Potassium Fertility

Potassium (K)

There are two distinct approaches to managing potassium (K); one is referred to as the “Sufficiency Approach” and the other is the “Build (or Target) and Maintain Approach”.  OMAFA K guidelines for corn as outlined in this section adhere to the Sufficiency Approach, for a more detailed explanation of the two approaches and how they influence K decisions, see Chapter 9, Soil fertility and nutrient use[BR1] .

Adequate potassium is necessary for optimum corn growth and yield, although the response to these nutrients is not as evident as with nitrogen. Potassium deficiency symptoms appear on the lower leaves of the plant first, showing as yellowing and browning beginning at the tip and proceeding back along the outside margin of the leaf (Figure 1). Both of these nutrients will exhibit “hidden hunger,” where yields are reduced by a deficiency even though no deficiency symptoms are visible.

Potash (K₂O) guidelines for corn are presented in Table 1.

Table 1. Potash (K2O) Guidelines for Corn

For information on the how to use these tables or if an OMAFA-accredited soil test is not available, see Fertilizer guidelines.

Where soil tests indicate that large amounts of potassium is required, the major portion may be broadcast and incorporated in the fall or spring.

Maximum Safe Rates of Fertilizer

Applying too much fertilizer to corn may result in crop injury, either from excessive salts or ammonia (Figure 2). The more concentrated the fertilizer and the closer it is to the seed, the greater the risk of crop injury and the lower the safe rate. Maximum safe rates are given in Table 9–24. Note that slight reductions in crop growth and yield are possible with these application rates under adverse weather conditions.

Potassium (K): Band vs. Broadcast

Potassium (K) included in starter fertilizers can result in profitable corn yield increases, especially when soil test K levels are less than 90 ppm.  Table 2. Corn yield response to broadcast potash (K) applications with various starter fertilizer options, contains results from Ontario research trials, which evaluated corn yield response to various starter fertilizers. When soil-test K levels were less than 90 ppm, and no broadcast K was applied, applying a MAP/Potash blend in a 5 cm x 5 cm (2 in. x 2 in.) starter band increased corn yields significantly.  In these same circumstances, seed placed liquid fertilizers that also contain a small amount of K, produced higher corn yields than where no starter fertilizer was used or where starter fertilizers contained only P. On these lower testing soils when K was broadcast prior to planting (fall or spring), yields were improved significantly by the broadcast K and the magnitude of the yield response due to the starters was reduced.

These data generally indicate that broadcasting K on the lower testing soils is advised. However, in situations where land tenure is in question and broadcasting a significant amount of K to build soil tests is risky, a producer with the capability to band dry fertilizer P and K blends can generate yields equivalent to other options.

On higher testing soils, the amount of yield response to any applied K is much lower.  Some K in a starter band can improve yields, but generally speaking the advantage to higher K rates in dry 5 cm x 5 cm (2 in. x 2 in.) bands compared to lower in-furrow rates is marginal.

If broadcast K is to be applied either in the fall or spring prior to corn planting, the need for K in the starter is significantly reduced unless soils are low testing (HR) (e.g., less than 61 PPM). In these low K fertility situations, broadcasting to build soil fertility levels and banding to help meet the crops immediate requirements are likely both profitable.

Table 2. Corn Yield Response to Broadcast Potash (K) Applications with Various Starter Fertilizer Options