Substrate compaction is one of the most important factors influencing plant growth. Seedling growth is influenced by the substrate’s physical properties, including its compaction level, typically expressed as bulk density. Substrate compaction determines the air−water ratio, water and nutrients availability, and microbial activity. Several authors have studied the effect of soil compaction on the growth and development of various plants. However, the influence of a wide range of different levels of substrate compaction on the growth of Scots pine seedlings in container nurseries has yet to be investigated. This paper presents the results of a study of the influence of different levels of peat substrate compaction on the growth parameters of Scots pine seedlings. The seedlings were grown in 9 variants of bulk density, ranging from 0.208 to 0.342 g·cm^–3 (dry bulk density: 0.083−0.137 g·cm^–3) in containers with multiple 120 cm³ cells. Each variant was prepared in 3 replicates (a total of 1080 seeds was sown). The experiment lasted one growing season. The substrate density was found to affect seedling height, root collar diameter, needles’ dry weight , shoots and roots, and total root length. The results showed that the growth parameters of Scots pine seedlings grown in nursery containers could be modified by selecting appropriate peat substrate density. Low (0.208−0.242 g·cm^–3) and high (0.308−0.342 g·cm^–3) substrate densities reduced seedling growth. In terms of the desirable seedling parameters, namely shoot−to−root ratio (S/R) and sturdiness quotient (SQ), the seedlings achieved the best values at medium substrate densities, i.e., from 0.26 to 0.29 g·cm^–3. We conclude that peat substrate compaction in the range 0.26−0.29 g·cm^–3 is recommended for growing Scots pine seedlings in 120 cm³ containers.

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