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Does your forage fertility plan have enough potassium?

Matt Laubach for Progressive Forage Published on 28 February 2017

High-yielding, quality alfalfa and corn silage place a high demand for potassium on soil’s nutrient- supplying power. To maximize animal performance and profitability, it’s important to look and keep a close eye on the potassium (K) level in your forage fields.

In recent years, the price of potash (K20) has risen to very high levels, and the overall market has seen greater volatility. Because of the increased price of potash, combined with lower-than-average grain or milk prices, many growers have not applied the full rates of potassium in an effort to cut costs.

At the same time, many growers are transitioning to more corn silage acres and higher yields of corn silage and alfalfa. These higher-yielding crops require higher potassium amounts, which further impacts soil potassium levels. Many growers have not adjusted their potassium fertility programs for these changes.

A recent DuPont Pioneer study, conducted across 12 states, confirmed that phosphorus and potassium deficiencies can lead to a loss in yield potential and grain quality. Comprised of more than 22,000 soil samples, the study demonstrates that phosphorus and potassium were deficient in a significant number of tested fields.

Those numbers may be further exacerbated by the higher nutrient requirements of today’s more productive hybrids and varieties.

Role of potassium

Required by both alfalfa and corn, potassium helps regulate the enzymatic processes of these crop staples. What does that mean? It means potassium can influence:

  • Photosynthesis
  • Cell division
  • Carbohydrate production
  • Protein synthesis
  • Root development
  • Tolerance to temperature extremes
  • Tolerance to drought through regulating water use
  • Minimize susceptibility to disease

In alfalfa, potassium helps the plant endure cold temperatures during the frigid winter months, while also aiding in nitrogen fixation and utilization in growing plants.

In corn, potassium deficiency can lead to shortened leaf photosynthesis, compromising the integrity of the plant and limiting the usefulness of nitrogen.

The real impact of potassium can be seen in times of plant stress. When in proper balance with nitrogen, phosphorus, sulfur and other micronutrients, potassium reduces a plant’s vulnerability to stress while increasing yield and improving quality.

Both alfalfa and corn silage remove a tremendous amount of potassium from the soil profile. A corn crop takes up nearly as much potassium as it does nitrogen, yet management of each nutrient is entirely different. Managing potassium for corn silage production is different than for high-moisture grain, snaplage or earlage harvest.

Managing potassium for alfalfa

When applying potassium to alfalfa, the best amount will depend upon yield goals and harvest schedules, as potassium needs increase with harvest frequency and higher yields. Alfalfa requires the most potassium just before the winter months, which helps improve the winter hardiness of the plant.

It is also needed during this period to improve the storage of soluble carbohydrates in the plant’s roots.

Avoid applying high levels of potassium in the spring before first cut. This can lead to excessively high potassium levels in first-growth alfalfa, increasing the chances of milk fever in dairy cattle.

If soil fertility requires a boost, consider splitting the nutrient application into two parts to decrease the chances of salt injury or luxury consumption from the alfalfa.

Multiple applications of potassium ensure there is enough of the nutrient during the most critical times of plant growth. The very highest- yielding alfalfa will have potassium concentrations as high as 3 percent. When that concentration drops below 2 percent, the plant is much more susceptible to winter injury.

As alfalfa ages, its response to potassium increases. Therefore, it is critical to continue fertilizing aging stands to ensure maximum productivity.

Managing potassium for corn silage

No matter which parts of the corn plant are harvested, the growing plant has the same potassium needs. Nutrient uptake mirrors plant growth. The highest demand period for potassium is during the grand growth phase from V6 through silking. Having high available potassium at this time is critical to ensuring successful plant development and growth.

The difference comes from the replacement of the nutrients removed. Corn silage fields may take as much as five to six times more potassium than fields where just grain was removed, because potassium accumulates in the vegetative material of a growing plant.

Most growers will apply potassium ahead of planting, or during the previous fall just prior to tillage. This is often supplemented with starter fertilizer applications in the seed zone made during planting.

Maintaining very high yields of alfalfa and corn for grain or silage requires optimum to high soil test levels of potassium. These levels are best maintained through a renewed emphasis on soil testing and making adjustments for K removal rates from a field.

Many states have modified recommendations for phosphorus and potassium fertility in recent years, and growers need to make sure their programs are keeping up with the changes.

Finally, a potassium fertility program for your farms requires a balanced approach. It is important to provide all the essential nutrients in the proper amounts.  end mark

Matt Laubach
  • Matt Laubach

  • Dairy Account Manager
  • DuPont Pioneer - based in Minnesota





Take it to the field

  • Test: Soil test your fields regularly to track your potassium levels, especially after alfalfa and corn silage crops.

  • Calculate: Measure or estimate annual crop removal of potassium. Account for manure and other fertilizer sources applied to the field.

  • Plan: Make a plan to replace potassium based on removal rates, manure use and soil test levels.

  • Alfalfa: Apply twice per year – once after first harvest and once after last summer harvest.

  • Corn: Apply needed potassium prior to planting and work it into the soil.