Urban Programs Resource Network

News Releases

Index

Soil Amendments Impact Soil Test Results

FOR IMMEDIATE RELEASE
March 13, 2013

Gardeners already know the importance of testing their garden beds every few years. If soil amendments are added annually, then the soil test should be done more often to be sure that nutrient levels and, just as important, the pH level are still within an acceptable range, said University of Illinois Extension horticulture educator Richard Hentschel.

"Because amendments are so important to the continued health of the soil, there has been a substantial increase in their use. For the most part, adding well-composted organic matter will not cause a growing problem in the garden beds," he said.

"Organic matter today can come from your own compost bins, the store, or other sources such as municipal composting programs," he continued. "There is something very satisfying about knowing that the spent plant parts from your own yard are being reused in your own beds in the form of composted organic matter."

As soil organisms break down organic matter during composting, the pile or bin can become more acidic. If this material is added to gardening beds and incorporated into the soil profile, over time it will change the soil pH.

"If your soil is alkaline anyway, this is a good thing, helping to keep pH levels between 6.0 and 7.0," Hentschel said. "If you have a garden soil that is normally acidic, it may be necessary to add garden lime as well as organic matter to maintain the pH level."

Only a soil test will indicate the need for lime, so do not assume it is necessary. Addressing the pH is important, as this will influence the levels of other nutrients available to the garden plants. In general, if the pH becomes too acidic or alkaline, nutrients will not be readily available for uptake by the plant. There are exceptions: Irish potatoes prefer an acidic soil, and asparagus prefers an alkaline soil.

"Besides composted organic matter from your own yard, there is a long list of amendments that can be used," he said. "These include peat moss, corn cobs, rice hulls, aged manure, wood ashes, and spent leaves.

If organic matter is used frequently, a gardener might want to invest in a soil pH meter for use in between commercial tests. There are many kinds available; the easiest are probe types where the probe is inserted into the soil profile and a reading is available immediately. Some are a single piece; others use a probe connected to the meter. The key to getting an accurate reading is to sample the garden bed in several places, inserting the sampling probe to the same depth and the same moisture content. There are other tests that the home gardener can perform but, because pH levels influence so much in the soil, this test may be the most important.

Commercial soil tests are most often done in the late summer or fall to allow enough time for the organic matter added in the spring to react with the garden soil and ensure that the test results accurately reflect pH and nutrient levels.

"Soil test results should measure soil pH and the major nutrients used by plants, such as nitrogen, phosphorus, and potassium. They may also include calcium, sulfur, and magnesium as secondary nutrients. If organic matter levels are available, get them, too," he said. "Organic matter levels between 3 and 5 percent would be the target."

Source: Richard Hentschel, Extension Educator, Horticulture, hentsche@uiuc.edu