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Soil Fertility and How to Properly Sample

Fall soil sampling is a relatively inexpensive tool to help plan for the upcoming crop season. Understanding soil sampling results informs nutrient decision making on a field-by-field basis.

Soil Sampling
Good soil sampling techniques ensures accurate test results. Avoid field edges or low areas and decide how many samples you will take per field to gain an accurate representation of soil conditions. To get a sample for a portion of your field 20 acres or less, take a vertical column of soil 6–8” deep and mix it with other soil cores.

​​​​​​​Get the most out of your soil samples by:
  • Taking a sample from a field at least once every 4 years
  • Sampling fields at the same time every year
  • Sampling after harvest when there are no crops in the field
  • Avoiding sampling where inputs were recently applied, such as lime or fertilizer 
  • Allowing enough time for planning adjustments by sampling 3–6 months before the next crop

Source: Colorado State University– CMG Garden Notes #222

Understanding Soil pH
Soil pH is described as the measure of acidity or alkalinity of the soil. The pH scale is 1 to 14, where 7 is neutral. The type and amount of clay and organic matter content in the soil influences the hydrogen ion activity, which is the basis of the soil pH result. Soil tests may show a buffer pH result to indicate the amount of agricultural lime required to neutralize the hydrogen ions from the soil. “Buffer” refers to the ability of the soil to release acidity ions into the solution. For example, high clay soils are highly “buffered” and require more lime to raise pH to a certain level than sandy soils.

Field crops perform best at a soil pH between 6.0–6.8 depending on the crop, as the availability of some plant nutrients are affected by soil pH. Soil pH can decrease or become more acidic due to nutrient removal by crops, leaching of basic nutrients like soil cations or 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 and create a healthy environment for critical soil microbes.

Potassium (K) and Phosphorus (P) Soil Activity
K is typically unaffected by pH, but it may be limited by factors such as soil type, wetting and drying cycles, soil aeration and moisture. Considerations to keep in mind with K:
  • K is an exchangeable ion that easily binds with charged soil particles
  • The only other nutrient absorbed in larger quantities than K is nitrogen
  • K is vital to many plant functions and cycles back into the soil from crop residue with precipitation
  • Dry conditions limit the movement of K in soil
P is another critical nutrient. Soil test results report P as an estimate of the nutrient available to plants, not the total P in soil. Several helpful tidbits to keep in mind about P:
  • P is commonly a limiting nutrient in crop production
  • The amount of available P is low because mineralization of the nutrient is slow, even while most soils have adequate P
  • P moves very little in soil and doesn't leach even with large amounts of precipitation
  • The pH of the soil solution impacts P availability because it changes the P form, whether the usable or unusable ionic form
  • Soil pH between 6.5–7.0 allows the most P availability in the soil solution
Understanding key nutrients and soil factors such as pH, K and P guide potential soil inputs you may need to apply to maximize crop yield. Soil sampling allows you to fine-tune your nutrient management plan to build healthy, resilient soils from one season to another.

Contact your Golden Harvest Seed Advisor help answer questions about managing your soil fertility and provide further agronomic insight.

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