Andrew Kness, Extension Educator, Harford County
Potassium (K) is one of the three primary macronutrients that all plants mine from the soil that is necessary for proper growth. On a fertilizer grade analysis, it is the third number listed (in the form of potash) behind phosphorus and nitrogen; yet, potassium is sometimes overlooked as an important plant nutrient until problems arise.
This year, potassium deficiency symptoms appear to be occurring more than in years past. The growing conditions that we have experienced this year have contributed to potassium deficiency. One obvious factor that affects potassium deficiency is low soil K; if there’s little to no K in the soil, plants will show deficiencies. However, K deficiency can appear even in soils that have adequate levels of soil K if certain criteria are met.
Potassium deficiency tends to show up and/or become exaggerated during periods of drought and in compacted fields. Under drought and compacted soil scenarios, plants have smaller, restricted root systems that cannot grow deep enough to access soil K located deeper in the soil profile. Planting into wet fields in the spring can cause soil compaction and glazed sidewalls, leading to K deficiency if dry conditions manifest later in the season. This is exactly what we’ve seen this year in Maryland. Potassium deficiencies do not often appear early in the season, especially in corn. Symptoms typically begin to appear a few weeks after planting, and in corn, begins to appear during jointing.
Potassium deficiency starts out as chlorosis (yellowing) of the leaf margins (edges), progressing down the leaf blade with the midrib staying green (Figure 1). Eventually the tip will begin to die, advancing down the leaf until it is dead. This is opposite of nitrogen deficiency symptoms, which starts as chlorosis in the middle of the leaf and progresses towards the margins. Potassium is mobile in the plant; meaning that plants can sacrifice K in the lower leaves and move it up to the growing point (newly emerging leaves) to support growth. Therefore, symptoms of K deficiency appear first in the lower leaves and progress up the plant as it grows taller.
Managing a potassium deficiency mid-season can be achieved by topdressing with a K fertilizer if the plants are still small enough. Keep in mind that a granular K fertilizer will need to be dissolved and soaked down into the soil profile, so a mid-season application will only help if it rains or you irrigate following application. Successful management of K should start with your soil test. Test your field to see how much K you have and if you’re low, then fertilize accordingly. Avoid practices that can cause soil compaction, such as working or planting wet ground. If you have compacted soils, consider subsoiling in the fall or planting deep-rooted, diakon-type radishes to shatter the hardpan. Control weeds, as they can compete with your crop for potassium.
Most crops require a large amount of potassium; roughly the same or more than nitrogen. Keep in mind that forage crops, including hay, remove substantial amounts of potassium from the soil (upwards of 200 lbs/acre/yr), so it is necessary to fertilize with K to replace what’s been removed, especially if you plant behind a hay or forage crop.
