Take Soil Samples in Alfalfa Fields

While it’s regular practice to sample corn fields, I often get asked by farmers if they should sample their alfalfa fields. My answer is always the same, “Absolutely!” Alfalfa has a high demand for nutrients compared to other crops, making soil sampling even more valuable.

Soil is the basis for successful plant growth, so understanding your soil is the key to managing your fields properly. Field sampling and soil testing are important for assessing the fertility of your soil, as well as assisting you in choosing the proper fertilizer. More importantly, it can assist in diagnosing specific production problems.

The best time to sample your alfalfa field is mid-October or in early spring if time allows.

When collecting your soil samples, be sure to take the soil from 0-30 cm from approximately 15 to 20 sampling sites. Ideally, samples should be taken from an area where yield is typically average. This will ensure that your soil samples will be representative of the entire field.

The most beneficial tests of soils test for the following:

  • Nitrate-nitrogen
  • Available phosphorus
  • Available potassium
  • Extractable sulfer (15-60 cm)
  • Soil pH and salinity (electrical conductivity)

Remember to inspect soil nutrient and pH values.  An optimal soil pH should be between 6.0 and 6.5.

Timing is critical when it comes to applying nutrients to your alfalfa. Keep the following tips in mind for annual top-dress applications:

  • Fertilize immediately after harvest and before regrowth resumes, but avoid fertilizer contact with wet foliage.
  • Topdress following the first cutting to stimulate second and third cutting regrowth, or topdress in early September to increase winter hardiness.
  • Avoid topdressing plants when soils are soft, such as early spring, to avoid physical damage to alfalfa crown.
  • Split the application to avoid salt damage if using more than 500 pounds/acre of fertilizer material in any year. Base fertilizer purchases on cost per unit of plant food provided and need for all nutrients contained in the fertilizer.

Tillage prior to establishment is the last opportunity to incorporate nutrients such as nitrogen, phosphorus, potassium and sulfur during the life of the stand. By adding nutrients that are immobile, you can drastically increase their effectiveness.  Recyclable nutrients from manure can also be added at this time if proper weed control practices are followed.