Every now and then I have the opportunity to provide guidelines on soil preparation to ensure landscapes not only survive but thrive. With the number of samples that come into this office that lack proper soil preparation, I thought it was about time to list a number of steps that should help you do the right thing for your landscape and garden plants.
As Don Mundy of High Country Lawn Care mentioned to me the other day, you don't skimp on the foundation for your home, so why should you skimp on the foundation for your landscape plants.
For those of you who don't prepare the soil properly prior to planting you can expect more insect and disease problems and more trouble keeping those plants alive when water use is restricted.
I want to take this discussion step by step starting with the subsoil. This is the soil upon which the planting soil is placed and into which roots penetrate to anchor trees and shrubs in the ground. The planting soil may only be a few inches deep so it is critical to amend the subsoil to allow roots to penetrate and water and oxygen to enter. For this to occur the subsoil should be amended with composted organic matter with a particle size of 20 to 30 mm (0.8 to 1.2 inches in size). Avoid the use of any organic matter that is finer as this will block drainage and result in root rot. If subsoil drainage is deemed inadequate, the organic matter should be increased up to no more than 50% by volume until drainage is no longer a problem.
To determine if drainage is a problem, cut both ends off a can. Push one end an inch or two into the soil. Fill the can with water. If the water in the can drops two inches or more in an hour, the subsurface soil has adequate drainage. If the water in the can is slower in entering the soil, the soil will need more amending with organic matter. If the layer of planting soil applied is less than 6 inches thick, the subsoil should be amended to a minimum depth of eight inches.
Adding sand is not appropriate as this can result in compaction and poor drainage unless you use sand with particles that are two millimeters in size or larger, silica-based so it is slow to break down, and has been washed to remove all fine particles. The amount of this sand needed depends on the amount of clay in the soil. The more clay, the more sand is needed to keep the soil from setting up like concrete.
The planting soil should have all roots, clots and stones larger than 1 inch in greatest dimension, noxious weeds, and other litter, removed. The compost for the planting soil should consist of particles between 2 and 5 mm in size for non-traffic areas, and 5 mm to 15 mm in size for areas subjected to foot traffic.
Ideally, a layer of the amended planting soil should be applied on the subsoil and the two mixed together. Then more layers of the amended planting soil should be applied until the desired depth is achieved. Blending the planting soil into the subsoil is critical to prevent the development of a barrier between the two soils. Not blending the soils together can create a water table developing on top of the subsoil. This results in water saturated subsoil into which roots cannot penetrate.
I have not yet addressed tests for the composted organic matter or soils. The first test necessary is to determine if soluble salts are going to be a problem. If landscape plants or a lawn is going to be established on this soil, the soluble salt level of the compost and soil used should be 5 mmhos/cm or lower based on the Saturated Paste Extraction method. This is a measure of electrical conductivity where the higher number means more salts. If this soil is going to be used for a vegetable or flower garden the soluble salt number needs to be even lower.
The next test that should be conducted is one to determine the nutrient content of the planting soil. While a low nitrogen level is easily corrected after planting, a phosphorus or potassium deficiency is not. Ideally, these two nutrients should be worked into the soil prior to planting.
Dr. Curtis E. Swift is the area horticulture agent with the CSU Extension. Reach him at Curt.Swift@mesacounty.us, visit WesternSlopeGardening.org, or check out his blog at http://SwiftsGardeningBlog.blogspot.com.