The facts and fallacies of foliar feeding and other mistaken beliefs

Environmental conditions can dramatically affect the uptake of foliar-applied nutrients

Foliar feeding has been frequently advocated in recent years as a way to boost crop yields, or in the case of nitrogen (N) as a boost for grain protein in wheat.

In reality, plants take in very little fertilizer directly through their leaves or stems. In a way, it is a process that does not occur in nature for acquiring trace or minor amounts of plant nutrients. Could you hose down your hogs or cattle with mineral nutrients and expect them to be fully absorbed or utilized? If you have a given crop growing on land that is known to be deficient in zinc, molybdenum, copper or boron, or due to a high pH soil where iron and manganese are deficient, a foliar application of these micronutrients is perfectly in order. You might even consider a repeat application if the timing of the second micronutrient application is within an effective time frame.

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For example, a 60-bushel crop of wheat in an acre of land would require almost a half ounce of copper for its full nutritional requirement. A 100-bushel crop of corn in an acre would require two ounces of zinc. Likewise, a 35 bushel an acre of canola would require an ounce or so of boron, and if the canola is grown in acidic soil perhaps an eighth of an ounce of molybdenum to meet the crop needs for these essential elements.

Foliar fertility is for micronutrients since plant leaves and stems can only absorb tiny amounts. Golf course managers are quite guilty of talking about foliar feeding of those perfect grass greens. Idle talk since when they apply “foliar” fertilizers they also irrigate the greens and wash these nutrients directly into the soil (i.e., to the grass root zones which are at or just below the soil surface).

If I were to spray 20 pounds (10 kilograms) of nitrogen in the form of urea in an appropriate volume of water to an early boot stage crop of wheat and the soil was virtually bone dry, most of the nutrients, as usual, would run down the wheat leaves and stems to the soil below. If dew occurred during the night and rolled down the tillers, more of the nitrogen would be taken down to soil level. If the weather remained dry (a prolonged drought of several weeks) less than 10 per cent of this nitrogen would even enter the wheat plant and perhaps most of the nitrogen in the form of urea would break down in sunlight on the dry soil surface and be lost.

However, if the soil was wet or moist at the early boot stage and perhaps it rained within hours of the N application, 90 per cent or most of the nitrogen from the urea would be taken up by the wheat crop within days. The surface roots of the wheat crop on the wet or moist soil would rapidly uptake this bonus nitrogen and likely raise the protein level and perhaps the overall yield of the wheat crop.

Foliar feeding, whether you use nitrogen, phosphorus, potassium or sulphur, is dependent on good soil moisture if your intent is to boost yield or correct a nutrient deficiency problem involving any of these four macronutrients.

So-called foliar feeding, especially with nitrogen, is a waste of time and money during periods of drought when the intended crop is struggling to obtain sufficient moisture. Just call foliar feeding supplemental macronutrient application during good to excellent crop growing conditions with adequate soil moisture and, hopefully, periodic rainfall.

More nitrogen-fixing plants

Legumes are not the only plants on the Prairies that fix nitrogen. Did you know that the Russian olive, wolf willow and sea buckthorn are all members of the Elaeagnus family of plants and all members of this wide-ranging family fix their own nitrogen as effectively as any legume?

Wolf willow has nitrogen-fixing bacteria in root nodules that on a per acre basis can fix as much nitrogen as alfalfa. Many Prairie cattle owners do not realize that they fix nitrogen. The fallen leaves and stems of the wolf willow contributed significantly to the Prairie productivity of the nitrogen cycle. The vigorous invasive growth of Russian olive and sea buckthorn are due to their nitrogen-fixing nodules. Caragana, a legume, fixes its own nitrogen.

Another Prairie family of plants that fix nitrogen are the various alder shrubs or trees that colonize poor soils. In Northern Europe in the 16th century, it was common to interplant alder seedlings alongside young spruce trees. The Scandinavians did not know why but they realized that spruce trees grew much faster and bigger in the presence of the deciduous alders. These alders fed leaked nitrogen to the spruce trees. If you have ever gone picking blueberries on the more acidic soils of the Prairies, you will notice that the best and biggest blueberries grow right next to an alder shrub — nitrogen compounds likely leak out of the big, woody, alder nodules.

Rhizobial bacteria exist naturally on the Prairies in many types and strains. They live freely in the soil. When they come into contact with the roots of a legume, they can form nitrogen-fixing nodules. Different species of nodule bacteria do well with specific species of legume plants. There is no one size fits all. So, when you seed a legume crop it is very good advice to purchase an inoculant that does best with your legume species, such as alfalfa, clover, pea, bean, lentil or soybean, which require nodule bacteria that do not exist naturally on the Prairies. Seeing nodules on your peas, beans or lentils does not necessarily mean they are doing their nitrogen-fixing job. Some strains and species of nodule bacteria are actually parasitic and steal energy from the legume host. You can often see this if you cut open the nodule — if it’s white it’s a thief, if it’s pink or red then it’s likely fixing nitrogen for its host plant.

To be safe, buy the appropriate recommended strain of nodulating bacteria and treat the peat moss pelleted or liquid inoculant as you would a bunch of fresh flowers — keep the inoculant cool and plant alongside your legume seed as soon as possible, since you are dealing with very fragile bacteria that can die off in days.

About the author

Contributor

Dr. Ieuan Evans is a forensic plant pathologist based in Edmonton, Alta. He can be reached at [email protected]

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