1. Introduction
China is a significant player in the field of facility horticulture, with the area of facility horticulture accounting for over 80% of the total area of facilities worldwide [
1]. As the scale and intensity of facility horticulture continue to grow, there are increasing demands for advanced seedling operation machinery (including transportation, pallet loading/molding, media digging for plug trays, etc.) [
2,
3]. The discrete element method is frequently utilized to accurately represent the motion behavior of materials and simulate their interactions with machinery [
4,
5]. The application of this method in studying the interaction between seedling substrates and working machinery has the potential to enhance work efficiency, minimize research and development costs, and provide valuable theoretical insights for optimizing the design of seedling substrate production and working machinery [
6]. An accurate and reliable simulation is contingent upon the selection of appropriate discrete element-simulation parameters (such as eigenparameters, basic contact parameters, and contact models), which serve as essential prerequisites for effective simulation [
7]. However, due to the diverse materials, particle-size distribution, and shapes of the objects being studied, it is essential to select an appropriate contact model and parameters that align with the characteristics of the research objects to enhance the simulation’s accuracy [
8,
9].
Currently, numerous scholars have conducted comprehensive research on agricultural granular particles using the discrete element method. The discrete element simulation parameters of fallen jujube fruit were calibrated using the Hertz–Mindlin (no slip) contact model [
10]. Wang et al. [
11] recommended selecting soil particles with a radius of 7 mm for Hertz–Mindlin with a bonding model when conducting research on soil subsoiling. Discrete element models of soil particles have been widely reported. For instance, discrete element-simulation models were developed based on Edinburgh Elasto-Plastic Adhesion (EEPA) for consolidated soil [
12], no-tillage soil [
13], broken soil [
14], and sandy loam soil [
15], which has been established that the discrete element method can be effectively utilized in simulating agricultural machinery operations and optimizing agricultural machinery design. However, there are relatively few simulation studies on growing media. Tong et al. [
16] selected Hertz–Mindlin with the bonding model when the end efferent of the simulated transplanter captured the growing media (composed of peat, perlite, and vermiculite with a moisture content of ~50–72%) and used the soil parameter as an alternative to achieve a qualitative analysis of the cohesion between the growing media. Wang et al. [
17] modeled and calibrated one of the growing media’s raw materials (coconut husk particles) based on the Hertz–Mindlin (no slip) contact model with the discrete element method, providing a reference for the design of special equipment for mixing coconut husk-growing media. Ding et al. [
18] conducted a discrete element parameter calibration for the plug tray-growing media (mainly composed of peat, wood chips, etc., with a moisture content of about 31%), and selected the EEPA model.
To sum up, the research object of seedling breeding substrate by researchers at home and abroad is mainly a certain growing media material or plug tray-growing media. In actual seedling breeding practices, due to the inherent limitations in the physical and chemical properties of individual growing media, it is often necessary to utilize a composite growing media that combines both organic materials (such as peat, straw, organic fertilizer, coconut bran, etc.) and inorganic materials (such as perlite and vermiculite) to achieve optimal seedling breeding results [
19,
20]. Furthermore, currently, there is a lack of simulation analysis on the flow behavior and compression molding of growing media in the preparation of seedling nursery blocks.
According to the previous study, it has been proved that the rabbit manure compost-growing media has good physical and chemical properties and a seedling-rearing effect [
21]. The growing media, being a typical granular particle, exhibit a notable rebound following compression molding, accompanied by a certain degree of cohesiveness and elastoplasticity. Therefore, compared to the Hertz–Mindlin (no slip) model, which is suitable for incompressible non-viscous particles [
10], the Hertz–Mindlin-with-JKR model, which is suitable for incompressible viscous particles [
22], the Hysteretic Spring model, which is suitable for compressible non-viscous particles [
23], and the Hertz–Mindlin-with-bonding model, which is suitable for materials such as rocks and concrete that do not rebound after breaking [
24], the EEPA model may be more suitable for simulating this growing media [
14,
15]. When calibrating the parameters of the EEPA model, the Plackett–Burman screening test is considered to be able to quickly identify parameters that have a significant impact, while the Box–Behnken test can further find the optimal parameter combination, thereby achieving accurate simulation of the growing media [
12,
13].
In this study, the growing media used in the production of seedling nursery blocks were selected as the focus object. The discrete element method was utilized, with the EEPA model employed, to conduct Plackett–Burman (PB) screening tests and Box–Behnken (BB) tests for the calibration and optimization of the discrete element parameters of the growing media. After verifying the maximum forming load during the uniaxial closed compression test, the construction of a discrete element model for the growing media was finally completed, providing valuable data references for the design and optimization of the molding device for seedling nursery blocks.
4. Conclusions
In this study, a discrete-element model of the growing media was obtained through calibrating the contact parameters of the growing media based on accumulation tests and uniaxial closed-compression tests.
The measured density of growing media was 1080 kg/m3, and the geometric average particle size was 0.43 mm. The repose angle of the growing media was 44.64° by means of the cylinder-lifting method. The maximum axial load during growing media compression (compression ratio of 4:1) was 2644.41 N by the uniaxial closed-compression test.
According to the characteristics of the growing media, the EEPA model was selected to calibrate the contact model of the growing media of seedling nursery blocks. Based on the sensitivity analysis of the PB test results, it was found that the parameters that had significant effects on the growing media repose angle were the interparticle collision-recovery coefficient, the interparticle collision-recovery coefficient between the particle and geometric model, the dynamic-friction coefficient between the particle and geometric model, and the tangential stiffness factor. Then, the four parameters were optimized by the BB test, and the discrete-element parameters of the growing media were obtained as follows: interparticle collision-recovery coefficient, static-friction coefficient, and dynamic-friction coefficient were 0.5066, 0.65, and 0.5, respectively; the collision-recovery coefficient, static-friction coefficient, and dynamic-friction coefficient between the particle and geometric model were 0.714, 0.55, and 0.381, respectively. The surface energy, plastic deformation ratio, adhesion branch index, and tangential stiffness factor were 17 J/m2, 0.55, 3.5 and 0.375, respectively.
The growing media model was used to conduct the simulation test of uniaxial closed compression, and the physical verification test was conducted. The results showed that the relative error between the maximum axial load on the punch and the measured value is 4.23%, which had high reliability and could provide a reference for the simulation of growing media. This study also verified that the discrete-element method could be employed to simulate the interaction between the growing media and the seedling machine, thereby supporting the design of the precise seedling machine and the enhancement of the operation precision of the facility horticulture.