Replacement of existing soil with amendments is practical and suitable only for short term subjects such as flowering annuals and annual vegetable plants. Plant your ferns instead in loosened existing soil and apply compost to soil surface afterward, as mulch.

If the existing soil really seems completely unsuitable for these kinds of ferns then instead dig out a bed of it and replace the excavated soil with an actual soil (containing a proportion of mineral material such as sand and silt) for them to grow in.

Do not depend on organic matter alone to produce a lasting beneficial situation. And do not plant long term subjects in small areas of highly modified - as in half added organic matter - soil surrounded by unmodified existing soil. In addition to the problem of the organic matter decomposing and disappearing plants placed in amended planting holes and small narrow beds are subject to problems with how water movement into and out of the hole is affected by the differences in soil textures between the amended part and the unchanged area around it. (This may occur mostly around the sides and at the bottoms of amended holes, so that annual plants with their often tiny root systems are not affected - unless planted right at the edge of a narrow bed).

Not all California native plants are alike. We have deserts, mountains, coastline, moist areas (a few) and dry places. Try to reproduce the environment where these ferns grow naturally. That can best be determined by doing a Google search of the ferns you plan to grow. Their needs may vary.

Ferns like peat moss mixed in with the native soils. Just make sure you don't keep things sopping wet.