By Doon Pauly, Agronomy Research Scientist
Alberta Agriculture and Forestry

Soil sampling and analysis produce a “snapshot” of the nutrient status of soil at a given moment in time. The results and corresponding recommendations can then be the basis for fertilizer application decisions. Soil analysis is most relevant when sampling occurs close to seeding because the nutrient “snapshot” is similar to what is available for the crucial early-season nutrient uptake period. Since most Alberta crops are spring-seeded, this would imply that spring is the optimal time for soil sampling, but this is not always the case and there are logistic and agronomic pros and cons for both fall and spring soil sampling.

Fall Sampling:

Pros:

• The “window” for soil sampling in the fall is generally wider than is the case in the spring. Unless sampling equipment can work through frozen soil, spring sampling must happen in the relatively brief time between spring thaw and crop seeding.
• Results and recommendations are available in the fall so informed fertilizer-related decisions, including fertilizer purchases, can be made during winter months.
• In many cases, fall sampling seems so much more convenient than spring.

Cons:

• Soil is a dynamic system and nutrient levels, particularly nitrogen (N) change throughout the year. Fall soil test nitrate-N may denitrify and be lost during spring snow-melt events. This is especially true for the black and grey soil zones which tend to have sufficient snow to produce denitrifying conditions in the spring. Over-winter denitrification losses are often, but not always, minimal in the brown and dark brown soil zones where winter snowfall is frequently insufficient to create water saturated surface soils in the spring.
• Fall soil analysis-based fertilization plans usually include a “fudge” or “guess” factor to compensate for potential losses.

 

Spring Sampling:

Pros:

• Spring soil test nutrients are unlikely to be lost from soil between sampling and the time the crop is growing and able to utilize them.
• Fertilizer-related decisions based on spring sampling can be made with greater certainty and less guesswork than is the case with fall analysis.

Cons:

• The entire process of field sampling, shipping, and lab analysis may take five or more business days, and may delay seeding operations, or at least add stress to decision makers.

 

Pointers for both fall and spring soil sampling:

• Microbial activity will affect soil test N levels. There is minimal N mineralization when soils are 5-7°C or less, so delay fall sampling until soils have cooled to these temperatures. In spring sample as early as possible when soils are thawed but still within this 5-7°C or less temperature range, even for late-seeded crops like dry beans or soybeans.
• Some sampling equipment can auger or core through frozen soils. Frozen soil sampling can produce reliable results if samples are handled properly and the lab receives representative material.
• Refrigerate soil soon after it is taken from the field. Deliver samples directly to a local lab or ship with “Next Day” or “Overnight” delivery to minimize the potential for samples sitting in warm conditions.
• Sulphur (S) and N are both soluble and move in soil as water moves. After growing seasons with above average or well above average growing season rainfall, both N and S may have moved from the nutrient-rich surface soil to sub-surface horizons. Sample deep enough to capture these nutrients at depth.