How to Build a Garden in Unstable Soil

Create a trench along the sloped land’s base. Make the trench a few inches deeper and wider than the block’s dimensions.

Pack the soil at the bottom of the trench by striking it repeatedly with a hand tamper.

Fill the trench with 2 inches of gravel and then compact the gravel with the hand tamper. Lay the first course of blocks. If necessary, cut the final brick to fit by breaking it with a chisel and 3-lb. hammer. Regularly place a level across the tops of bricks checking for evenness. Add or subtract gravel from the trench bed for necessary adjustments.

Lay the second course of blocks. Stagger the seams with the blocks’ first course. Sever a block in half to start the staggering. Continue building up courses of blocks in the same manner until reaching a height at least the same as the slope’s midpoint.

Pack soil into the remaining trench cavity on the wall’s front side.

Cover the retaining wall’s inside with weed-blocking fabric, securing the top edge between the last and second-to-last block courses.

Filling the space behind the wall with a few inches of gravel provides garden drainage.

Removing soil from the slope’s high point and adding soil to the lower point levels the land just below the retaining wall’s top.

Nourish the soil by blending in approximately 25 percent compost. Conduct a soil test using a soil test kit. Add any further amendments as indicated by the soil test. Covering the ground with mulch holds in topsoil. Wait until the next growing season before planting so that the amendments have time to take effect.