The New Soil Conditioner DewEco Could Improve Sandy Soil’s Properties for Efficient Maize Growth

Abstract

Sandy soil, one of the most abundant soil types in the world, often has lower crop productivity because of poor water and fertilizer retention capacity. The objective of this study was to investigate the effects of the new soil conditioner DewEco (fermented organic material consisted mainly of salt of L-lysine and citric acid) on sandy soil quality and plant growth. Serial dosages of DewEco and nitrogen (N) fertilizer were mixed into sandy soils and planted maize in a greenhouse. DewEco application increased large soil particle composition and decreased small soil particle composition. Soil porosity and the liquid phase increased as the DewEco dosage increased. DewEco also decreased soil pH and increased soil electrical conductivity, soil organic matter content, total nitrogen and available potassium. DewEco significantly enhanced the soil water-holding capacity and soil effective water content although it also increased the wilting coefficient. Finally, DewEco markedly promoted maize growth while improving water use efficiency (WUE) and nitrogen use efficiency (NUE). In addition, there was an interaction effect between DewEco and nitrogen fertilizer, such that the combined effects of DewEco and N exceeded the sum of their respective effects promoting plant growth. Thus, DewEco application can significantly enhance soil water content and nutrient levels by alleviating sandy soil’s physical and chemical properties, thereby promoting plant growth, WUE and NUE. This study indicates that DewEco is a useful eco-friendly sandy soil conditioner for arid and semi-arid regions.

1. Introduction

Land degradation is a global development and environmental issue that could decrease the amount of arable land, threaten food production and influence water security, economic development and environmental integrity [1,2]. Sandy soils are widely distributed, covering more than 900 million hectares worldwide [3]. In recent years, with changes in global climate and increased human activities, more and more arable lands have become sandy soil [4,5]. Soil sandification not only decreases plant productivity but in severe cases also leads to land abandonment and even desertification [6].

Land sandification occurs worldwide, particularly in arid and semi-arid regions, which cover about 41% of the total land surface of the earth [7]. In China, land has desertified rapidly due to high-intensity utilization in the past three decades. The area of land affected by desertification and sandification has already reached 7.1 × 10⁵ km² [3]. Although the inappropriate development and utilization of sandy soils are likely to lead to further sandification and desertification, sandy soils are still widely used in agricultural production due to increasing food demand [3]. Therefore, discovering ways to improve sandy soil’s properties, prevent sandification and increase plant productivity is important to enable sandy soil utilization and sustainable agricultural production.

Sandy soil is characterized by high large-sediment concentrations, large soil particles and large soil interparticle pores [8]. Because of these distinctive properties, sandy soils are not optimally conducive to agricultural production because they have limited the ability to retain water and fertilizer as well as low organic matter content, imbalanced nutrients and large temperature variations [8,9]. They do offer some advantages, however, including good gas permeability and tillage for agricultural production [9,10]. Previous studies and practices have shown that improving sandy soils can permit high crop yields and effectively prevent soil sandification [11]. The key to improving sandy soils is enhancing soil water and fertilizer retention capacity through improving certain soil properties including soil texture, organic matter content, and nutrient levels [12].

At present, there are three main categories of methods to improve sandy soil, namely, physical, chemical and biological methods [13]. Physical improvement methods consist mainly of engineering strategies, i.e., soil replacement, preloading and sand drains [13,14]. Engineering strategies do improve sandy soil quality quickly and effectively, but they can have a significant negative effect on environmental conditions as well as huge costs [13]. Biological improvement methods typically increase soil organic matter content and nutrients by planting certain crops at certain times, but these methods take a long time and diminish the area’s food production capacity during the improvement process [15]. Chemical improvement methods mainly focus on using synthetic soil conditioners (i.e., polyacrylamide, hydrogels, industrial/municipal solid waste), which can efficiently improve soil physical and chemical properties and increase water and fertilizer retention capacity. Although soil conditioners have the advantages of convenience and effectiveness, it remains uncertain how long these improvements can last, and whether the intermediate or final products of soil conditioners are harmful to soils, plants, groundwater and human health [16,17,18].

People have consistently worked to produce new effective soil conditioners to improve sandy soils [19,20]. The components of most soil conditioners can be divided into three classes: natural organic materials, inorganic or mineral materials, and synthetic materials, which consist primarily of chemical polymers and surfactants [21]. Organic conditioners such as manure, peat, crop residues, and biochar have been widely used to increase soil infiltration and retention, promote aggregation, and provide a substrate for micro- and mesobiological activity [21]. Many organic conditioners increase soil infiltration and soil water retention evidently and immediately, whereas it can take weeks or months to promote aggregation through chemical and biological processes [21]. Inorganic or mineral materials such as bentonite, zeolite, and gypsum or lime can modify soil chemical or physical processes [22]. Synthetic materials mainly include surfactants, superabsorbent hydrogels (SH), and polyacrylamide (PAM) [21,23]. PAM is widely used as a soil conditioner for improving sandy soils; it effectively improves soil water content because PAM itself has a strong water retention capacity [24]. Yet, previous studies have shown that PAM has weaker effects on soil texture, organic matter content and nutrient levels [21]. Moreover, PAM’s cost is too high to allow it to be widely used in agricultural production, and its environment harmfulness, including its impacts on soils, plants, and even human health, remains unknown [16].

To date, there have been few sandy soil conditioners that are simultaneously efficient, economical and eco-friendly, and even fewer that meet those criteria and can also be used in arid and semi-arid regions. Soil conditioners intended for sandy soils should have strong water and fertilizer retention capacity and should improve soil nutrient profiles, boost micro- and mesobiological activity, improve soil texture, and prevent further sandification [10]. Thus, the development of new sandy soil conditioners that can effectively enhance sandy soil’s water and fertilizer retention and its soil organic matter content at low dosages without harm to environment is urgently needed.

DewEco is a liquid-type product consisting of the low molecular weight of organic materials produced through fermentation, invented by CJ (CJ CheilJedang Corp.; Seoul, Rep. of Korea). The main component of DewEco is the salt of L-Lysine and Citric acid, and it contains 50% organic matter and 4% N. DewEco can provide the soil particle adhesion, i.e., their aggregation at the surface and inside the soil structure. Thus, DewEco is located between soil particles at the surface of the soil and/or inside the soil to bind them to improve the soil. Its strong water retention capacity, strong capacity for cementation, relatively high amino acid contents and environmentally friendly impact give it potential as a soil conditioner for use on sandy soils, especially in arid and semi-arid regions. In this study, the effects of DewEco as a soil conditioner for sandy soils were evaluated according to the following metrics: (1) the effects of DewEco on sandy soil’s physical and chemical properties, (2) the effects of DewEco on water retention ability, (3) the effects of DewEco on fertilizer use efficiency (NUE) and water use efficiency (WUE), and (4) the effects of DewEco on plant growth and development. Soil pH, electrical conductivity (EC), soil porosity, soil particle composition, WUE, NUE, plant growth and biomass were investigated in pot experiments using maize seedlings.

2. Materials and Methods

2.1. The Experimental Condition and Materials

The experiments were carried out in 2018 in a glass greenhouse at the Institute of Soil and Water Conservation, Chinese Academy of Sciences, Yangling, Shaanxi Province, China (34°12’ to 34°20’ north latitude and 108°0’ to 108°7’ east longitude). The day/night temperatures of the greenhouse were controlled at 28/22 ± 5 °C and a natural photoperiod (approximately 14 h light a day) was in place during the experimental period. Serial dosages of DewEco which is prepared according to the manufacturing method of Patent no. PCT/KR2019/011181, including 0, 0.4, 0.6, and 0.8 g·kg⁻¹ soil (equivalent to 0, 300, 600, and 900 kg·ha⁻¹ in the field), were applied in this study. The basic properties of the sandy soils were measured before the experiment was started; the contents of clay, silt, and sand particles in the soil were 10% (<0.002 mm), 19% (0.02~0.002 mm), and 71% (2~0.02 mm), respectively. Soil pH, available phosphorus, available potassium, and organic matter content were 8.02, 3.46 mg·kg⁻¹, 106.7 mg·kg⁻¹, 0.72 g·kg⁻¹, respectively.

2.2. The Effect of DewEco on Soil Properties and Soil Water Parameters

Different dosages (0, 0.4, 0.6 and 0.8 g·kg⁻¹) of DewEco were mixed into sandy soils, and 1 kg of uniformly mixed soil was placed in each pot. Each dosage was tested in ten replications (ten pots per dosage). All pots were placed in a greenhouse and soil moisture was maintained at 80% of the field water-holding capacity by measuring pot weight and supplying water every two days. After 15, 20 and 25 days, soil samples were collected, air-dried and passed through a 1 mm sieve to measure soil physical and chemical properties and soil water parameters.

(1) Soil particle size distribution and soil porosity measurement

The sieving and specific gravity tests were conducted to measure soil particle size distribution [25]. A sample of 100 g (accurate to 0.01 g) air-dried soil was passed through a 2 mm sieve, then weighed and placed in a 500 mL conical flask. A total of 50 mL of 0.5 M hexametaphosphate and water were added to a final volume of 250 mL, then lightly boiled for 1 h. Samples larger than 0.25 mm were then measured by the sieving method, while those smaller than 0.25 mm were measured by the specific gravity method.

The ring knife method was used to measure soil porosity [26]. Specifically, soil was sampled with a ring knife, and the quality of the soil and ring knife were recorded as W₀. The soil and ring knife were then placed in a basin for wetting overnight, and the weight of water saturated was recorded as W₁. After natural drying (without water leaking from the pot bottom), the soil and ring knife were then dried at 105 °C for 24 h, and the quality of the soil and ring knife was recorded as W₂. Soil porosity was calculated by the following equations:

Soil porosity = (W₁ − W₂)/V × 100%

(1)

Soil bulk density = (W₂ − W₀)/V × 100%

(2)

where V refers to the volume of the ring knife.

(2) Soil pH and soil electrical conductivity (EC) measurements

Soil pH was measured according to Wilke (2005)’s electrode method (1:2.5 soil/water ratio). A sample of 10 g air-dried soil was passed through a 1 mm sieve, then placed in a 150 mL triangular bottle. An amount of 25 mL distilled water was added and the bottle was shaken for 30 min at 180–200 r/min; pH was then measured with an acidimeter. Soil electrical conductivity was measured according to Wilke [27]’s conductivity method (1:5 soil/water ratio). A sample of 5 g air-dried soil was passed through a 1 mm sieve, then placed in a 150 mL triangular bottle. An amount of 25 mL distilled water was added and the bottle was shaken for 5 min at 180–200 r/min. The filtered supernatant was then used for measuring EC with a conductivity meter. Each soil sample was measured three times and each treatment had three replicates.

(3) Soil organic matter content, total nitrogen, available phosphorous and available potassium measurement

Soil organic matter content was measured according to Walkley and Black [28]. A sample of 0.5 g soil was placed in a 250 mL conical flask, to which 10 mL of 0.17M K₂Cr₂O₇ was added with a pipette. The soil and dichromate were mixed by gently swirling the flask, followed by the addition of 20 mL H₂SO₄. The flask was again swirled gently to allow the soil to have good contact with the reagent. The contents of the flask were allowed to stand for 30 min, then diluted with 200 mL of water and titrated with 1M FeSO₄ and three or four drops of O-phenanthrolineas indicator. Each soil sample was measured three times and each treatment had three replicates.

Soil total N was measured according to the Kjeldahl method [29]. A sample of 0.5 g soil was placed in a digestive glass tube and wet with water; 10 mL H₂SO₄ was then added. After resting overnight, H₂O₂ was added as a catalyst and digested at 38.5 °C for 3 h; the total N content was then measured with a Kjeldahl meter. Available phosphorous was measured according to the Olsen method [30]. A sample of 0.5 g soil was placed in a 150 mL Erlenmeyer flask to which 50 mL of 0.5 M NaHCO₃ was added. The flask was then shaken at 180–200 r/min for 30 min and the sample was filtered through phosphorus-free filter paper. Then, 10 mL of the solution was placed in a 50 mL volumetric flask to which 5 mL molybdate and 1 mL ascorbic acid solution were added, followed by distilled water for a final volume of 50 mL. Available potassium was measured according to the flame photometer method [31]. A sample of 5 g soil was placed in a 150 mL triangular bottle and shaken at 180-200 r/min with 50 mL of 1 M NH₄OAc for 1 h at 25 °C. The phosphorus and potassium contents in the extractant were determined using a flame photometer. Each soil sample was measured three times and each treatment had three replicates.

(4) Soil water-holding capacity, wilting coefficient and soil effective water content measurement

Soil water-holding capacity (WHC) was measured according to [32]. A sample of experimental soil was placed in a pot with a hole to allow unabsorbed water to escape. Soil quality was recorded as V₀ and the soil was then wetted overnight. After natural drying (without water leaking from the pot bottom), the quality of the soil and pot were recorded as V₁. The soil and pot were then dried at 105 °C for 24 h, and the quality of the soil and pot were recorded as V₂. Soil WHC was calculated according to the following equation:

Soil water-holding capacity (%) = [(V₂ − V₁)/(V₂ − V₀)] × 100

(3)

The wilting coefficient was calculated according to Briggs and Shantz [33] by the following equation:

$$\mathrm{Wilting}\mathrm{Coefficient}(\%)=\left[\mathrm{Soil}\mathrm{maximum}\mathrm{moisture}\mathrm{absorption}/\left(0.68\pm 0.012\right)\right]\times 100$$

(4)

The soil effective water content calculated according to Rawls et al. [34] using the following equation:

$$\mathrm{Soil}\mathrm{effective}\mathrm{water}\mathrm{content}(\%)=\mathrm{Soil}\mathrm{water}-\mathrm{holding}\mathrm{capacity}-\mathrm{Wilting}\mathrm{Coefficient}$$

(5)

(5) Soil three phase ratio (TPR) was calculated from soil porosity and soil water content [35] as follows:

Solid phase (%) = 1 − soil porosity

(6)

Liquid phase (%) = soil porosity − soil water content

(7)

Gas phase (%) = 1 − solid phase − liquid phase

(8)

2.3. The Effect of DewEco on Plant Growth, Water Use Efficiency and Nitrogen Use Efficiency

For further investigation of the effect of DewEco on plant growth, a complete randomized pot experiment was carried out with four DewEco and four nitrogen dosages (Figure 1) during June and July 2018. A combined series of dosages of DewEco (0, 0.4, 0.6 and 0.8 g·kg⁻¹ soil, equivalent to 0, 300, 600 and 900 kg·ha⁻¹ in field application) and N (0, 0.029, 0.058 and 0.087 g·kg⁻¹ soil, equivalent to 0, 20, 40 and 60 kg·ha⁻¹ in field application) were included in this study. In addition, all treatments were supplied with 60 kg P₂O₅·ha⁻¹ and 60 kg K₂O·ha⁻¹. Urea (46% N), superphosphate (12% P₂O₅) and potassium chloride (60% K₂O) were used as the N, P and K fertilizer sources. The various dosages of DewEco and nitrogen fertilizer were mixed with 8 kg sandy soil and placed into a plastic pot (20 cm height by 11 cm inner diameter). Seeds of a maize cultivar (‘Zhengdan 958’) were sown on June 1 and the plant was sampled on 25 July. Each treatment had six pots and each pot contained three plants. During the experimental period, soil water content was maintained at 80% of the field water-holding capacity as confirmed by weighing each pot every two days. At the end of the experiment, the fifth leaf of each maize plant (from bottom to top) was used to assess photosynthesis (Pn) and chlorophyll content (SPAD index). Maize roots were washed, then each whole plant was separated into root, stem and leaves and dried to measure the biomass.

After measuring the biomass, the N content in plant tissue was measured by the Kjeldahl acid-digestion method [36]. The nitrogen use efficiency (NUE) was calculated according to the following equation [37]:

NUE (%) = (N₁ − N₀)/N₂ × 100%

(9)

where N₀ is the nitrogen content of the whole plant without N application, N₁ is the nitrogen content of the whole plant with N application, and N₂ is the amount of N applied. N₀, N₁ and N₂ are expressed in grams N per kilogram soil.

During the experimental period, the amount of water consumption was recorded, and water use efficiency (WUE) was measured and calculated according to the following equation [38]:

WUE (%) = (biomass/total water consumption) × 100%

(10)

2.4. The Analysis of Interaction Effects between DewEco and Nitrogen Fertilizer

Based on the biomass parameter, the interaction effects between DewEco and N for maize growth were analyzed using the following equations [39]:

$$Z_{ij}=y_{ij}-{\overline{y}}_{ij}$$

(11)

y_ijk = μ + a_i + b_j + (ab)_ij + ε_ijk

(12)

where μ = $\overline{y}$, a_i = ∑Z_ij/4, b_j = ∑Z_ij/4 and (ab)_ij = Z_ij − a_i − b_j, where i = 1, 2, 3, 4; j = 1, 2, 3, 4; k = 1. Z_ij represents the difference between y_ij and y_ij. a_i represents the main effect of DewEco. b_j represents the main effect of N fertilizer. (ab)_ij represents the interaction between DewEco and N. y_ijk represents the k_th observation, and ε represents the value of error.

2.5. Statistical Analysis

All data were subjected to statistical analysis using SPSS version 22 (IBM Company, Chicago, IL, USA). Analysis of variance (ANOVA) was used to compare the mean values of different parameters in the different treatments. Significant means at 5% probability level were detected by least significant deference testing (LSD). All treatments included three replications.

3. Results

3.1. The Effect of DewEco on Sandy Soil Properties

The soil particle composition was affected by DewEco application (Figure 2). The particle sizes in sandy soils were mainly distributed between 0.01 and 0.25 mm. As DewEco dosage increased, the proportion of large soil particles (>0.05 mm) increased while that of small soil particles gradually decreased. Under 0.8 g kg⁻¹ DewEco application, for example, large soil particles were increased by 27%, and small soil particles were correspondingly reduced by 27% (Figure 2d).

Soil porosity (SP) and soil three-phase ratio (TPR) also reflected soil physical properties. SP increased significantly from 46.11% to 49.02% when the DewEco dosage was increased from 0 to 0.8 g/kg soil. The TPR was also affected by DewEco application. Compared with control, the solid phase was decreased by 2.81% under 0.8 g DewEco application; correspondingly, the liquid and gas phases were increased by 0.45% and 2.46%, respectively (Figure 3).

3.2. The Effect of DewEco on Soil pH, EC, Organic Matter and Nutrient Contents

As DewEco dosage increased, soil pH gradually decreased from 8.02 to 7.74. EC, on the other hand, gradually increased from 53.3 µS/cm to 73.7 µS/cm. EC was 38.2% greater under 0.8g·kg⁻¹ DewEco treatment than under control conditions. Soil total nitrogen, available phosphorus, available potassium and organic matter were all increased with DewEco application (

Table 1. Effects of different dosages of DewEco on soil pH, EC, organic matter and nutrient contents.

DewEco g kg Soil−1	pH	Electric Conductivity (µs·cm−1)	Total Nitrogen (g·kg−1)	Available Phosphorus (mg·kg−1)	Available Potassium (mg·kg−1)	Organic Matter (g·kg−1)
0	8.02 a	53.3 a	0.22 c	3.46 b	106.7 c	0.72 a
0.4	7.89 b	55.7 b	0.24 bc	3.59 b	111.7 bc	0.74 a
0.6	7.82 c	61.5 c	0.26 b	4.24 b	120.1 ab	0.77 ab
0.8	7.74 d	73.7 c	0.32 a	5.84 a	126.9 a	0.82 b

Values within a column followed by different letters are significantly different at p ≤ 0.05.

). Under 0.8 g·kg⁻¹ DewEco application, for example, soil total nitrogen, available phosphorus, available potassium and organic matter increased by 0.1 g·kg⁻¹, 2.4 mg·kg⁻¹, 20.2 mg·kg⁻¹ and 0.1 g·kg⁻¹, respectively, relative to the control.

3.3. The Effect of DewEco on Soil Water-Holding Capacity, Wilting Coefficient and Effective Soil Water Content

At a dosage of 0.4 g·kg⁻¹, DewEco significantly increased the water-holding capacity (WHC); at a dosage of 0.8 g·kg⁻¹, it increased the WHC by 16%. Meanwhile, the wilting coefficient (WC) also increased from 2.47 to 3.03% as the DewEco dosage increased. Thus, the effective soil water content was increased from 7.91% to 8.99%, rising by 3%, 8% and 14% under DewEco dosages of 0.4, 0.6 and 0.8 g·kg⁻¹, respectively.

3.4. The Effect of DewEco on Maize Growth, WUE and NUE

Maize plant height, chlorophyll level (SPAD), photosynthetic ability (Pn) and biomass were significantly enhanced with DewEco or N application (

Table 2. Effects of different dosages of DewEco and N application on maize growth, chlorophyll level (SPAD value) and photosynthesis rate (Pn).

Treatments DewEco N g kg Soil−1 g kg Soil−1		Plant Height (cm)	Biomass (g·Plant−1)	SPAD Value	Pn (Mmol·H2O·m−2·S−1)
0	0	35.57 d	13.47 d	14.76 d	14.03 c
0.4		39.43 c	28.20 c	21.84 c	15.18 b
0.6		45.00 b	37.98 b	25.22 b	16.17 ab
0.8		54.70 a	43.64 a	32.30 a	17.04 a
0	0.029	42.8 d	30.31 c	23.60 c	15.69 d
0.4		48.57 c	46.94 b	30.54 b	17.08 c
0.6		53.93 b	83.52 a	32.36 ab	20.32 b
0.8		66.33 a	91.31 a	33.66 a	22.27 a
0	0.058	58.87 dc	53.81 d	30.14 b	17.40 c
0.4		64.87 b	83.82 c	32.44 ab	20.37 b
0.6		72.47 a	114.80 b	33.64 a	23.29 a
0.8		77.03 a	127.59 a	35.20 a	24.04 a
0	0.087	56.10 c	56.97 d	30.16 b	18.78 d
0.4		69.70 b	86.00 c	33.98 ab	21.11 c
0.6		77.03 ab	123.63 b	35.22 a	24.01 b
0.8		80.13 a	143.44 a	38.08 a	27.65 a

Values within a column followed by different letters are significantly different at p ≤ 0.05.

). Under N60 treatment combined with 0.8 g·kg⁻¹ DewEco, plant height, biomass, SPAD and Pn were increased by 42.83%, 151.8%, 26.26% and 47.23%, respectively, compared to control values (Table 2).

WUE increased as the DewEco or N fertilizer dosage increased (Figure 4). Holding the N dosage constant at 0.087g N, 0.8g DewEco application increased WUE by 51.18% compared to the control value. The N contents of maize roots, stems and leaves were all increased as the DewEco and N dosages increased (Figure 5). Holding the DewEco dosage constant at 0.8 g, for example, the N content in whole plants increased from 5.88 (0 N application) to 7.86 (0.087 g N application). Meanwhile, under 0.087 g N application, 0.8 g DewEco application increased the N content by 49.43% compared to no DewEco. NUE also gradually increased as DewEco dosage increased. The highest NUE value of 64.89% was found under 0.8g DewEco and 0.029 g N treatment; this value was 51.12% higher than that recorded in the absence of DewEco (DewEco 0 and N 0.029). This shows that DewEco application dramatically increases NUE and saves N fertilizer.

3.5. The Interaction Effects between DewEco and N

Z values could be positive or negative as an intermediate parameter for calculating interaction effect values (

Table 3. Z values calculated from biomass.

Treatments		y1 (g)	y2 (g)	y3 (g)	$\overline{\mathbf{y}}$ (g)	Z1
DewEco g kg soil−1	N g kg soil−1
0	0	11.89	14.33	14.21	13.47 j	−0.58
0.4		27.9	28.02	28.68	28.20 i	−0.3
0.6		36.32	43.73	33.89	37.98 h	−1.66
0.8		42.46	45.41	43.06	43.64 gh	−1.18
0	0.029	26.40	31.36	33.17	30.31 i	−3.91
0.4		47.49	41.29	52.03	46.94 g	0.55
0.6		89.48	78.39	82.69	83.52 e	5.96
0.8		94.69	83.92	95.31	91.31 d	3.39
0	0.058	51.07	53.36	56.99	53.81 f	−2.74
0.4		83.27	80.46	87.7	83.81 e	−0.54
0.6		119.37	114.68	110.35	114.80 c	4.57
0.8		123.6	131.71	127.45	127.59 b	−3.98
0	0.087	54.93	59.05	56.94	56.97 f	−2.04
0.4		83.48	87.4	87.12	86.00 de	−2.52
0.6		117.12	125.06	128.71	123.63 b	−6.51
0.8		143.39	139.67	147.28	143.40 a	−0.06

Note. Z values were calculated according to Equation (11). y₁, y₂ and y₃ refer to three repetitions of maize biomass. Values with different letters are significantly different at p ≤ 0.05.

). The magnitude of Z values reflected the distance between y_ij and the corresponding $\overline{\mathrm{y}}$. The interaction effects between DewEco and N are shown in Figure 6. We calculated the interaction effect values (ab_ij) according to Formula (l). The data show that all ab_ij values are not equal to 0, indicating that the different dosages of DewEco and N admixed into sandy soils have an interaction effect on the biomass of maize seedlings, and that the size of this interaction effect can be calculated from the size of the ab_ij values. The interaction effect of DewEco 0.6 and N 0.058 treatment is the largest, with a value of 4.65.

4. Discussion

In this study, we tested the new soil conditioner DewEco and found that it can improve sandy soil’s properties and plant growth significantly, indicating that DewEco can be used as a novel soil conditioner in sandy soils to ensure sustainable agricultural development.

4.1. Application of DewEco Improves Sandy Soil’s Physical and Chemical Properties

Soil particle composition, which reflects the source of the mother matter and certain characteristics of the soil-forming process, is an important indicator of soil physical properties and has a great impact on fertility and water characteristics [40]. Our study showed that DewEco application increased the proportions of large particles and decreased the proportions of small particles. For example, the proportion of small soil particles (>0.02 mm) was improved by 225% under 0.8 g·kg⁻¹ DewEco application compared to control (Figure 2). Moreover, the three-phase ratio was also changed by DewEco application (Figure 3). These results indicate that DewEco can markedly improve sandy soil’s physical properties.

In this study, the test soil was an alkaline sandy soil and the pH of the soil decreased as DewEco dosage increased (Table 1). Alkaline soil is adverse for plant growth and nutrient absorption; thus, the ability to neutralize alkalinity improves conditions for plant growth [41]. Other than the strong acidity of DewEco itself (its pH is about 4.4), which directly decreases soil pH, this neutralization is also due to the high level of organic compounds in DewEco, which decreases the pH through chelating metal ions such as K⁺, Ca²⁺ and Mg²⁺ [42]. Soil EC is a measure of the total ion conductance in the soil solution, which is closely correlated with crop growth and yield [43]. EC that is too low results in lower nutrient content in the soil, while EC that is too high causes salt stress and inhibits plant growth [44]. The application of DewEco significantly increased the EC of sandy soil, perhaps because of existing strong water absorbent -COO- groups forming a 3D structure in DewEco [45,46].

Soil organic matter and nutrient levels are important indicators of soil quality, and they are also affected by the application of soil conditioners [47,48,49]. In this study, DewEco application enhanced soil total N, available P and available K as well as organic matter contents (Table 1). The increased N and organic matter may have been directly caused by DewEco in part. The application of DewEco could transfer large amounts of amino acids into the soil, which could stimulate soil microbial activity and thereby lead to enhanced soil nutrient levels. On the other hand, it has been shown that some kinds of soil conditioners can decrease nutrient leakage by adsorbing inorganic ions and nutrients [50]. In addition, the enhanced EC and decreased pH associated with DewEco application could also contribute to enhanced soil nutrient levels. It has been reported that available P is increased ten times for every one-unit decrease in pH, and increased EC can also contribute to increased nutrient levels [43].

4.2. DewEco Improves Sandy Soil’s Water Retention

Previous studies have proved that soil conditioners such as PAM, illite and biochar can effectively increase the water-holding capacity, decrease deep percolation and reduce evaporation in sandy soils [51,52,53,54]. Improved water retention through soil conditioners will improve plant growth and increase plant water use efficiency [23] (Guilherme et al.; 2015). In our current study, application of DewEco increased the sandy soil’s water-holding capacity and the effective soil water content, although the wilting coefficient was also enhanced (

Table 4. Effects of different dosages of DewEco on soil water-holding capacity, wilting coefficient and effective soil water contents.

DewEco g kg Soil−1	Water-Holding Capacity (%)	Wilting Coefficient (%)	Effective Water Content (%)
0	10.38 c	2.47 c	7.91 c
0.4	10.95 b	2.78 b	8.17 ab
0.6	11.47 b	2.95 a	8.52 b
0.8	12.02 a	3.03 a	8.99 a

Values within a column followed by different letters are significantly different at p ≤ 0.05.

, Figure 4). Although DewEco cannot create a gel in the soil as PAM does, the numerous -COO- groups in DewEco could also contribute to the soil’s ability to retain water [16,23,45,46] In addition, application of DewEco significantly increased the soil porosity and enhanced the liquid phase ratio (Figure 2 and Figure 3), each of which is also known to increase water retention.

4.3. Application of DewEco Promotes Plant Growth, WUE and NUE

Plant growth and yield are the most reliable indicators for evaluating the performance of soil conditioners [55,56,57]. In this study, application of DewEco increased maize seedling height, biomass, photosynthetic rate (Pn) and chlorophyll level (SPAD) (Table 2), suggesting that DewEco has a great potential as a useful soil conditioner for sandy soils. Furthermore, our results show an interaction effect between DewEco and N fertilizer such that the combined effects of DewEco and N exceed the sum of their respective effects promoting plant growth (Table 3). In addition, the application of DewEco markedly enhanced WUE and NUE. The increase in WUE may be ascribed to the increase in water retention ability and the decrease in water leakage and evaporation. Meanwhile, DewEco application also enhanced soil and plant tissue N contents, leading to higher NUE, which in turn contributed to higher WUE.

5. Conclusions

Taking all our findings together, we conclude that DewEco application can improve soil texture, increase soil porosity, large particles content and water retention capacity, and enhanced soil fertility. The application of DewEco can also decrease soil pH and increase soil EC and nutrient contents, which together contribute to enhanced NUE. Therefore, the application of DewEco effectively enhances plant growth by enhancing soil water content and nutrient levels (Figure 7). In addition, compared with other commonly used organic conditioners such as manure, peat, crop residues and biochar, the necessary dosages of DewEco are far lower, while the beneficial effects on plant growth emerge far more quickly. Compared with existing synthetic soil conditioners such as polyacrylamide (PAM), DewEco excels at improving soil nutrient levels and soil texture. Overall, this study indicates that DewEco can be used as an eco-friendly sandy soil conditioner for improving sandy soil quality for sustainable agricultural production in arid and semi-arid regions.