 19%, pH 7.5, EC 1.92 dS/m) was collected from the Postgraduate Agricultural Research Station (PARS), Jhang Road, Faisalabad (31°26'N, 73°06'E), Pakistan. The soil was air dried, ground, and passed through a 2 mm sieve. The soil fractions of less than 2 mm were used in the experiments.