University of Illinois Extension
Working the Soil - Bed Preparation and General Maintenance - Stepping Stones to Perennial Garden Design - University of Illinois Extension

Working the Soil

Roto-till or dig the soil to a depth of six to ten inches. Adding any type of organic matter at this time is recommended. Your perennial garden, unlike an annual garden, is a long term planting. You only have this one pre-plant time to properly till and amend the soil. Once the plants are in, it is virtually impossible to totally work the soil without removing the plants. Organic matter such as compost, peat moss, leaf mold or aged manure is fine. Add a 3- to 4-inch layer over the garden and work it in.

Fertilizer may be needed at this time. Follow your soil test recommendations. If you didn't test the soil, apply a complete fertilizer such as 5-10-5 at a rate of 2 pounds (4 rounded cups) per 100 square feet. Add this when you add your organic matter and work everything in together, just like a cake batter.

If your soil pH needs to be adjusted, now is the time to do so. Perennials prefer a pH between 6.0 and 6.5. If your soil pH is lower than 6.0, it is acidic or sometimes called sour. You need to add ground limestone to adjust it. If your soil pH is higher than 7.0, it is alkaline, or sweet, and you will need to add an acidifier such as garden sulfur.

Chart 1. Pounds of ground limestone needed
per 100 square feet to raise pH to 6.5

Soil pH Sandy Loam Loam Clay loam
5.0 8 10 15
5.5 6 8 10
6.0 3 4 6

Chart 2. Pounds of sulfur needed
per 100 square feet to lower pH to 6.0

Soil pH Sandy Loam Loam Clay loam
7.5 1.8 2.5 3.9
7.0 1.4 2.0 3.1
6.5 0.8 1.2 1.9

Again, all these amendments can be spread over the bed and worked in all at the same time.