Page 108 - Nutrient Management Planning Guide

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Chapter 3.4

tip

Contact the soil testing laboratory with specific questions about the analytical techniques used to measure individual soil.

much from year to year and are typically quite low (less than 15 ppm for 0 to 15 cm depth). On fields with a history of manure application, the N and P levels may be considerably higher. Potassium levels of Alberta soils are relatively stable, often quite high (more than 150 ppm in 0 to 15 cm depth) and may exceed 500 ppm on Brown and Dark Brown soils, even without a history of manure application. Sulphur levels are variable and can range from less than 5 ppm to more than 50 ppm for 0 to 15 cm depth. Large year-to-year changes in soil nutrient levels should be investigated to determine the cause (e.g., management changes, change in analytical method, or mishandling of samples).

Excess nutrient levels may be suggested on a soil analysis report when nutrient levels are reported as being greater than a lab threshold (e.g., K is more than 600 ppm). Unless a dilution is performed, the lab will not be able to provide an exact nutrient level. While this has minimal influence on crop production, it can suggest nutrient levels that pose potential environmental risk. If high levels of nitrate (NO

3

- ) or P are suspected in a field, ask the lab to dilute the extract to get exact NO

3

- and P levels.

Soil analysis reports often include a subjective rating of nutrient levels (#6 in Figure 3.4.1) based on the probability that a particular nutrient will limit plant growth and production. Often these ratings are depicted as bar graphs for each nutrient. These subjective ratings may also help identify potential environmental risk.

For most soils, micronutrient levels are usually in the marginal range but are occasionally adequate or deficient. The probability of crop response to micronutrient application is not clear in many instances.

Soil quality factors including pH, salinity, organic matter, and texture (#7 in Figure 3.4.1) provide information useful for the site assessment and crop selection. Often soil quality factors will have a rating system that may flag potential problems.

Often kg/ha and lb/ac are interchanged and considered to be equal. However, the actual conversion is kg/ha x 0.8924 = lb/ac.

s i d e b a r

Labs based in western Canada do not emphasize the total cation exchange capacity or the composition of exchangeable cations (#8 in Figure 3.4.1). These analyses are usually included at additional costs. Other labs may recommend nutrient additions to “balance” exchangeable ions but there is little research evidence to support this practice.

Fertilizer recommendations are usually based on yield response curves or yield expectations (#9 in Figure 3.4.1) for a crop based on soil moisture and growing season precipitation. Some labs will provide more than one set of recommendations to account for different rainfall conditions (e.g., average and excellent). Fertilizer application rates can then be adjusted or selected based on expected rainfall.

Crop Nutrients

One of the basic principles behind formulating fertility requirements relates to the probability of a crop response to nutrient application (Figure 3.4.2).

From Beegle, 2006

Figure 3.4.2 Yield Response in Relation to Soil Nutrient Levels