The Effects of Different Soil Substrates on the Growth and Root Coixol Content of Local Coix Varieties in China

Abstract

Coix lacryma-jobi L., an annual or perennial plant belonging to the Poaceae family, has long been cultivated as a food and medicine plant in China. In recent years, coix cultivation for high yields and good quality has become a research hotspot in Southwest China. Soil optimization is essential for improving crop growth. To ensure the robust establishment of coix plants, eight soil substrates, prepared from three typical soils, i.e., red clay soil, peat soil, and sandy soil, were used to cultivate two local coix varieties (Pu coix from Fujian Province, China; Qi coix from Hebei Province, China), and the plant growth and root coixol content of the two coix varieties were investigated. It was found that coix plants could maintain growth when cultivated with peat soil or sandy soil, but red clay soil was unfavorable for coix growth. The mixtures of sandy soils and peat soils resulted in synergistic benefits for coix growth and root coixol levels over the effects of sandy soil or peat soil alone. In conclusion, the mixtures of sandy soils and peat soils with appropriate proportions (sandy soils/peat soils = 2:1) were suggested as an ideal soil substrate for coix cultivation. The results provide valuable guidance for the establishment of coix plants, which could contribute to high yields and good quality in coix cultivation.

1. Introduction

Coix lacryma-jobi L. is an annual or perennial C4 plant of the Poaceae family with strong adaptability and regenerative capabilities [1]. Coix is a multifunctional herbaceous plant that is used in medicine, food, and feed and has been officially included in China’s list of “both food and medicine” [2]. Coix seeds are rich in protein, fats, carbohydrates, vitamins, minerals, and other nutrients, making it an edible minor cereal crop [1]. Both the roots and seeds of coix are used in traditional Chinese medicine to treat various types of diseases because of their pharmacological effects [3], such as their effects in strengthening bones and muscles, supporting spleen and stomach health, reducing edema, and promoting intestinal health. Owing to their high biomass and good palatability, the stems and leaves of coix are processed into green and silage feed [4].

China is the primary producer of coix seeds, with the largest planting area and coix yields in the world. The production of coix seeds in China has been increasing annually, mainly due to the increase in the planting area and the promotion of new coix varieties in Guizhou Province [5]. The production of coix seeds in Guizhou Province accounts for approximately two-thirds of the national production. Guangxi, Yunnan, and Fujian Provinces are also representative areas of coix production in China [6]. However, the total production of coix seeds in these representative regions is less than 30% of the whole nation. Overall, the production of coix seeds varies widely across provinces in China, which may be attributed to soil variation, the popularization of different varieties, the climate, and local government decisions.

Different varieties have different soil requirements and have substantial effects on crop growth, yield, and quality [7,8]. Different types of soil exhibit specific effects due to their unique attributes, which can vary from sandy soil to clayey soil to loam soil. Sandy soil has a loose texture and good air permeability, which are beneficial for root respiration, but the poor water retention capacity and low fertility of sandy soil restrict robust plant growth [9]. Clayey soil has good water retention capacity and high contents of organic matter and nutrients [10], which are beneficial for plant growth and development; however, clayey soil has poor air permeability and is thus not suitable for root growth.

In China, there seem to be no strict standards or requirements for soil selection for the cultivation of coix, and arable land soil is generally used for coix cultivation [11]. Small farmers in the karst mountainous areas of Guizhou and Guangxi Provinces have a long tradition of coix cultivation, which is an important part of local coix seed industry development [5,12]. Higher coix yields and better quality of coix seeds are more frequently obtained in Guizhou than in Guangxi. Soil management and optimization are among the hotspots of crop research, and soil optimization significantly improves crop root system structure, increases water and fertilizer utilization efficiency, enhances photosynthetic performance and grain-filling capacity, and helps in the achievement of high crop yields and quality [13,14]. The soil compositions vary across geographical locations. It is assumed that soils with different properties may contribute to large variations in coix growth among locations.

As a minor cereal crop, the cultivation techniques of coix are relatively underdeveloped, and the yield potential has not been fully exploited [5]. Previous research on coix has focused mostly on nutrition and chemical components and on medicine and food development [15,16,17]. The cultivation and breeding of coix varieties have been covered by only a few researchers, and previous studies have focused on the effects of cultivation-related factors, such as planting density and fertilizer inputs, on the yield and quality of coix varieties [18,19]; however, few studies have discussed the effect of soil in coix cultivation [20]. The present study aimed to analyze the effects of different soil types on the growth and effective chemical components of coix roots with the goal of identifying suitable soil types for coix growth.

2. Materials and Methods

2.1. Soil Substances

Red clay soils are distributed across large areas and represent the typical farmland soil in Guangxi Province. Typical red clay soils identified by Dr. Liangpu Lyv (Nanning Normal University) were collected from a hilly red-soil region at the Guangxi Institute of Botany, Chinese Academy of Science, Guilin city, Guangxi Province, China (25°4′ N, 110°18′ E, and 175 m above sea level). Peat soils, which cover approximately 3% of the Earth’s land mass, are formed from partially decomposed plant material in waterlogged environments. In the present study, peat soils were obtained from a private mining business. Sandy soil with good air permeability was also included. Twenty-gram soil samples (red clay soil, peat soil) were collected and air dried to determine the soil properties. The soil pH was measured electrometrically with a glass electrode pH meter (Ritz PHSJ-3F, Shanghai Yidian Scientific Instrument Co., Ltd., Shanghai, China). The available nitrogen content was determined via oxidative hydrolysis, and the available phosphorous and potassium contents were determined via acidic ammonium lactate extraction as reported in our previous study [21]. Conductivity measurements were performed using a conductivity meter (DDS-11A, Shanghai Yueping Scientific Instrument Manufacturing Co., Ltd., Shanghai, China). The physical properties of sandy soils were evaluated, but chemical properties were not analyzed because of extremely poor nutrients found in sandy soils.

Table 1. Properties of red clay, peat soil, and sandy soil used in the present study.

Soil Type	Red Clay Soil	Peat Soil	Sandy Soil
Bulk density (g cm−1)	1.27 ± 0.09 b	0.60 ± 0.04 c	1.45 ± 0.07 a
Total porosity (%)	59.2 ± 1.4 b	76.5 ± 4.3 a	42.9 ± 2.9 c
Electrical conductivity (uS cm−1)	14.9 ± 0.1 c	278.0 ± 18 a	49.0 ± 1.1 b
pH value	6.12 ± 0.02 b	5.75 ± 0.09 c	—
Organic matter (%)	10.0 ± 0.63 c	18.9 ± 0.41 a	—
Available phosphorus (mg kg−1)	6.20 ± 1.24 b	6.82 ± 0.93 b	—
Available nitrogen (mg kg−1)	38.6 ± 2.39 b	197.7 ± 4.92 a	—
Available potassium (mg kg−1)	12.7 ± 0.20 b	70.5 ± 2.91 a	—
Total potassium (%)	0.93 ± 0.02 c	1.27 ± 0.03 a	—
Total phosphorus (mg kg−1)	597.1 ± 8.01 c	1002.9 ± 7.58 a	—

The values are shown as the means ± SDs. Different letters indicate significant differences between red clay soil, peat soil, and sandy soil at the p < 0.05 level according to the least significant difference test.

shows the physical and chemical properties of red clay soil and peat soil. Generally, the sandy soil showed the highest bulk density, followed by the red clay soil and peat soil, and peat soil showed both the highest total porosity and electrical conductivity, followed by the red clay soil and sandy soil. The red clay soil (pH = 6.12) and peat soil (pH = 5.75) were relatively acidic, and the available phosphorus content in red clay soil was comparable with that in peat soil; however, the organic matter, available nitrogen, available potassium, total potassium, and total phosphorus levels in peat soil were significantly greater than those in red clay soil. In the present study, eight soil substrates, which were prepared by evenly mixing red clay soil, peat soil, and sandy soil in different volumetric ratios (

Table 2. Volumetric ratios of red clay soil, peat soil, and sandy soil for different types of soil substrates.

Soil Type	Red Clay Soil	Peat Soil	Sandy Soil
T1	1	1	1
T2	0	1	2
T3	0	2	1
T4	2	0	1
T5	2	1	0
T6	0	0	3
T7	0	3	0
T8	3	0	0

), were used for coix cultivation. Specifically, the T1, T2, T3, T4, T5, T6, T7, and T8 soil substrates included red clay soil, peat soil, and sandy soil at ratios of 1:1:1, 0:1:2, 0:2:1, 2:0:1, 2:1:0, 0:0:3, 0:3:0, and 3:0:0, respectively. Five replications (1 pot for each replication) of each type of soil substrate for each coix variety were included.

2.2. Crop Husbandry

Pot experiments were conducted at the Guangxi Institute of Botany from June~August 2023. Two local coix varieties, the Qi coix variety from Anguo city, Heibei Province, and the Pu coix variety from Longyan city, Fujian Province, which are listed as national geographic indication products due to their industrial advantages, high visibility, and industrial-scale production in China, were used in the present study. Seeds of the Pu coix variety were supplied by Dr. Fang Liu (Longyan University), and seeds of the Qi coix variety were purchased from the Anguo traditional Chinese medicine market, one of the largest commercial markets for traditional Chinese medicines nationwide. Coix seeds were soaked with tap water for 12 h on 19 June 2023 and incubated in a constant-temperature incubator (DHP-360, Saidelisi Technology Co., Ltd., Tianjin, China) at 37 °C to hasten germination after breaking dormancy at 45 °C for 6 h.

On 22 June 2023, three germinated seeds of each coix variety were sown into 40 plastic pots (15.5 cm height × 22.5 cm top diameter × 14 cm bottom diameter) and thinned to two uniform plants per pot two weeks after sowing. A total of 0.5 g of urea was top-dressed per pot on 5 July 2023. The coix plants were cultivated in a net-covered shed, which provided approximately 44.2% of the original light intensity, for one month. The occurrence of diseases and pests was controlled intensively using pesticides, and weeds were controlled manually. The coix plants were sampled on 5 August 2023, and two coix plants were sampled for each replication.

2.3. Agronomic and Physiological Characteristics

The plant height was determined with a ruler by measuring the distance from the top of the blade to the stem base of the stretched plants, and the tillers of each coix plant were counted. The diameter of the main stem of each coix plant was measured via a Vernier caliper. The leaf nitrogen concentration and SPAD value were measured via a portable plant nutrition analyzer (TYS-4N, Zhejiang TOP Cloud-agri Technology Co., Ltd., Hangzhou, China). Fresh leaves were nipped to measure the total leaf area via a leaf area meter (LI-3000C, LI-COR, Inc., Lincoln, NB, USA). The roots were washed with tap water, and 2.0 g samples of fresh white roots were collected and stored at −80 °C. The stems, leaves, and roots were collected and dried to a constant weight at 105 °C in an oven (101-0, Shaoxing Supe Instrument Co., Ltd., Shaoxing, China). The biomass of roots and shoots (stems and leaves) and the root-to-shoot ratio were calculated according to the dry weights of the stems, leaves, and roots.

The total coixol in the roots was extracted and quantified according to the methods of Yang et al. (2017) [22] via high-performance liquid chromatography (HPLC, Shimadzu LC-16, Shimadzu Co. Ltd., Kyoto, Japan) with some minor modifications. Briefly, 0.2 g of root tissue was ground with 5 mL of cold extraction buffer (100% methanol, Sinopharm Chemical Reagent Co., Ltd, Shanghai, China). The homogenates were transferred to 10 mL centrifuge tubes, extracted for 1 h with ultrasonication in an ice and water bath, and centrifuged at 10,000× g for 10 min. The supernatants were collected, and the pellets were extracted once again following the same procedure as described above. The supernatants were pooled, and 10 mL of the mixture was filtered through an organic 0.45 μm microporous filter membrane.

The diluted solution was separated by HPLC equipped with a C18 column (WondaCract ODS-2 C18 column; 250 mm × 4.6 mm, 5 μm) by isocratic elution [25% acetonitrile (Sinopharm Chemical Reagent Co., Ltd, Shanghai, China)/75% ddH₂O] for 20 min at a flow rate of 0.9 mL min⁻¹ under a constant column temperature of 25 °C with UV detection at 232 nm. The calibration standards were prepared at concentrations of 3, 5, 7, 10, 15, 20, and 25 ng mL⁻¹ of the coixol standard, and a standard curve was generated. The coixol content in the coix root samples was calculated according to the derived standard curve and expressed in units of mg g⁻¹ fresh weight (FW).

2.4. Statistical Analysis

Analysis of variance (ANOVA) for all the variables was performed via Statistix 8.0 (Analytical Software, Statistix® v.8.0, 2003, Tallahassee, FL, USA). The treatment means of the five replications (pots) were compared via the least significant difference (LSD) test at the p < 0.05 level. Two outliers in coixol concentration, which are marked with dotted circles, in the Pu coix variety under T1 soil substrate and the Qi coix variety under T6 soil substrate, were excluded as assessed by the Altman’s Z-Score Model (Z-Score > 3.0) when ANOVA was performed. The subordinate function value of a certain index could be calculated according to the following equation:

$$R (X_i)={\displaystyle \frac{X_i-X_{min}}{X_{max}-X_{min}}}$$

where X_i represents the index of interest and X_max and X_min are the maximum value and minimum value of the index of the eight soil substrates for each coix variety, respectively. A comprehensive evaluation of the effects of the eight soil substrates on each coix variety was performed according to the average subordinate function values as reported previously [23].

3. Results

3.1. Effects of Different Soil Substrates on the Growth of Coix Varieties

The plant growth indices of coix varied significantly among the eight soil substrates and between the two varieties, and significant interactions occurred between the soil substrates and coix varieties for all the agronomic indices except the stem diameter (

Table 3. Agronomic performance of coix varieties in different soil substrates.

Variety	Treatments	Tiller Numbers (Tillers Plant−1)	Plant Height (cm)	Leaf Area (cm2 Plant−1)	Stem Diameter (cm)
Pu coix	T1	2 ± 0.38 b	62.3 ± 3.44 bcd	171.3 ± 28.9 cd	6.21 ± 0.50 bcd
	T2	2 ± 0.16 b	73.9 ± 5.90 a	246.1 ± 26.5 b	7.21 ± 0.66 ab
	T3	3 ± 0.33 a	72.5 ± 2.77 ab	316.3 ± 38.1 a	7.36 ± 0.19 a
	T4	1 ± 0.00 c	47.7 ± 3.99 fg	128.6 ± 27.8 cdef	4.55 ± 0.47 efg
	T5	1 ± 0.34 bc	65.4 ± 3.94 abc	148.2 ± 21.7 cde	5.45 ± 0.33 def
	T6	1 ± 0.10 bc	43.3 ± 2.93 fgh	77.8 ± 19.1 fgh	4.10 ± 0.38 gh
	T7	2 ± 0.19 b	49.2 ± 4.70 ef	81.4 ± 19.4 fgh	5.32 ± 0.36 def
	T8	1 ± 0.10 bc	38.3 ± 4.78 gh	34.8 ± 8.22 hi	3.70 ± 0.29 gh
Qi coix	T1	1 ± 0.00 c	51.8 ± 3.30 def	115.4 ± 7.45 defg	5.6 ± 0.30 de
	T2	1 ± 0.00 c	63.7 ± 3.82 abc	175.4 ± 8.89 c	6.85 ± 0.56 abc
	T3	1 ± 0.30 bc	58.4 ± 3.99 cde	167.0 ± 17.2 cd	6.94 ± 0.14 ab
	T4	1 ± 0.00 c	41.6 ± 1.50 fgh	59.7 ± 2.76 ghi	4.47 ± 0.11 fg
	T5	1 ± 0.20 bc	46.9 ± 4.63 fg	103.7 ± 36.5 efg	5.39 ± 0.43 def
	T6	1 ± 0.00 c	51.3 ± 2.28 ef	116.7 ± 8.78 cdefg	5.83 ± 0.30 cd
	T7	1 ± 0.00 c	42.5 ± 2.75 fgh	33.3 ± 14.6 hi	4.54 ± 0.47 efg
	T8	1 ± 0.13 bc	33.0 ± 2.29 h	6.90 ± 0.8 1i	3.33 ± 0.21 h
Variety (V)		23.51 **	18.00 **	25.56 **	0.38 ns
Treatment (T)		4.72 **	18.12 **	24.69 **	20.64 **
V × T		2.58 *	2.2 *	3.06 **	2.07 ns

The values are shown as the means ± SDs (n = 5, indicate the 5 replications). *, ** and ns indicate significance at the p < 0.05 and p < 0.01 levels and nonsignificance, respectively. Different letters indicate significant differences among the eight soil substrates for the Pu and Qi coix varieties at the p < 0.05 level according to the least significant difference test.

and

Table 4. Plant biomass of coix varieties in different soil substrates.

Variety	Treatments	Root Biomass (g Plant−1)	Shoot Biomass (g Plant−1)	Root-to-Shoot Ratio
Pu coix	T1	3.83 ± 0.51 abc	8.74 ± 1.44 c	0.48 ± 0.07 cd
	T2	3.95 ± 0.34 ab	13.12 ± 1.58 b	0.32 ± 0.05 d
	T3	4.88 ± 0.69 a	16.43 ± 1.77 a	0.31 ± 0.06 d
	T4	2.43 ± 0.55d ef	5.15 ± 1.04 de	0.50 ± 0.09 bcd
	T5	2.48 ± 0.34 de	7.28 ± 1.19 cd	0.36 ± 0.04 cd
	T6	1.81 ± 0.48 efg	3.10 ± 0.7 efg	0.59 ± 0.05 bcd
	T7	1.96 ± 0.35 efg	3.10 ± 0.52 efg	0.64 ± 0.05 bc
	T8	1.00 ± 0.17 gh	1.38 ± 0.21 g	0.77 ± 0.14 b
Qi coix	T1	2.39 ± 0.2 def	5.19 ± 0.46 de	0.47 ± 0.04 cd
	T2	3.33 ± 0.16 bcd	8.46 ± 0.47 c	0.40 ± 0.04 cd
	T3	3.26 ± 0.35 bcd	7.91 ± 0.74 c	0.41 ± 0.02 cd
	T4	1.10 ± 0.08 gh	2.21 ± 0.17 fg	0.51 ± 0.05 bcd
	T5	1.72 ± 0.67 efgh	4.61 ± 1.53 def	0.35 ± 0.03 cd
	T6	2.73 ± 0.27 cde	4.86 ± 0.39 def	0.56 ± 0.03 bcd
	T7	1.37 ± 0.29 fgh	1.53 ± 0.51 g	1.17 ± 0.32 a
	T8	0.62 ± 0.15 h	0.56 ± 0.15 g	1.13 ± 0.12 a
Variety (V)		13.77 **	35.64 **	6.33 **
Treatment (T)		15.51 **	34.01 **	10.89 **
V × T		2.09 ns	4.86 **	2.02 ns

The values are shown as the means ± SDs (n = 5, indicate the 5 replications). ** and ns indicate significance at the p < 0.01 levels and nonsignificance, respectively. Different letters indicate significant differences among the eight soil substrates for the Pu and Qi coix varieties at the p < 0.05 level according to the least significant difference test.

). In general, the Pu coix variety grew better than the Qi coix variety regardless of the soil substrate, and the agronomic characteristics of the coix varieties, such as tiller number, plant height, leaf area, stem diameter, and biomass, appeared to be greatest when the plants were cultivated in the T2 and T3 soil substrates.

The Pu coix variety was slightly superior in the T3 soil substrate compared with the T2 soil substrate, and the Qi coix variety grew better in the T2 soil substrate than in the T3 soil substrate, but the variation between the T2 and T3 soil substrates was not significant for the two coix varieties. Although robust coix plants were not obtained, the T1, T4, T5, T6, and T7 soil substrates basically supported the growth of the coix varieties. The T8 soil substrate contained the weakest plants of the Pu and Qi coix varieties (Table 3 and Table 4).

3.2. Effects of Different Soil Substrates on the Leaf Physiological Indices of Coix Varieties

The leaf nitrogen concentration and SPAD value of coix leaves varied significantly among the different soil substrates and between the two varieties, and significant interactions occurred between the soil substrates and coix varieties (Figure 1). Compared with the Qi coix variety, the Pu coix variety presented substantially greater leaf nitrogen concentrations and SPAD values regardless of the soil substrate. Among the soil substrates, the leaf nitrogen concentration and SPAD value were significantly greater in the T7, T5, and T3 soil substrates than in the other soil substrates for the two coix varieties. The lowest leaf nitrogen concentration and SPAD value were observed in the T8 soil substrate for the two coix varieties.

3.3. Effects of Different Soil Substrates on the Root Coixol Content of the Coix Varieties

The concentration of coixol in roots varied significantly among different soil substrates and between the two varieties, and significant interactions occurred between the soil substrates and coix varieties (Figure 2). Under most conditions, the concentration of coixol in the roots of Qi coix was significantly greater than that in the roots of Pu coix, except in the T7 and T8 soil substrates. Among the soil substrates, the coixol concentration was relatively higher in the T3, T1, and T2 soil substrates than in the other soil substrates for the two coix varieties. The lowest coixol concentration was observed in the T7 and T8 soil substrates for the two coix varieties (Figure 2).

3.4. Ranking of Soil Substrates for Coix Growth via Subordinate Function Analysis

Figure 3 shows the rankings obtained from the comprehensive evaluation of the performance of coix varieties growing in the soil substrates created in the current study. According to the results of the subordinate function analysis, the performances of the agronomic and physiological indices were basically consistent between the two coix varieties. For the Pu coix variety, the rankings from the comprehensive evaluation of the soil substrates were as follows: T3 (0.88) > T2 (0.61) > T1 (0.52) > T5 (0.50) > T7 (0.42) > T4 (0.16) > T6 (0.15) > T8 (0.13). For the Qi coix variety, the rankings of the soil substrates were as follows: T3 (0.91) > T2 (0.79) > T5 (0.61) > T6 (0.59) > T1 (0.54) > T4 (0.43) > T7 (0.30) > T8 (0.05).

4. Discussion

Coix seeds, which have long been used for both medicine and food in China, are gaining favor worldwide [1,6]. Coix cultivation for high yields and good quality has become a research hotspot in Southwest China [20] since the demand for coix seeds has exceeded the supply in recent years. As a fundamental factor affecting plant growth, soil optimization helps achieve robust plants, which ensures a high yield and quality of crops [24]. However, few studies have explored the potential effects of soil on coix growth [20]. Previously, soils with different properties were shown to strongly affect the grain yield of main crops such as rice, wheat, and corn [25,26].

In the present study, obvious variations were detected in two traditional Chinese coix varieties cultivated in different soil substrates (Table 3 and Table 4), which preliminarily suggested the suitability of the soil substrates for vegetative growth and root coixol levels of coix plants (Figure 3). Generally, the T3 (peat soil/sandy soil = 2:1) and T2 (peat soil/sandy soil = 1:2) soil substrates exhibited the best comprehensive effects on coix growth and root coixol levels in the Pu and Qi coix varieties, followed by the T5 (red clay soil/humic substrate = 2:1), T1 (red clay soil/humic substrate: sandy soil = 1:1:1), T6 (sandy soil), T7 (peat soil), and T4 (red clay soil/sandy soil = 2:1) soil substrates. The T8 (red clay soil) soil substrate consistently produced the weakest plants of the Pu and Qi coix varieties (Figure 3).

The soil substrates with appropriate proportions of peat soil and sandy soil, such as the T2 and T3 soil substrates, could ensure the satisfactory growth of coix plants in the current study (Table 3 and Table 4; Figure 3). However, sandy soil or peat soil alone could maintain the growth of coix plants. Interestingly, the Pu coix variety grew better in peat soil than in sandy soil, whereas the Qi coix variety performed relatively better in sandy soil than in peat soil (Table 3 and Table 4). In Eleutherine palmifolia plants, better growth was observed in peat soil than in sandy soil [27]. The properties of peat soil differ from those of sandy soil [28], and the higher abundance of mycorrhizal species and fewer spores in peat soil than in sandy soil may contribute to differences in plant growth [27]. In any case, red clay soil was unfavorable for coix growth, as supported by the fact that the worst growth of coix plants was observed in the T8 soil substrate.

Peat soil consists of a high percentage of organic matter content from plant materials and has received attention from scientists in many disciplines [29]. However, peat soil is acidic, retains high amounts of water, and is recommended for growing acidophilic plants, such as blueberry and cranberry plants [30,31]. According to experience, a fertile, moist, neutral or slightly acidic clay loam with good water retention seems to be the most suitable type of soil for coix cultivation [32]. Although high in organic matter and moisture contents, peat soil was not the most favorable for coix cultivation, as supported by the fact that the unsatisfactory growth of coix was observed (Figure 3) in the T7 substrate containing only peat soil (Table 1). In the present study, the pH value of the peat soil used for coix cultivation was 5.75 (Table 2), which can be considered relatively acidic. Moreover, increasing the soil pH by reducing chemical fertilizers enhances the yield of coix varieties [33]. The acidic property of peat soil may be one of the primary factors limiting coix growth.

Sandy soil is loose, gritty, dry, warm, and low in nutrients. In agriculture, sandy soil is perfect for root crops, such as cassava and radish [34,35]. In the present study, coix plants seemed to grow better in sandy soil (T6 substrate) than in peat soil (T7 substrate), especially in terms of the accumulation of root biomass in the Qi coix variety (Table 3 and Table 4). However, the Pu coix variety was slightly superior, but not significantly, in terms of plant biomass in the T6 substrate containing peat soil than in the T7 substrate containing sandy soil (Table 4). The Pu coix variety and Qi coix variety were domesticated and cultivated in southern and northern China [36], respectively, where the surface soils presented significant regional differences. The loose texture and good air permeability of sandy soil should be beneficial for root respiration, thus promoting root growth in the Qi coix variety (Table 4); however, the poor sandy soil may restrict aboveground growth. Nevertheless, sandy soils have some advantages for coix growth, although limitations exist.

Red clay soils are dense, heavy, and sticky and restrict water and air penetration [37,38]; thus, these soils are suitable for the cultivation of rice, tea, and sugarcane [39,40]. However, red clay soils seemed inappropriate for coix growth, as supported by the fact that the root biomass and aboveground biomass in the Pu and Qi coix varieties were the lowest among the eight soil substrate treatments (Table 3 and Table 4). Some soil substrates, including those with high proportions of red clay soils, such as the T4 and T5 substrates, exhibited relatively poor coix growth and root coixol levels (Table 4; Figure 2). The high compaction of clayey soil is associated with poor air permeability, which may not be suitable for root growth, thereby also limiting the growth of the aboveground parts of coix plants.

The mixtures containing sandy soils and peat soils at a proportion of 2:1 (T2 substrate) or 1:2 (T3 substrate) resulted in better coix growth and root coixol levels than the individual sandy soil or peat soil did. The synergistic effects of the T2 or T3 substrate may be attributed to the complementary advantages of sandy and peat soils. Similarly, the incorporation of biochar into sandy and clay loam strongly affects soil fertility [24], which may contribute to plant growth. However, the effects of the sandy soil and peat soil mixtures on coix growth were significantly weakened when the red clay soil was mixed in, e.g., in the T1 substrate (Table 3 and Table 4; Figure 3). These results suggest that red clay soils, which represent typical surface soils in South China, may not be suitable for coix cultivation. As discussed above, we concluded that the mixtures of sandy soils and peat soils with appropriate proportions were an ideal soil substrate for coix cultivation. The optimal proportions of sandy soils and peat soils need to be further determined.

5. Conclusions

The coix plants could maintain growth when cultivated with peat soil or sandy soil, but red clay soil was unfavorable for coix growth. The mixtures of sandy soils and peat soils resulted in synergistic benefits for coix growth and root coixol over the effects of sandy soil or peat soil alone. The mixtures of sandy soils and peat soils with appropriate proportions (sandy soils/peat soils = 2:1) were suggested as an ideal soil substrate for coix cultivation. The results provide valuable guidance for the establishment of coix plants.