I've already germinated giant sequoia (Sequoiadendron giganteum) seeds for 6 weeks.
What's the ideal soil conditions for planting the saplings?
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Sequoias require moist, rich, balanced ph, and well-drained soil. Permanently swampy or muddy soil will not work.
Based on this, you should make sure your soil is neither sandy nor hard clay... you'll want a loam type of soil.
Study their native range.
Giant Sequoia (Sequoiadendron Giganteum) grows in a sandy loam soil in California it's a granitic residual.... Though adaptable to many soil conditions You can replicate it. If you have silt/clay soil it's fine. Break it up real good and mix it with 25% sand with granite dust (a countertop maker which cuts granite will give it to you), 40% potting mix/compost blend and 40% the soil you broke up. Well drained.
Another recommendation is to give your tree a mycorrhizal inoculation.The mycorrhizal fungus basically serves as an extension of the plant root system, exploring soil far beyond the roots' reach and transporting water and nutrients to the roots; by producing micro filaments thousands of times smaller than a normal root, these filaments soak up nutrients like a sponge and give it to the plant, In exchange for a steady diet of sugars. For example: Pine Tree on the right, same age but grown without a myco inoculation
Tree on the left, Same Age grown WITH.