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The production of phosphoric acid produces “waste heaps” that have not yet been tapped, but which have the character of weak fertilizers and can perhaps be reintroduced into the elemental cycle in the forests. Two variants of mixing with organic ash and with sewage sludge were carried out. One-year-old pine seedlings (Pinus sylvestris L.) from the Trzebieź forest district (northern Poland) were planted in pots with soil that also came from the same field. Preparations containing phosphogypsum were applied topically to the soil in four doses (1, 2, 3 and 5 t/ha). The trial, which lasted one growing season, was conducted in four replicates. At the end of the trial, the height of the above-ground parts and root length, needle and root area, root neck diameter and photosynthetic performance were measured. The phosphogypsum-based preparations used showed no harmful (toxic) effects on the potted pine seedlings during the six-month trial period. The loosely prepared preparation made from a mixture of phosphogypsum and organic ash began to have a positive effect on the development of the seedlings’ root system, and it was also easier to mix with the soil surface than phosphogypsum with sewage sludge, which also contained a sticky form. The photosynthetic performance of one-year-old pine seedlings decreased after one growing season following the application of phosphogypsum preparations and most of the growth parameters tested did not differ from the control, so observations over a longer period (at least two to three growing seasons) are required. However, dosages of 1 and 2 t/ha seem to be the most promising, and these lower dosages are more economical to manage in nurseries or plantations, especially on poor sites. Formulations should be tested for heavy metals and their effects on seedling development. Testing should also be continued to monitor changes in the microbiome.

The present study was inspired by scientific reports describing the positive effects of silicon preparations on fruit and vegetables in horticulture. The use of silicon preparations in forest protection represents a new research application, especially in the cases of oak seedlings that are damaged by oak powdery mildew. Additionally, these preparations increase the photosynthetic efficiency of the seedlings, and thus promote increased biomass and better nutritional value of the root systems. The new idea of using silicon formulations in forestry, based on the initial test results, is particularly important for seedlings in the post-planting period in forest plantations. In particular, these preparations increase yield and plant growth, and improve plant health. So far, no comparable studies have been conducted on forest tree species. To this end, one year-old pine (Pinus sylvestris L.) and two year-old English oak (Quercus robur L.) seedlings were sprayed with silicon preparations of different concentrations, and changes in growth parameters and photosynthetic efficiency were observed. The main objective of the study was to determine the effect of the silicon preparation on the seedlings during their first life span, i.e., the first year after planting. In this study, 50 pine and 50 oak seedlings were sprayed three times with the silicon preparation, in five variants (control; concentrations of 1%, 2%, 3% and 5%), as well as the chlorophyll fluorescence, height, thickness and biomass of the roots, were measured. It was found that the treatment with a concentration of 2% was better and statistically, significantly different from the other variants, e.g., in terms of increasing photosynthetic efficiency.