Page 117 - Nutrient Management Planning Guide

112

Practical Use of Soil Analysis Results

In contrast, the Black soils developed under a cooler, moister aspen parkland condition resulting in greater production of vegetation and organic matter accumulation. Dark Brown soils developed in the transition zone between the Black and Brown zones and has an intermediate organic matter content.

In parts of the province where trees have been the natural, dominant vegetation, Dark Gray or transitional soils developed. In regions where forest cover dominated for longer periods, Luvisolic (forest) soils developed.

Organic or peat soils are found in low-lying areas throughout the Black, Dark Gray and Gray soil zones. These soils formed where organic residues accumulated at a greater rate than they decomposed. These areas are characterized by waterlogged conditions for much of the year.

Typical soil organic matter levels for Alberta cultivated soils range from two to 10 percent (Table 2.2.2, Chapter 2.2). Specific soil organic matter levels will vary based on management history and landscape position.

The most common laboratory procedure for determining organic matter content is through loss on ignition whereby organic matter is incinerated and only the ash residue remains. Organic matter content is the difference in weight before and after the procedure.

More precise methods are used to determine organic carbon content. This involves correcting total carbon content in a sample for the presences of non-organic carbon (e.g., carbonate). Organic carbon is then used to calculate C to N ratios in the sample.

Estimated Nitrogen Release

Organic matter content is an important source of several key crop nutrients including N. Estimated N release (ENR) is an estimate of the amount of N expected to

become available from organic matter (i.e., mineralized) over the growing season. This estimate takes into account soil organic matter level, soil moisture, and temperature during the growing season. These are the major factors influencing the rate of mineralization from organic matter (refer to the discussion of organic matter in Chapter 2.2).

Typical ENR values for cultivated Alberta soils are provided in Table 3.4.9 and are based on typical soil organic matter levels for each area. Testing labs use ENR when developing N fertilizer recommendations. Consequently, labs may recommend lower N fertilization rates for individual situations where soil analysis ENR is higher than the expected typical range for that soil zone.

Table 3.4.9 Expected ENR Values for Alberta Soil Groups.

Soil Group

Cultivated Soil kg/ha lb/ac Mean Range Mean Range

Brown 31 30–33 28 27–29 Dark Brown 38 34–47 34 30–42 Black 56 39–81 50 35–72 Dark Gray 45 43–47 40 38–42 Dark Gray (Peace River Region)

41 34–54 37 30–48

Source: AF Field Research, Kryzanowski & Kelbert (2005)

Variability in growing season nitrogen release (mineralization) will exist from field to field depending on management history. Management practices such as direct seeding, rotation with forages or livestock manure application tend to build the more labile (easily decomposable) fraction of soil organic matter. This helps to improve the nutrient supplying power for a specific field situation. The average ENR’s in Table 3.4.9 may underestimate the actual field values.