A New Method to Recycle Dairy Waste for the Nutrition of Wheat Plants

Dairy products are vital components of human food, however, they are rapidly spoiled due to their high content of organic matter which encourages the growth of decomposing microbes. The recycling of dairy wastes is an ideal solution to preserve the environment, as it is in line with the principles of sustainable agriculture. In this experiment, an organic fertilizer was extracted from dairy wastes and was used for the nutrition of wheat grown on sandy soils under two-year field studies. The application rate of the extracted organic fertilizer was 8 ton ha−1. Moreover, the same rates of N, P, and K were added from inorganic fertilizers. The extracted organic fertilizer significantly (p < 0.05) enhanced the wheat growth and increased chlorophyll by 11% and 16% in the first and second season, respectively, in comparison to the inorganic fertilization. The extracted organic fertilizer significantly minimized the soil pH from an initial value of 8.00 to 7.05. The tested organic fertilizer increased the uptake of N, P, and K by 55%, 49%, and 51% above the inorganic nutrition, respectively. The wheat straw and grain yield increased by 16% and 29% as a result of the addition of the organic fertilizer extracted from dairy wastes. The dairy wastes organic fertilizer caused a notable improvement in the soil quality. The extracted organic fertilizer was able to supply wheat with its nutrient requirements as it showed a remarkable superiority over the mineral fertilization. The disposal of expired dairy waste can be managed in a way that preserves the environment by converting it to organic fertilizers. Laboratory and field experiments have proven the efficiency of the extracted organic fertilizer in nutrition of wheat plants in sandy soils with low fertility.


Introduction
Wheat is the most strategic cereal crop for ensuring global food security and is a major source for human food and livestock feed. The high growth and yield of wheat depend mainly on suitable agriculture management, especially concerning soil fertility [1]. The ideal solution to the global food shortage is the cultivation of the desert. The main problem for the cultivation of desert land is its inability to retain water and its low fertility, which increases the need for mineral fertilization, which thus increases the costs along with the environmental pollution [1][2][3]. Increasing the productivity of the degraded sandy soils requires the use of organic fertilizers to reduce the unfavorable soil conditions. Dairy wastes are rich materials with organic carbon and essential elements, and they must be recycled to reduce environmental pollution [4]. Due to their importance as a human food, dairy products must garner more interest from scholars. The world produces more than 850 million tons yearly, and a large percentage of dairy products loses its validity before use; hence, the disposal of these huge quantities becomes an environmental problem [4]. The disposal of expired materials must be given special care and be conducted well to preserve the safety and health of humans and the environment [5]. The disposal of the dairy industry wastes is a crucial issue facing the dairy industry due to the large and renewable quantities involved [6]. Dairy factories deal with the expired products in several ways, including: addition to agriculture soils, disposal at landfills of municipal wastes, and transferring to special station units for solid and liquid waste stations [7]. The main problems that restrict the direct application of diary wastes to agriculture soil are the high microbial content and the rich soluble organic matter that may encourage the growth of some unfavorable microbes, thus causing harm to the growing plants [4,6,7]. Moreover, most of the dairy products contain a high levels of fats-up to 8% (dry weight bases)-and most of the fatty compounds are slow to dissolve in the soil, which reduces the benefit from diary wastes [4,8].
Suárez et al. [9] used dairy wastes in the nutrition of Italian ryegrass (Lolium multiflorum Lam.) grown on acid soil for a nine-week trial. The dairy wastes increased the growth and nutrients uptake by Italian ryegrass compared to the mineral nutrition [9]. In the same trend, Eissa et al. [4] confirmed these results by using wheat plants in a pot experiment grown on sandy soil for only 60 days. The use of pot experiments in the evolution of dairy wastes as a fertilizer is not enough and the evaluation must depend on the result of field experiments. Only Vasilyev et al. [10] used the dairy wastes in nutrition of wheat under field scale.
Most of the previous studies concentrated on the use of dairy wastes from the manufacturing process, and little is known about the use of expired dairy products. Moreover, most of these studies used the dairy wastes directly without any treatments. This study presents a simple and novel method for extracting an organic fertilizer from expired dairy wastes. Our study aims also to evaluate the efficiency of this fertilizer compared to mineral fertilization under the scale of pot and field conditions.

Preparation and Characterization of the Organic Fertilizer from the Dairy Wastes
Expired milk, yogurt, and cheese were mixed thoroughly, and then water was added at a ratio of 1:5. The mixture was left for 10 h without stirring on bench at 25 • C. Based on the gravity theory, three layers generated from top to bottom including: fat, water, and protein. The protein layer was extracted after removal of the fat layer by filtration through clean fine clothes and then oven dried (60 • C) and ground. The procedure of the extraction is described in Eissa et al. [4].
The chemical properties of the extracted organic fertilizer were evaluated based on the standard methods of Burt [11] and Parkinson and Allen [12]. The total N, P, and K were analyzed by the standard methods of Burt [11]. The main chemical characteristics were as follow: pH of 4.50, organic carbon of 450 g kg −1 , total N, P, and K of 50, 15, and 10 g kg −1 , respectively.

Incubation Experiment
The soil used for the incubation experiment was collected from a private farm with a sandy texture and classified as Typic Torripsamments [13]. The air-dried soil was placed in plastic pots. Some soil physical and chemical properties of the experimental site are summarized in Table 1. The tested treatments were: untreated soil without any addition, inorganic fertilization (a mixture of inorganic fertilizers at a rate of 400 mg N + 150 mg P + 100 mg K/kg soil), and the third treatment was the newly extracted organic fertilizer at a rate of 8 g/kg soil. The amount of N, P, and K in the inorganic fertilization and dairy product treatment was equal. Commercial inorganic fertilizers of urea (46% N), super phosphate (6.8% P) and potassium sulphate (40% K) were used in the second treatment. The treatments were applied and mixed with the soil, then incubated for 3, 6, 10, 15, 20, 25, 30, 40, 50, and 60 days at 25 • C. The pots were irrigated to near field capacity based on the pot weight. After the incubation period, the soil samples were collected to measure the available nitrogen, phosphorus, and potassium according Burt [11]. The soil pH and organic carbon were measured in the incubation experiment based on the standard methods of Burt [11]. Table 1. Main properties of the studied soil.

Field Experiments
Seeds of wheat (Triticum aestivum L. variety Giza 164) were cultivated in experimental unites of 20 m 2 at a rate of 100 kg ha −1 . The experiment was repeated for two seasons in same the experimental units during 2018 and 2019 seasons. The studied treatments were: untreated soil without any addition, inorganic fertilization (a mixture of inorganic fertilizers), and the organic fertilizer extracted from the dairy wastes. Rate of nitrogen, phosphorus, and potassium additions were 400, 150, and 100 kg ha −1 of N, P and K. The expired dairy products were applied at a rate of 8 ton ha −1 . The treatments of the field experiments contained the same amount of nitrogen, phosphorus, and potassium. The climatic conditions of the field experiment were as follow: maximum and minimum temperature of 19-30 and 7-18 • C, relative humidity of 40-50%, and no rainfall during the experimental periods. Wheat plants were cultivated on December of 2018 and 2019 growing seasons and were harvested on May. At harvest, the grain and straw were recorded as well as some yield attributes.

Soil and Plant Analysis
The soil samples were air-dried and sieved by a 2-mm sieve. The tested physiochemical properties of the studied soil were determined according to Burt [11]. Total soil nitrogen was determined using the micro-Kjeldahl method [11]. Available soil phosphorus was extracted by 0.5 M sodium bicarbonate solution at pH 8.5 according to and phosphorus was determined by spectrophotometer [11]. Available soil potassium was extracted by NH 4 OAC (1 N, pH 7), method and was measured by flame photometer [11].
Wheat plants samples, (1/2 m 2 ) from each experimental unit, were collected after 70 days of cultivation. The uptake of N, P, and K were measured in the collected plant samples by calculating the dry matter multiply in the nutrient concentration. The oven dried (70 • C) plant samples were ground and digested with H 2 O 2 and concentrated H 2 SO 4 [14]. Nitrogen, phosphorus, and potassium in the digest of plant samples were determined according the standard methods described by [11]. Chlorophyll content (SPAD value) in leaf was measured using the SPAD device (SPAD-502-m Konica Minolta, Inc., Tokyo, Japan).

Data Analysis
The experimental design of the incubation experiment was randomized complete design, while the field trials were laid out in a randomized complete block design. Each treatment replicated five times in both incubation and field experiment. The analysis of variance (ANOVA) and Duncan multiple range tests (p ≤ 0.05) were performed by SPSS, version 15 after homogeneity test.

Effect of Organic and Inorganic Fertilization Treatments on the Growth and Yield of Wheat Plants
The extracted fertilizer was added to wheat plants grown on the sandy soil under field scales. The growth parameters recoded from the field trials are shown in Table 2, while the yield of straw and grain is shown in Figure 1.  The growth of wheat plants responded significantly (p < 0.05) to the organic and inorganic nutrition compared to the untreated soil. The extracted organic fertilizer significantly (p < 0.05) enhanced wheat growth and increased chlorophyll by 11% and 16% in the first and second season, respectively, in comparison to the inorganic fertilization. The extracted organic fertilizer enhanced the growth of wheat plants significantly over the inorganic fertilization and the untreated soil. The application of the extracted organic fertilizer increased the number and length of spikes compared to the inorganic fertilization and the control soil. The weight of 1000 grains increased significantly as a result of organic fertilization. The extracted organic fertilization increased the growth of wheat as well as the yield attributes.
Based on the data of the field studies, wheat plants that were fertilized with the dairy wastes organic fertilizer produced higher grain and straw yield than that were received the full mineral nutrition (Figure 1). The tested organic fertilizer enhanced the wheat grain yield by 22% in the first year and by 35% in the second year above the inorganic fertilization. In the same trend, the addition of organic fertilizer derived from the dairy wastes caused increases in wheat straw yield by 15% and 17% above the inorganic fertilization.

Effect of Organic and Inorganic Fertilization on Some Chemical Properties and Nutrients Uptake
The influence of organic and inorganic treatments on the availability and uptake of N, P, and K was investigated in an incubation and field experiment, Figure 2 and Table 3 illustrated the obtained results. Addition of the tested organic fertilizer significantly increased the uptake of N, P, and K. The extracted organic fertilizer caused 57%, 47%, and 52% increases in the uptake of N, P, and K uptake above the mineral nutrition in the first season, while these increases were 53%, 51%, and 49% in the second season. Although the inorganic fertilization increased the nutrients uptake over the control, the extracted dairy wastes organic fertilizer exhibited a great superiority over the mineral nutrition in that respect.  The soil and plant samples were collected from the field experiment after 70 days of cultivation. Means (± SD, n = 5) with different letters indicate significant difference. P = the p-value of the ANOVA test.
The added organic fertilizer had clear effects on improving the organic carbon in the sandy soil ( Figure 2). Moreover, the extracted organic fertilizer had a clear effect in the soil pH ( Figure 3). The soil pH reduced significantly when the soil was amended with the extracted organic fertilizer compared to the untreated soil and the inorganic fertilization. The extracted organic fertilizer significantly minimized the soil pH from to 7.05 in the end of the incubation experiment. The highest pH values were recorded in the control and inorganic fertilization treatments. Addition of the investigated organic fertilizer enhanced the soil organic-C from 3.6 to 7.0 g kg −1 (Figure 2). Addition of organic fertilizer to the tested sandy soil increased the soil available N and P by 13% and 83% compared to the full mineral nutrition at the end of the incubation experiment. The obtained results of the incubation experiment for the release of N, P, and K are shown in Figure 3. Nitrogen and potassium release from the organic and inorganic treatments showed the same behavior, and both of the two treatment were higher than the control. The P availability of the organic fertilizer was higher than the inorganic one in the incubation periods above 25 days.

Response of Wheat to the Organic Fertilizer of Dairy Wastes
The developing countries suffer from the population increase and increased demand for food, thus they must focus on the cultivation of sandy soils which are a promising solution for horizontal expansion and increasing the agricultural land area. One of the most negative characteristics that limit the reclamation of these lands is their low organic matter content [2][3][4][14][15][16]. The response of wheat plants to an organic fertilizer derived from the dairy wastes was investigated under field experiment scale. The extracted organic fertilizer has clearly shown its superiority over mineral fertilization in enhancing the growth of wheat cultivated on the sandy soil. Based on the obtained results, it was clearly that the superiority of the extracted organic fertilizer was due to its ability to improve the organic matter by about 38% over the inorganic fertilization. Increasing the soil organic matter enhanced the growth and yield [4,17,18]. The growth of Italian ryegrass (Lolium multiflorum Lam.) increased significantly in the soil amended with dairy wastes over the untreated soil [9]. The ability of the dairy waste to supply Italian ryegrass with its nutrient requirements was equal to the mineral fertilization, but it was a pot experiment for only nine weeks. Eissa et al. [4] found the same results by using 60 day-old wheat plants in a pot experiment. The results obtained from Eissa et al. [4] and Suárez et al. [9] need to be confirmed under field conditions. The results of our field studies, which continued for two growing seasons, confirm the possibility of using the fertilizer extracted from dairy wastes in nutrition of wheat plants during the full growth cycle and until reaching seed production. Vasilyev et al. [10] used an organic fertilizer extracted from the residues of cleaning milk pipes in nutrition of spring wheat under field condition and found that the extracted organic fertilizer is able to provide the tested plant by its nutrient requirements. Despite the importance of field studies carried out by Vasilyev et al. [10], they used dairy wastes of the manufacturing process. The current study was carried out using the expired dairy products in nutrition of wheat plant under laboratory and field scales. This study presents an innovative and simple method for extracting nitrogen-rich compounds from the expired dairy products. The extraction method depends on simple procedure only the use of gravity and thermal treatment. The extracted organic fertilizer was able to provide wheat plant with its nutrient requirements, moreover, it showed significant increases over the mineral nutrition. Our findings are consisted with those of Suárez et al. [9], Eissa et al. [4], and Vasilyev et al. [10].

Soil Fertility as Affected by the Organic Fertilizer of Dairy Wastes
The soil pH is the main factor that is determining the degree of nutrients availability, as well as the optimum root growth nutrients uptake by plants from the soil solution [19][20][21][22][23]. The initial pH value of the studied sandy soil was 8.00 and the inorganic fertilization failed to reduce it compared to the extracted organic fertilizer. The organic fertilizer reduced the soil pH to 7.02, thus it was more able to increase the available N, P, and K in the tested soil. Increasing the availability and uptake of nutrients increased the photosynthesis process and enhanced wheat growth [4,24]. The optimum soil pH for maximum P availability is below 7.20 [19]. Low soil organic matter content combined with high soil alkalinity severely reduces phosphorus readiness for uptake by plants [25][26][27][28]. Based on the incubation experiments, the tested organic fertilizer exhibited superiority in increasing the availability of soil phosphorus as well as the plant uptake. The extracted organic fertilizer raised the availability and uptake of P by 66% and 73% above the inorganic fertilization. According to the obtained results, the increase in phosphorous availability was due to two main mechanisms, the first was through decreasing the soil pH and the second was the remarkable increases in the soil organic matter. These results are consistent with the results of [4,19,25]. The main effect of the organic compounds in increasing the soil available P may be through the increase of the dissolved organic compounds, which act as ligands to protect the P from the precipitation in the soil solution [24,25,29].

Conclusions
In this study, organic fertilizer was derived from dairy wastes and was evaluated as a source of nutrition for wheat plants in incubation and field experiments. The obtained results confirm the possibility of using this organic fertilizer to supply wheat plants with their requirements of nitrogen, phosphorus, and potassium. The high content of organic materials in the extracted organic fertilizer helps in improving the soil quality and enhancing wheat growth. The organic fertilizer clearly outperformed mineral fertilizers, and it led to an increase in the grain yield by 22-35% and in the straw yield by 15-17%. The obtained results represent a new strategy for recycling agricultural waste to reduce environmental pollution as well as to maximize its use in agricultural production. The new organic fertilizer exhibited clear positive results in improving wheat growth and the fertility of sandy soil, however, more studies are required on the effect of this fertilizer on soil quality, e.g., soil microbes and enzymes. The response of other plant species under different climatic conditions and soils should also be studied to achieve a better understanding of the performance of this fertilizer.