Fall Applications of P, K, and Lime

If required, phosphorus (P), potassium (K), and lime are typically applied in the fall because of the need for application prior to tillage and the likelihood of dry soils. Application rates should be based on current soil samples taken at least every four years. Follow sampling techniques recommended by your state university extension system.

Lime Application. Lime neutralizes soil acidity and adds calcium, which is a micronutrient essential to plant growth. Plant nutrients are most available in a recommended soil pH range between 6.0 and 7.0. Nutrient deficiency symptoms may appear in crops if soil pH falls below 5.5. If needed, lime should be applied about three to six months before planting as it takes time to dissolve in the soil and neutralize soil acidity. Applying lime in the fall allows time for it to breakdown before the next growing season. Lime should be applied and incorporated at least a month before adding fertilizers since it can interfere with the availability of fertilizer nutrients, especially P. Attention should be paid to the different sources of liming material. Differences among products in their neutralizing efficiency (calcium carbonate equivalent and particle size composition) do exist and can influence optimum lime application rates.

P and K Application. Fall is typically the time for P and K applications because of dry soils, available fertilizers, workload, and the need for application before fall tillage. Fall is also a good time to apply P for environmental reasons, since there is generally a lower risk of P runoff with the typical fall weather and soil conditions. P and K fertilizer should not be applied on frozen ground because applications made to fields with any slope may result in significant runoff and fertilizer loss

In order for the soil to supply adequate quantities of nutrients to support optimum plant growth, soil test recommendations should be followed to maintain soil P and K above their respective “critical levels. Generally, with adequate existing soil fertility (above critical levels) crop removal fertilizer applications (also referred to as maintenance fertilizer) should be made. For example, each bushel of harvested corn grain removes the equivalent of 0.35 pounds of P2O5 and 0.25 pounds of K2O.1 These values are used to determine maintenance fertilizer application rates.

Operating at low soil test levels (less than critical levels) of these nutrients may reduce yield potential, especially concerning K. Therefore, P and/or K “buildup fertilizer applications may be recommended in addition to crop removal rates.

It is common to apply P and K fertilizer once for two years of crop production. In a corn-soybean crop rotation, all of the K can be applied after soybean harvest in the fall ahead of corn. Enough K will be returned to the soil from the corn residue for the soybean crop in the rotation. However, recommended rates of fertilizer should be applied annually to soils that are below critical levels of these nutrients. As stated earlier, these applications include P and K soil build-up and crop removal needs.

Very high P and K soil test levels may not need any crop removal fertilizer applied for several years due to the high levels of available P and K. Follow recommendations by your state university extension system.​