One of the important physical parameters of soil, especially relevant when considering the movement of water through soil layers, is the flow velocity
v. This parameter describes the ratio of the volume of flowing water
Q to the surface
A perpendicular to the flow direction. According to Darcy’s law (2),
v is influenced by the material’s permeability coefficient
Ks, flow path
L, and hydrostatic hydraulic height Δ
H. The outflow speed
v is vital for agricultural and forest soils due to its association with the drainage of excess rainwater into deeper soil layers and eventually to groundwater. When permeability is low, either from natural impervious layers, compacted soils, or human-made factors, the outflow speed diminishes, potentially causing ponding or surface runoff. Such situations occur with intensive use in agricultural areas [
22] and forest areas [
23,
24,
25,
26]. In container production, where the container and a minimal amount of substrate are elevated from the ground, water outflows freely. Water inflow arises either from natural rainfall or artificially via ramp systems for irrigation and fertilization. In this case, the outflow speed is related to the type of substrate and the shape (surface, height) of the container cell in which the substrate is located. As determined using the proposed prototype stand, the water outflow velocity
v from the examined V150 and V300 containers was significantly influenced by the substrate’s
ABD (
r = −0.524) and
DBD (
r = −0.523). This correlation arose from notable bulk density differences: the larger V300 cell exhibited a lower density (
ABD = 0.322 g∙cm
3,
DBD = 0.079 g∙cm
3) compared to the denser V150 cell (
ABD = 0.418 g∙cm
3,
BD = 0.103 g∙cm
3). Consequently,
v was affected more by container type than repetition or cell distribution. Differences in
v across container types were likely due to the filling time and vibration needed for the V300’s larger cell versus the V150 and the compaction susceptibility of the more considerable substrate mass on a vibrating table. Both the size of the inlet surfaces (V150 −16.64 cm
2; V300 −21.2 cm
2) and the cell heights (V150 −15 cm; V300 −18 cm) might have played significant roles (
Table 1). Due to the fact that both container types had consistent filling times and vibration levels (constant line efficiency of 400 containers per hour and maximum vibration acceleration of 12 G), the substrate in V300 cells did not achieve the same bulk density as in V150. There was also variation in the measured parameters in individual containers (
ABD;
DBD;
v) and within the container (
v) for V300. Therefore, line operating speeds and/or vibrating table intensities should be selected for specific container types. The demonstrated correlation between bulk density and liquid outflow speed may be of key importance when considering the relationship between substrate density and the growth of plants in containers. As the literature indicates, both too-high and too-low density values in a container cell may affect the production effect in the form of differentiated seedlings. This is particularly visible in the high variability of parameters such as shoot height, root collar diameter, root system architecture, and the degree of root overgrowth [
27,
28]. Individual species have different preferences as to substrate density, which may be caused by the availability of water and fertilizers that the plant can absorb before they flow out of the container cell. For example, research carried out for Scots pine (
Pinus sylvestris L.), a major species cultivated in Poland (comprising 58.6% of its forested area) [
29], indicates its heightened sensitivity to substrate compaction. The density level of pine significantly influences its growth attributes, including height, root collar thickness, dry mass of needles, shoots, and roots, and the average length of skeletal roots (>2 mm in diameter) and fine roots [
27]. Both excessively high and low densities restrict the growth of this species’ seedlings. Conversely, for the common beech (
Fagus sylvatica L.), another prominent species in Poland covering 8% of the forested area [
29], Pająk et al. [
28] showed that high substrate density in containers negatively affects the growth of seedlings of this species. Other studies by Pająk et al. [
30,
31] indicate that for both pine and beech, changing the substrate density in nursery containers influenced the content of macroelements in seedlings, and high density causes a reduction in the uptake of elements, especially a reduction in the content of macroelements in the assimilation apparatus. The reason for this may be the rapid outflow of elements and the short availability time for the plant root system. That is why the density of the substrate in the container cell is so important. This density–runoff velocity relationship might also be crucial for seed germination. Typically, low and variable bulk densities cause uneven germination since seeds, upon intensive irrigation, shift to varying depths, naturally settling due to gravity. They subsequently access water differently, contingent on substrate water retention [
11,
32]. Low density may cause water to drain out quickly, which may result in low moisture around the seed in the event of germination because the loose substrate releases water quickly due to its high outflow rate, and the moisture around the seed may be low or short-lived. In turn, too much water at high density and poor outflow from the cell may favor the appearance of pathogenic factors, especially those related to the increased number of fungi, as indicated by [
33]. The data underscore the significance of correct bulk density level (which can be influenced) in container production. This influences the water outflow rate from containers, consequently shaping optimal plant growth conditions. A judiciously selected bulk density can also streamline water and fertilizer usage, curtail chemical runoff from containers, and minimize groundwater contamination. With well-planned and carefully considered irrigation, fertilization and monitoring of the rhizosphere and substrate moisture can yield high-quality seedlings, ensuring optimized costs of container production and minimization of groundwater contamination. This pertains to tunnel cultivation and open-area cultivation alike, as demonstrated for black spruce (
Picea mariana (Mill.) Britton, Sterns, and Poggenb.) [
34] and for white spruce (
Picea glauca (Moench) Voss [
35] and Stowe et al. [
36]). However, as shown by [
37], excessive irrigation and rainfall can cause nutrient losses in container production. The proposed technical solution, beyond regulating the liquid outflow velocity
v in individual existing nursery containers, can guide the design of containers with varied shapes, volumes, or dimensions. This device also facilitates measurements on container plants throughout their growth phases, allowing for intermittent
v parameter monitoring during production without the need to damage the growing plants. This study confirmed that using the Urbinati Ypsilon line to fill different container types with consistent performance parameters and vibration intensities results in variable substrate
BD and subsequent liquid outflow velocities. Hence, filling parameters should be selected individually for specific container types.