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Sprayer pressure impact on microbes?

Posted by Jake Riley 
Sprayer pressure impact on microbes?
March 15, 2020 12:07PM
We just bought a new sprayer that can build enormous amounts of pressure and it got me thinking. Last season we sprayed beneficial nematodes throughout the orchard. The instructions cautioned against spraying with pressures exceeding 100 psi, if I remember correctly, which wasn’t much of an issue with the back pack sprayer. Does anyone have any insight on how spray pressures may impact beneficial microbes ?

Lesley Run Orchard, Zone 5b/6a, West Alexandria, Ohio

Edited 1 time(s). Last edit at 03/17/2020 11:10AM by Michael Phillips.
Re: Sprayer pressure impact on microbes?
March 17, 2020 11:20AM
You bring up a very important point, Jake. I use a PakTank sprayer with a hydraulic (handheld) gun. I have the pressure set at 100 psi which oscillates down to more like 90 psi when the gun is engaged. Similarly, last year when I applied parasitic nematodes as a ground application, I dropped the pressure down to 60 psi. Nor do microbes like to be squeezed. The adjustment on a handheld gun has limited range in this regard as even spraying a finer stream still consists of distinguishable water droplets. Air blast applicators must pay far more attention here as misting at high pressure is designed to achieve chemical coverage with minimal water rather than keeping biology viable.

I'd love to see someone to post specific references about this. And for air blast applicators to explain what they do to ensure better microbe survival.

Lost Nation Orchard
Zone 4b in New Hampshire
Re: Sprayer pressure impact on microbes?
March 29, 2020 02:20PM
A friend was asking about Venturi-type sprayers in this regard. (I urged him to add his thoughts to this thread.) In the meanwhile, here's some numbers to chew on. Finding search wording to get biological perspective about sprayer parameters has proven difficult.

This comes from a pdf about Gearmore Venturi Sprayers:
"Laboratory tests show that there is a substantial difference in the size of spray particles between the two methods of atomizing the liquid. With the High-Volume Sprayer, the droplets have a diameter of 250 to 300 microns. This size can not be reduced, even by using higher pressure. The Venturi Air Sprayers reduce the liquid to diameters of approximately 50 microns, i.e. fog size. The difference of the particle size is extremely important. Unlike the High-Volume Sprayer, the particles from the Venturi Air Sprayer do not run together and drip off the plant. Much less water is used because the air is carrying the spray particles to cover the plant with a homogeneous foglike spray. We specifically designed the Gearmore Venturi Air Sprayer as a Low-Volume or Concentrate sprayer. If required, our sprayer will also do High-Volume or Dilute spraying."

This comes from an overview of spray application techniques from University of Massachusetts Extension:
"One way to distinguish between a hydraulic sprayer and low-volume sprayer is by droplet size. Hydraulic sprayers produce a spray with most droplets in the 200-400 micron diameter range (thickness of the human hair is about 100 microns). Low-volume sprayers develop a mist (50-100 microns) or fog (0.05-50 microns). Small droplets from a mist or fog applicator can result in more uniform coverage and greater likelihood of contact with the insect or disease. In contrast to the hydraulic sprayer, spray material is usually applied to "glisten" as it is difficult to see the individual droplets on the leaf."

Lost Nation Orchard
Zone 4b in New Hampshire
Re: Sprayer pressure impact on microbes?
March 30, 2020 04:55PM
I attended John Kempf's two day Regenerative Agriculture course in December at the ACRES conference. Looking back at my notes he suggested using a droplet size as coarse as possible and having your psi no higher than 50.

There is a webinar from back in May where John talks about How to Design Foliar Sprays.

Running Hills Farm
Mount Horeb, WI
Zone 5a
Re: Sprayer pressure impact on microbes?
March 30, 2020 05:56PM
Microbes come in various sizes and shapes, and I will assume physical resiliencies. There are so many factors that go into the ability of a substance - inert or ert - to survive a modern sprayer are immense. However, in terms of mere physical construct we have the following VERY basic guidelines.

1. Bacteria - generally less than 2 microns in diameters and up to 10 microns in length.
2. Fungi - generally 2-10 microns in diameter and up to 50 microns in hyphal length.

According to the South Dakota Extension Service, the average spray droplet size is 150 microns or roughly 3 times as large as the largest typical hyphal length or 15 times larger than the longest rod-shaped bacteria (according to a quick Google search). They define "coarse" as 326-400 microns in size.

In other words, from my intuition, spray droplet size is "unlikely" to have a significant effect on the physical maceration of biological components compared to other issues such as EC, pH, other spray tank additives, etc. Now, what interests me are the physical dynamics of any material - including a living organism - being thrown through a spray pump, in a vortex, bashed against the sides of a sprayer's rough-hewn components, swirled through various nozzling devices, perhaps against violent fan blades, etc.

I'd love to see some research on this. But my general feeling is that the droplet size and pressure do not have as much of an effect on the survival rate of biological organisms through a sprayer as the pH, EC, and physical exertion. Droplet size and pressure could very well exacerbate these conditions - I am all for being gentle with all life forms - I am just not sure that those elements have as great an effect on biology as we may think. But I have been wrong before.

Mike Biltonen, Know Your Roots
Zone 5b in New York
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