Soil Management: Chemical Properties

Understanding the Basics: Soil Chemical Properties

To understand soil management, you need to know a little about the chemical aspects of the soil, such as soil pH, cation exchange capacity, and chemical properties of soil organic matter.

For a more detailed discussion of soil chemical properties, see the Best Management Practices book, Nutrient Management.

Soil pH

Soil pH refers to the level of acidity in a soil. The pH is a measure of the number of hydrogen (H+) ions that are in the soil.

The pH is recorded on a logarithmic scale that goes from 0 to 14. A pH of 7.0 is considered to be neutral. The higher the number, the less acidic or more alkaline the soil; the lower the umber, the more acidic the soil. With a logarithmic scale, a pH of 6.0 is 10 times more acidic than a pH of 7.0, while a pH of 5.0 is 100 times more acidic than a pH of 7.0.

Soil pH influences how efficiently a crop grows in a soil by affecting:

• nutrient availability (and potential toxicity)
• disease organism activity
• microorganism activity
• potential crop damage by some herbicides.

Most crops in Ontario grow best in soils that have a pH ranging from 6.0 to 8.0. Agricultural practices tend to lower the pH of soils over time, making them more acidic. This is the result of a number of activities:

• crops and plants removing nutrients
• fertilizer application, particularly banded ammonium fertilizers
• leaching or water flow through the soil, removing nutrients
• decomposition of organic materials
• acid rain.

Eventually, the drop in pH will become great enough to affect crop growth and yield, and you'll have to take steps to raise the pH. Soil pH can be raised using agricultural lime. Ontario Ministry of Agriculture, Food and Rural Affairs Publication 296, Field Crop Recommendations for Ontario gives the current liming recommendations with some discussion of lime quality.

Not all soils become acidic. In areas with alkaline (calcareous) subsoils, tillage practices tend to raise the pH. This is due to dilution with subsoil as a result of tilling too deep, tillage erosion, and wind and water erosion.

Soil pH should be tested regularly, as part of your normal soil testing program. Regularly soil test fields to which large amounts of nitrogen are being applied to monitor changes in soil pH.

Some plants need highly acidic conditions to grow, such as blueberries, rhododendrons, and chestnuts. In some instances it may be necessary to lower the soil pH. For example, to grow blueberries effec-tively, a pH of 5.0 or less is required. You can lower soil pH through the application of elemental sulphur. However, if the soil pH is high (above 6.5), this can be extremely expensive.

Poor crop growth is common in soils with a low pH.

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Cation Exchange Capacity

The cation exchange capacity is a measure of the capacity of the soil to hold some nutrients. It plays a role in soil fertility.

As soil minerals weather, cations are released into the soil water or solution. Cations are positively charged elements such as calcium, magnesium, hydrogen, and potassium (among others). These cations are attracted to the negatively charged surfaces of clay and organic matter particles. There's a constant exchange of cations between these surfaces and the soil water, called cation exchange. Cations aren't held tightly to these surfaces. Water can't remove them; however, they can be removed by changing places with canons discharged from plant roots.

The cations held on the organic matter and clay surfaces act as a reserve of nutrients, continually resupplying the soil solution with nutrients required by plants.

The size of the cation exchange capacity depends on the kind and amount of suitable surfaces for the canons to attach to. Organic matter supplies a much greater number of exchange sites for the cations than clay particles.

A high canon exchange capacity is desirable, because it indicates a fertile and resilient soil. However, the canon exchange capacity of a soil doesn't tell the whole fertility story - only the canon portion. That's why the Ontario system of fertility recommendation isn't based on cation exchange capacity.

High canon exchange capacities are associated with high clay contents and high organic matter levels. For sandy and loamy soils, it's not easy to change clay content. However, organic matter levels can be maintained and improved to enhance cation exchange capacity. Follow the best management practices for soil structure and organic matter.

Soil Organic Matter

Soil organic matter acts like a bank for many essential plant nutrients, by:

• providing exchange sites for cations such as potassium and magnesium
• releasing nitrogen during breakdown
• providing virtually all of the manganese and boron that crops require throughout the growing season.

If you've ever taken out an old fencerow to make a larger field, you'll know that the fencerow produces tremendous crops during the first few years in production. This is attributable to organic matter, both in nutrient release and soil structure.

The benefits of organic matter to soil structure, coupled with increased nutrient release, explain the dramatic yields. Tillage promotes greater aeration of the soil, which increases the breakdown of organic matter and releases a large quantity of nutrients to support the following crop. In fact, this bank of nutrients is what many farmers relied upon to sustain crop production before the advent of commercial fertilizers.

Unfortunately, tillage also reduces the level of organic matter over time to the point that it may become difficult to maintain good soil structure, and increased additions of fertilizer are required.

Available in Published Version of Soil Management

• Examples of Soil Texture and Cation Exchange Capacity - Chart

| Introduction | Physical Properties | Chemical Properties | Biological Properties |
| Information & Interpretations | Soil Structure | Erosion | Other Soil Management Problems |
| Best Management Practices for Soil | Table of Contents |

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