Don't let your potassium levels slip
Sunday, November 3, 2013
Low K levels can occur in any soil. Ignoring this nutrient can lead to some very costly yield losses
by KEITH REID
Potassium has been described as the Rodney Dangerfield of nutrients. To paraphrase, "it just don't get no respect!"
This lack of attention may stem from the long history of potassium (K) being much cheaper, per pound of nutrient, than nitrogen or phosphorus. This price gap has narrowed significantly in the past few years, but there is still some sense that lower price means lower value. Unfortunately, ignoring this nutrient has led to some very costly yield losses for farmers who let their K levels slip. Low K levels can occur in any soil, depending on several factors.
Soil type. Sandy soils do not have a large capacity to hold on to K, so levels can drop very quickly. In contrast, clay soils have a much greater capacity to hold on to K than sand. In addition, the types of clay minerals present in our soils already contain a large reserve of K that is released as the clay weathers.
The amount of potassium removed during harvest. The largest removal of potassium, by far, is with crops where the entire plant is harvested, since most of the K is in the plant sap rather than concentrated in the seeds. Forages and corn silage can each pull hundreds of pounds of K from the soil in a growing season. Within the grain crops, high protein crops like soybeans or canola will remove more K than a cereal or grain corn crop. Most of these crops will take up a large amount of K, but the part in the non-harvested crop residues will be recycled back to the soil very quickly. Potassium residues can get concentrated in windrows behind the harvester, which leads to an accumulation of K in some areas and depletion in others.
Fertilization program. Low capacity to supply K from the soil and large removals of K will not result in low K levels if it is regularly replaced through fertilization. Without regular K additions, however, even moderate removal will over time deplete the soil's K supply.
Potassium can be added through fertilizers or livestock manure. Many livestock farms are efficiently recycling potassium from the fields to feed, and back again as manure, so the net impact on soil test levels is small even though the removal each year is large. The situation is very different where forages are being sold off the farm, so large quantities of K are being exported as well.
Consequences of low potassium. Yield reduction from low K shows up far sooner than visible deficiency symptoms. It is this "hidden hunger" that you are trying to avoid with regular soil testing. Lack of K in the plant will reduce drought tolerance and, to some extent, resistance to disease and lodging. The net effect will be poor grain fill, lower protein and test weight and uneven maturity. In tomatoes, low potassium can lead to uneven ripening and yellow shoulders on the fruit.
Where deficiencies are severe enough to show up as visible symptoms, they will show up on the older plant tissue first, since K will move from the older leaves to the new ones as the plant grows. The outside edges of the leaves will be affected first. In corn, this will appear as a yellow or brown band extending from the leaf tip along the leaf margins. Soybean leaves will show yellow discolouration along the leaf margins, which eventually dry up and drop off, leaving a ragged margin. K deficiency in alfalfa appears as white speckles on the leaves, more frequent near the margins than near the centre of the leaf.
Correcting potassium deficiencies. A K deficiency develops when a lot of K has been removed from the soil, so a similar amount needs to be put back in to correct the deficiency. Adding a little bit as a foliar spray or a banded application may provide some temporary relief, but it cannot make up for the whole amount removed.
The most economical form of potassium fertilizer (next to manure) is muriate of potash (potassium chloride, or KCl), which is mined from extensive deposits in Western Canada. Muriate of potash can be easily broadcast, but you have to be careful not to apply too much in a band near the seed as it can result in fertilizer burn.
Potassium sulphate is a good source of K where you also need sulphur. It can be handled in the same way as muriate of potash, and is slightly safer in a fertilizer band. Sulfate of potash-magnesia (K-Mag) includes magnesium along with the potassium and sulfur, so it has a good fit in specialty fertilizers. Both of these forms will be more expensive than potassium chloride.
Most plants are quite tolerant of high levels of K in the soil, so over-fertilization is usually not a problem. The two exceptions to this rule are high application rates on very coarse textured soils that can lead to salt injury of new seedlings, and low magnesium soils where excessive potassium can compete with magnesium for uptake by the plant, leading to magnesium deficiency. BF
Keith Reid is Soil Scientist, Agriculture and Agri-Food Canada, Guelph.