The wording in the source

Timing

• Therefore, sampling in late fall to mid-spring is recommended to best capture growing season N availability.

  Soil Scoop, Timing

• If fall nitrate levels are very high (e.g., greater than about 60 lb N/acre) and soil depth is less than two feet, a second sampling in spring is strongly suggested because there is a high likelihood of overwinter nitrate losses.

  Soil Scoop, Timing

Method

• Grid sampling is good for fields with a history of manure or high fertilizer levels (especially P and K), and small fields merged to form a larger field.

  Soil Scoop, Location

• Zone based sampling is suggested for fields with unknown or long cropping history, historically low levels of fertilization, no manure applications, and high with-in field variability or a relationship of yield to landscape.

  Soil Scoop, Location

• Uniform fields can be randomly sampled throughout the entire field.

  MontGuide MT200803AG, Random Sampling

• All sampling points should be georeferenced with a global positioning system (GPS) receiver and sampled in the same locations in subsequent years to track soil nutrient variation in a field, to see long-term trends, and to guide fertilizer application location.

  MontGuide MT200803AG, Types of Sampling

Depth

• Pasture and forage get sampled to 12-inch depth, and annual crops to 6-inch for most characteristics and at least to 2-foot depth for N.

  Soil Scoop, Depth

Cores

• Ten subsamples per composite sample is a reasonable average. More subsamples require more time, but improve accuracy.

  Soil Scoop, Number

• if an accuracy level of ± 25% is deemed sufficient at a 90% confidence, then 12 subsamples per field (or zone) should be collected, composited and analyzed for nitrogen, phosphorus, and potassium.

  MontGuide MT200803AG, Number of Subsamples

Laboratory and analysis

• For example, soil P levels in Montana soils are best tested with the Olsen rather than Bray or Mehlich tests, which are not suitable for soil pH greater than 6.

  Soil Scoop, Lab Selection

• Also, Montana’s P fertilization guidelines are based on Olsen P values.

  Soil Scoop, Lab Selection

• Ask if the lab is enrolled in a ‘proficiency’ testing program.

  Soil Scoop, Lab Selection

• If a lab is reliable, stay with the same lab in subsequent years, because soil test results and recommendations can vary greatly among labs.

  Soil Scoop, Lab Selection

Field identifiers

• Keep records of sampling locations and timing.

  Soil Scoop, Location

• All sampling points should be georeferenced with a global positioning system (GPS) receiver

  MontGuide MT200803AG, Types of Sampling