Page 45 - Nutrient Management Planning Guide

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Chapter 2.2

Generally, the environmental factors that influence plant growth also impact the activity of soil organisms and the rate of organic matter decomposition. These factors — aeration, moisture, temperature, and nutrient availability — are described in more detail below:

Decomposition is an aerobic process (i.e., requires •

oxygen). Soil conditions that limit aeration (e.g., water logging, compaction) will slow decomposition. Decomposition is optimal when the soil is near or •

slightly wetter than field capacity. Extreme moisture conditions (i.e., too dry or too wet) can impede decomposition. Optimal soil temperature for decomposer microbes •

is in the range of 25 to 40°C. Soil temperatures in Alberta are typically below this range; therefore, decomposition is much slower during spring and fall. Soil organisms have specific nutrient requirements •

to function. Microbial growth will be limited if any nutrient is lacking in the system, resulting in a reduced rate of decomposition.

The rate of decomposition varies through the year, between years and even across the landscape as environmental factors change.

Carbon to Nitrogen (C:N) Ratio: Organic Matter Cycling and Nutrient Release

The C:N ratio in soils and residues has a significant impact on decomposition and nutrient release. The C:N ratio in soils is about 10:1. Adding organic residues to the soil changes the C:N ratio. Decomposition is slowed when C:N ratio is high (greater than 30:1) and rapid when C:N ratio is low (less than 20:1). Generally, N is released when C:N is less than 20:1, and N is immobilized when C:N is greater than 30:1 (Figure 2.2.2).

Adapted from Brady and Weil 2000

Figure 2.2.2 Effect of (A) High and, (B) Low C:N Ratio in Added Organic Residues on Soil Available N Level

Management Factors That Influence Organic Matter

Cultivation has the largest impact on soil organic matter content (Table 2.2.2). This management practice accelerates the loss of soil organic matter because:

Cultivation aerates the soil and this promotes the •

activity of decomposer organisms. Bare soil warms faster in the spring and this •

increases the activity of soil organisms and creates a wider window for decomposition during the growing season. Cultivation reduces ground cover and this increases •

the risk of soil erosion. Surface soil is the most susceptible to loss and contains the majority of soil organic matter. Cultivation physically mixes crop residues into the •

soil where decomposition occurs.

Cultivating fallow repeatedly promotes decomposition and organic matter loss throughout the season. Reducing tillage helps to preserve soil organic matter.

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