Water Sources and Foliar Feeding
One of the biggest obstacles to foliar feeding for some is their source of water. GMS can be sprayed on straight and was as the norm during the early days of the company when lower volume sprayers were more ubiquitous than they are today. However, with larger spray volumes necessitating mixing GMS with water, as well as many people’s preferences for adding water to their spray, some issues arise with how well GMS mixes with certain water sources and how well those waters are absorbed into plant foliage.
We have long known that the phosphorus (P) in GMS will combine with certain elements in hard water and precipitate out. More recent research has also shown that those elements in hard water can be left on the leaf surface if not absorbed. Having said all of this, there are many times that we have been surprised when a hard water does not react with GMS and appears to be very spray compatible. It seems that the water quality discussion is far from over. The following experiment exemplified all of this.
Matt Gooding has worked quite a bit with different water sources to learn more about what makes water, GMS, and plants tick. In this case, he wanted to see what the results would be of using different water sources, all mixed as a 10% solution with GMS (1 part GMS to 10 parts water), spraying them on a cucumber leaf, than allowing them to absorb/dry. The sources of water he used were city water from Tiffin, Ohio; well water from his farm; well water run through a water softener; and rainwater.
His (and our) expectation was that the well water (softened or not) would perform the worst, while rainwater would perform very well, and the city water would be somewhere in between. This is not entirely what happened. As you can see in Photograph A, all four sources left a residue behind on the leaf after drying for 4 hours. The amount of residue left is less in the rainwater, but the other three appear functionally equal.
In order to simulate dew, Matt misted the leaf with some rainwater and then let it sit for another 3 days. Photograph B shows the results. The city water actually performed the poorest, while the well waters were roughly equal, with a small amount of visible residue left. The rainwater results were right in line with our expectations.
You may be asking two questions: Why did the well waters outperform the city water? And why do these results matter at all? As I stated above, we have been surprised many times when a hard water works well with GMS and as a foliar spray. This is because it is not only the amount of minerals dissolved in the water that affect it, but which minerals are dissolved. Ground water especially (but surface water to a lesser extent) will dissolve those minerals it comes into contact with. Depending on the parent material and structure of the ground it is in, it will have very different dissolved minerals in it than ground water form somewhere else. Elements like calcium and magnesium will bond very readily with phosphorus, but other minerals, especially anions (negatively charged elements and compounds) will not. Additionally, different elements are mire or less water soluble, so when in a dried state will be more or less successfully resolubilized by the re-wetting of the leaf.
Undoubtedly the minerals in Matt’s well water are very different than those in Tiffin’s city water. Municipal water treatment is usually more concerned with pathogens than mineral content, so besides some softening probably haven’t removed them. It is likely that whatever elements are in Tiffin’s city water are not very soluble.
It is important to note here is that softening salt softeners work by replacing positively charged cations in the water with sodium, so they do remove calcium and magnesium, which reduces scaling problems, but often actually increase the amount of hardness in the water by adding a lot of sodium. It would appear that whatever elements are in Matt’s well water are more soluble than the city water, but sodium is also quite soluble. This would explain their mostly equal results.
To answer my second posed question, I believe these results are important because they, a) give us greater insight into how water responds to and affects foliar spraying and, b) emphasized the importance of testing a water source before using it in a foliar mix. It is not just a matter of mixing well with GMS, but also one of what effect it will have on the leaf surface. A leaf covered in minerals will lose photosynthetic capacity until they are washed off or resolubilized, something that we have seen is not always easy to do. We believe that having the correct water source greatly increases the efficiency and efficacy of whatever you are spraying, be it a fertilizer or chemical.
This is an excerpt from the Spring Growers Solution (2024) written by Zach Smith, Growers Product & Training Specialist.
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