Effect of Foliar Application of Potassium Nitrate and Potassium Sulphate on Horticultural Traits and Nutrient Uptake in Guava cv. Allahabad Safeda

In the world of fruit cultivation, the health of a tree is often a story of what it can pull from the soil and what it can hold onto. Potassium is a vital mineral for plants, acting much like a conductor that directs the flow of energy and water within the tree. It helps the plant move sugars from the leaves to the developing fruit, keeps cells firm, and ensures that the fruit grows to its full size rather than falling off prematurely. For farmers growing guava, a popular fruit known for its sweet flesh and aromatic scent, maintaining high levels of this mineral is essential. However, in many regions, the soil is sandy and drains quickly, meaning that nutrients applied to the ground often wash away before the tree's roots can absorb them, or they become locked in the soil where roots cannot reach. To solve this, growers sometimes turn to foliar feeding, a method where a nutrient solution is sprayed directly onto the leaves. The leaves can absorb these nutrients quickly, bypassing the soil entirely and delivering them straight to the parts of the tree that need them most during the critical stages of fruit growth.

Researchers in Amritsar, India, set out to find the most effective way to deliver this crucial potassium to guava trees. They focused on a specific variety called Allahabad Safeda, which is widely grown in the region but often struggles with small fruit size and heavy fruit drop during the winter harvest season. The team wanted to compare two common sources of potassium: potassium nitrate and potassium sulphate. While both provide the essential potassium mineral, they differ in the other chemical component attached to it. The researchers also needed to determine the right amount to spray, testing low, medium, and high concentrations to see if more was always better or if there was a point where adding more provided no extra benefit. Their goal was to find a simple, reliable method to help these trees produce larger, heavier fruit with better nutritional value, while reducing the number of fruits that fall to the ground before they can be picked.

The experiment took place over a single growing season in an orchard where twenty-one healthy guava trees were selected for study. The scientists divided the trees into groups and applied different treatments. Some trees received sprays of potassium nitrate at one, two, or three percent concentrations, while others received sprays of potassium sulphate at the same levels. A control group of trees was sprayed only with plain water to serve as a baseline for comparison. The sprays were applied twice: once in early August when the fruit was just beginning to develop after the monsoon rains, and again in mid-September as the fruit began to enlarge. The researchers carefully measured the results at harvest, looking at the size and weight of the fruit, how many fruits stayed on the tree versus how many fell off, and the chemical composition of both the fruit and the leaves.

The results showed a clear preference for one source over the other. Trees treated with potassium nitrate consistently outperformed those treated with potassium sulphate at every concentration level. The most successful treatment was a spray of potassium nitrate at two percent. Trees receiving this treatment produced the largest fruit, with an average length of 8.23 centimeters and a diameter of 7.68 centimeters, compared to the control trees which produced fruit averaging only 4.38 centimeters in length. The weight of the fruit also increased dramatically; the best-treated trees produced fruit weighing an average of 266.70 grams, more than double the 122.42 grams found on the untreated trees. Perhaps just as important as the size was the retention of the fruit. The trees sprayed with two percent potassium nitrate lost the fewest fruits, with a drop rate of only 7.18 percent, whereas the untreated trees shed nearly two-fifths of their fruit before harvest.

Beyond physical size, the quality of the fruit improved significantly. The fruit from the best-treated trees contained higher levels of total soluble solids, which are the sugars and other dissolved substances that make fruit sweet and flavorful. They also had higher levels of ascorbic acid, commonly known as vitamin C, reaching 260.48 milligrams per 100 grams, which is almost twice the amount found in the control fruit. The pectin content, a substance important for the fruit's texture and its use in jams and jellies, also doubled in the treated trees. The researchers analyzed the leaves of the trees and found that the potassium nitrate spray not only increased the potassium levels within the leaf but also boosted the levels of nitrogen, phosphorus, and calcium, as well as the amount of chlorophyll, the green pigment responsible for photosynthesis. This suggests that the spray improved the overall health and energy production of the tree, allowing it to support larger and more nutritious fruit.

Interestingly, the study found that increasing the concentration of the spray beyond two percent did not provide any additional benefits. Trees sprayed with three percent potassium nitrate performed just as well as those sprayed with two percent, but not better. Since the three percent solution requires more material and costs more to prepare without offering a statistical advantage, the researchers concluded that the two percent concentration is the most efficient choice. The data revealed a very strong connection between the amount of potassium found in the leaves and the total yield of the tree, indicating that the success of the treatment was directly tied to how much potassium the tree was able to absorb. While the study was conducted over a single season, the findings offer a clear path forward for growers in this region. By applying two sprays of potassium nitrate at two percent concentration during the late summer and early autumn, farmers can expect to harvest significantly larger, heavier, and more nutritious guavas with far less waste from fruit drop.