Foliar application – applying nutrients to plants by spraying their leaves – works. But how well?
It might not always be the best method, Viterra fertility specialist Rigas Karamanos told an agronomic update meeting here recently.
If you’re going to try foliar application, it’s best to analyze nutrient deficiencies first, said Karamanos. He outlined three methods of analyzing nutrient deficiencies – soil testing, looking at visual symptoms of nutrient deficiency and plant tissue analysis.
Soil testing and plant tissue analysis are complementary, but different, said Karamanos. Soil testing assesses nutrients available in the soil, while plant analysis examines nutrient uptake by the plant. Both tools are good but have limitations. Soil tests may not be very effective when measurements of sulphate and nitrate are needed in areas of high rainfall, said Karamanos. Tissue analysis can verify deficiency symptoms or to make sure that available fertilizers will sufficiently serve the crop until the end of the growing season.
Karamanos said there are several things to take into consideration when conducting plant tissue analysis. Every cultivar behaves differently, which can have an effect on plant tissue sampling.
As a plant matures, the level of nitrogen in the plant changes radically, said Karamanos. Parts of the plant have different levels of nitrogen, so it’s important to specify what growth stage and what part has been sampled before sending it to a lab.
Foliar nutrients work by penetrating the cuticle or the stomata in order to get into the plant leaf and into the plant’s cells. “The effectiveness of the product will depend very much on the ability of the plant to absorb it,” said Karamanos.
Foliar nutrition works when nutrients that are soluble in water are applied to leaves, where they are absorbed into the leaf through the plant’s natural processes. Absorption is influenced by environmental factors including temperature, humidity, and light intensity. Two to three applications of foliar nutrients are needed for the method to be effective.
The quantity of nutrients absorbed by plant leaves is small in comparison to the plant’s total demand. It is a challenge for the plant to get enough nutrients without causing foliar burning, said Karamanos. In some cases, foliar application is not the right method to use.
“If you want to solve a phosphate deficiency using foliar application, good luck,” said Karamanos. “You’re going to go broke before you fix it.”
Timing of the foliar application is also important, as is product formulation. Applying an incorrect product can also damage the crop. Foliar application of nitrogen is a high-risk enterprise, said Karamanos. There is only a four-to six-week window after seeding that allows for foliar application, and if it is done at the wrong time, the nitrogen could go to producing protein rather than contributing to the plant’s yield.
Karamanos said there is absolutely no window available for applying phosphorus or potassium to plant leaves and this should not even be contemplated due to the high cost. Sulphur has a small window of opportunity and ammonium sulphate is best for foliar application.
Micronutrients such as copper, zinc, manganese and boron are the best candidates for foliar application and have a very wide window for application. In canola, the nutrients can be applied at any stage between bolting and flowering. “The efficiency of foliar applications for micronutrients is sometimes even greater than those of soil applications,” said Karamanos.