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The pH Factor – Part I

A Basic Understanding of pH in water

What is pH?
A seemingly simple question, but many people don’t really understand what pH is,
although they are familiar with the pH scale:

 

 

 

 

Seven is considered to be the point of neutrality on the pH scale, neither acid nor alkaline. pH measurements below 7 are considered to be acidic, while those above 7 are considered alkaline. But what do these numbers actually mean? Technically, pH is the negative logarithm to the base 10 of the hydrogen ion concentration:

-Log ₁₀ [H⁺]

Thus, pH is a measure of the concentration of hydrogen ions [H⁺]. The relationship between pH and hydrogen ion concentration is as follows:

 

 

Remember, pH is the negative logarithm to the base 10 of the hydrogen ion concentration. A logarithm is the small number written as a superscript when using scientific notation. Base 10 is the number system we use every day. When we say ‘negative logarithm’ it means to change the sign. So if the logarithm is –1, that becomes pH=1. You can see that it is a lot easier to talk about a pH of 1, 4 or 7 as opposed to a hydrogen ion concentration of 1 x 10^-1, 1 x 10^-4 or 1 x 10^-7.. So why is this so important to understand? It’s important because looking at it this way shows what looking at the pH scale does not: this is a logarithmic scale. Each time you move one number on the pH scale you are changing the H+ concentration by a magnitude of 10 i.e. the difference between 6 and 7 on the pH scale is a tenfold difference not a difference of a single digit, while a pH of 5 is one hundred times more acid than a pH of 7 which is a much greater difference than most people realize.

pH in Water (spray tank)

Most pesticides are designed to be mixed with water for use. As a turf manager, the pH of the water available to you is something over which you have little control, at least not without great expense or effort. If the pH of your water is 6.0-7.0, you may not give it another thought, but if your water has a pH of 8, 9 or even 10 it can be cause for major concern. Even if the pH of your water is supraoptimal, there are steps that can be taken to ameliorate the situation (suboptimal water pH is rarely, if ever, an issue).

Use of high pH water often leads to a loss of effectiveness of the pesticide in use. Many pesticide labels caution against the use of alkaline water. In general, this loss of effectiveness is due to hydrolysis (decomposition of chemical compounds thorough reaction with water). The rate of hydrolysis is determined by:

1) pH - As stated before, pH is a logarithmic scale. Therefore, the rate of hydrolysis of an alkaline sensitive chemical will increase by a factor of ten for every pH unit.
2) Chemistry of the pesticide – Most chemicals will undergo alkaline hydrolysis (decomposition under alkaline conditions). However, some are acid sensitive and will undergo acid hydrolysis.
3) Time of exposure in the spray tank – What comes out of the spray tank during the first hour of spraying could be more effective than what comes out during the last hour of spraying.
4) Temperature of the water in the spray tank – An increase in temperature of 10^oC (18^oF) ) will double the speed of decomposition. The sun’s rays beating down on a spray tank will have some effect on the rate of hydrolysis, as will constant agitation which tends to warm up the spray mixture.

Various pesticide manufacturers have supplied data showing the effect of pH on the half life (time required for half of the material to degrade) of their pesticides and it is reported here in table form (See Table 1).

In general, insecticides are more susceptible to alkaline hydrolysis than fungicides, herbicides, or growth regulators. Of the insecticides, the organophosphate and carbamate classes tend to be more susceptible than the chlorinated hydrocarbons.

Aside from pesticides, there are tremendous amounts of iron, magnesium and other trace elements being used as adjuvants in spray mixtures. With the exception of boron (which is not truly metallic), all of the metallic salts will undergo hydrolysis at a pH above 7 and end up as hydroxides and oxides which are totally inactive. The classic example is ferrous sulfate, which hydrolyzes rapidly and will end up as inactive iron oxide rust (sometimes in sufficient amounts to clog the sprayers). When these metals are chelated, they become immune to hydrolysis and are totally and completely available to the plant.

Correcting the pH of the water in the spray tank is possible and achievable, but should not be done haphazardly. The accurate way to monitor pH is with a properly calibrated pH meter. A pH meter that has not been calibrated may be more of a problem than a lack there of. If you are adjusting the pH of the tank water, be sure to do it before you add the pesticide. There are numerous, commercially available buffering solutions that can be utilized for pH adjustment. Many of them contain fertilizers and other adjuvants, so be sure your pesticide is not going to react with these other product components.

A caution on pH and post-emergent herbicides: Herbicides such as 2,4-D, MCPP and dicamba are water insoluble acids that have been put into solution with amines. These solutions are always alkaline and if they are acidified the herbicides can drop out of solution as water-insoluble gums that can foul up your sprayer. For this reason, they are best sprayed with alkaline water.

Table 1 Half Life of Some Common Insecticides at Various pHs.

COMMON NAME	TRADE NAME	PH	HALF LIFE
Acephate	Orthene	3 65 days 9 16 days 6 100-150 days 7 24-30 days 8 2-3 days 9 3 hrs.
Carbaryl	Sevin

Next month we will present “The pH Factor – Part II: A Basic Understanding of pH in Soil”

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