Fall soil sampling is a great and relatively inexpensive tool to help plan for the next crop season. It is important to assess nutrient levels in your crop fields to manage fertility inputs. Knowing test results of your fields such as pH, phosphorus, and potassium, etc. will help you make informed decisions around soil inputs.

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Soil Sampling

The first step in soil fertility planning is to take a soil sample and have a local analytical laboratory test the soil. A quality sample is required to have accurate results.

• Sample after harvest when there are no crops in the field
• Sample fields at the same time of year, so test results are more comparable across time.
• To allow enough time for soil adjustments and planning inputs, sample 3-6 months before the next crop
• Avoid sampling fields where inputs were recently applied (lime or fertilizer)
• Take a soil sample from a field at least once every 4 years

Good soil sampling technique can make a difference in receiving accurate test results. Decide how many samples you will take per field to gain a representation of the field soil conditions, avoiding field edges or low areas. Take a vertical column of soil, 6 inches deep, and mix it with other soil cores to get a sample for a portion of your field, 25 acres or less.

pH

The basic definition of pH is described as the measure of acidity or alkalinity of soil. It is measured on a scale of 1 to 14, where 7 is neutral. Soil pH is a measurement of the hydrogen ion activity in the soil solution, which is influenced by the type and amount of clay and organic matter content of the soil.

Soil tests may show a buffer pH result – this indicates the amount of agricultural lime required to neutralize the hydrogen ions from the soil. “Buffer” refers to the ability to release acidity ions into the soil – for example, high clay soils are highly “buffered” and require more lime to raise pH to a certain level than sandy soils.

The availability of some plant nutrients are affected by soil pH. Field crops perform best at a soil pH between 6.0 – 7.5, depending on the crop. In many areas of Canada, soil pH can decrease or become more acidic due to factors including nutrient removal by crops, leaching of basic nutrients (cations in the soil), and using ammonia-based nitrogen fertilizers. Adding liming materials can raise soil pH levels for ideal crop production where nutrients are more available to the plant, as well as create a healthy environment for critical soil microbes.

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​​​​​​​Phosphorus (P) and Potassium (K)

Phosphorus is commonly a limiting nutrient in crop production. While most soils have adequate phosphorus, the amount of available phosphorus is low as mineralization of this nutrient is slow. Phosphorus moves very little in the soil and does not leach even with large amounts of precipitation. pH of the soil solution impacts phosphorus availability because it changes the phosphorus form (usable ionic form or unusable form) – soil pH between 6.5 to 7.0 allows the most phosphorus availability in soil solution. Soil test results report phosphorus as an estimate of what is available to plants, not the total P in soil.

Potassium is an exchangeable ion that easily binds with charged soil particles, locking it in mineral structures. Nitrogen is the only other nutrient absorbed in larger quantities than potassium (depending on the plant). Potassium is vital to many plant functions and cycles back into the soil from crop residue with precipitation. Dry conditions limit the movement of potassium in the soil. Generally, potassium in the soil is unaffected by soil pH, but may be limited by factors such as soil type, wetting and drying cycles, and soil aeration and moisture.

Regular soil testing enables tailored nutrient management to help build healthy, resilient soils over time. It educates you on the current status of each field and helps you to increase the effectiveness of fertility investments. Understanding and managing key nutrients and soil parameters like pH is very important in crop production. Your local NK agronomist can answer your questions and provide further information.

References
University of Nebraska Extension – Plant and Soil Sciences eLibrary
Nutrient Stewardship
University of Minnesota Extension
University of Nebraska Extension – Nutrient Management for Agronomic Crops in Nebraska

Speak to your local NK® Agronomist for more information, or join the conversation online – connect with @NKSeedsCanada on Twitter and Facebook.

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