Non-essential silicon (Si) is beneficial to plants. It increases the biomass of Si-accumulator plants by improving photosynthetic activity and alleviating stresses. Desilication, however, takes place because of natural soil weathering and removal of harvested biomass. Pyrolysis transforms Si-rich biomass into biochar that can be used to supply bioavailable Si. Here, we applied two biochar materials differing in Si content on soils differing in weathering stage: a young Cambisol and a highly weathered Nitisol. We studied the impact of biochar supply on the bioavailability of Si, cotton biomass, and Si mineralomass. The biochar materials derived from, respectively: Miscanthus × giganteus (Mi; 34.6 g Si kg−1 in biochar) and soft woody material (SW; 0.9 g Si kg−1 in biochar). They were compared to conventional Si fertilizer wollastonite (Wo; CaSiO3). Amendments were incorporated in soils at the rate of 3% (w/w). The content of bioavailable Si in soil was determined through 0.01 M CaCl2 extraction. In the Cambisol, the proportion (CaCl2 extractable Si: total Si content) was significantly smaller for Mi (0.9%) than for Wo (5.2%). In the Nitisol, this proportion was much larger for Mi (1.4%) than for Wo (0.7%). Mi-biochar significantly increased Si-mineralomass relatively to SW-biochar in both soils. This increase was, however, much larger in the Nitisol (5.9-fold) than in the Cambisol (2.2-fold). Mi biochar is thus an alternative Si fertilizer to Wo to supply bioavailable Si, increase plant biomass, and promote the biological cycle of Si in the soil-plant system in the Nitisol. Besides, it increased soil fertility and soil organic carbon content.