Growing Media & Water pH 



OFTEN NOT WHAT YOU 



Paul A. King & Robert Eikhof 



Reprinted from the Ohio State 

 Flower Grower's Hotline, March 

 1990. 



Measuring and adjusting the pH in 

 growing media and irrigation water 

 is much more comphcated than many 

 people realize. The results are highly 

 depedent upon how the mix sample is 

 handled during analysis and where 

 the water sample is taken. For ex- 

 ample, pH values of the same sample 

 can vary from 0.5 to 1.5 units. We 

 have observed a number of growers 

 with problems directly caused by 

 these differences. 



Our laboratory normally measures 

 the pH of a growing medium by plac- 

 ing the probe directly into the satu- 

 rated growing medium paste. We be- 

 lieve this provides an accurate pic- 

 ture of what plant roots experience. 

 Other laboratories vacuum filter the 

 sample before measuing pH. The 

 table below lists pH's, measured in 

 the paste, after filtration by gravity, 

 or filtration by vacuum. 



Table 1 



Differences in water pH analyses 

 as affected by fUtration 



sample in 



number paste 



grravity vacuum 

 fUtered filtered 



1 - 4.95 5.30 



2 - 4.85 5.65 



3 4.90 4.95 5.65 



4 5.05 5.00 5.60 



5 5.15 4.98 5.50 



We have often observed, with samp- 

 les analyzed at other laboratories, as 

 much as one full unit increase in pH 

 caused by filtration. To prove that 

 vacuum filtration causes an increase 

 in pH, we measured the pH in the 

 saturated paste and at various times 

 during vacuum filtration. The pH 

 progressively increased from 5.65 to 

 6.8 during a 25-minute period of 

 vacuum filtration. 



An equally dramatic change occurs 

 with inigation water samples. The 

 pH at the hose has one pH value, 

 while the same water collected fi-om a 

 mist nozzle will have a significantly 

 higher value. Table 2 contains mea- 



surement of water acidified to vari- 

 ous initial pH levels with both phos- 

 phoric and sulfuric acids. We would 

 expect nitric acid to give the same 

 results as sulfuric acid. 



Table 2 



Difference in pH of water treated with 

 sulfuric or phosphoric acid from hose 

 end and mist line 



phosphoric acid 



from hose end from mist nozzle 

 pH pH 



5.20 5.70 



5.80 6.40 



6.25 6.75 



These phenomena are easily ex- 

 plained. The system is almost exactly 

 analogous to the buffer system of our 

 blood. The body conti'ols the pH of 

 blood by controlling the amount of 

 dissolved carbon dioxide in it. As our 

 blood becomes acidic, more carbon 

 dioxide (C02) escapes through the 

 lungs and the pH rises. The peritnent 

 equation is: 



C02 + H20 ^ H-H -H HC03- 



Another example is the behavior of 

 seltzer or plain carbonated water. Ev- 

 eryone has seen what happens if you 

 shake a warm bottle of seltzer; the 

 excess dissolved carbon dioxide es- 

 capes. In fact 60% of the total acidity 

 (caused by the carbonic acid) is lost by 

 shaking. 



When the water is vacuum filtered for 

 five minutes, like a growing mix 

 sample, the pH rises and the water 

 looses up to 99.3% of its acidity. 



The escape of C02 during vacuum 

 filtration and the misting process is 

 also what cause the increase in pH. 

 "Normal" water and growing media 

 contain hundreds of times more bi- 

 carbonate (HC03) than is in equilib- 

 rium with the C02 in the air. There- 

 fore, when we acidify water, we shift 

 the equilibrium in the equation, i.e. 

 we make C02 out of bicarbonate and 

 it escapes from the mist droplets into 

 the air. The acids from sphagnum 

 peat moss and fertilizer dissolve the 

 lime in growing media and produce 

 bicarbonate which produces C02 

 which is then removed by the vacuum 

 filtration step. 



We have proven that the increase in 

 pHisduetolossofbicarbonate(C02), 

 measured by ion exchange chroma- 

 tography, after vacuum filtering. 

 This investigation will be continued. 

 For example, we will determine the 

 buffering capacity of water from the 

 mist nozzle. 



Ifyou believe the pHofyour mix is too 

 high, make certain you know how it 

 was measured before making 

 changes. Vacuum filtration causes a 

 false reading since it changes the 

 chemistry of the system. 



If you believe your plug mix is too 

 high in pH and you want to lower it, 

 check the pH of the mist water. You 

 may find that you are not misting 

 with what you thought you were. It 

 may be necessary to use more acid. 



Editor's Note: A pH of 7.0 is 10 times 

 more alkaline than a pH of 6.0. A pH of 

 9.0 is 1,000 times more alkaline than a 

 pHofe.O. 



Paul King and Robert Eikhof are 

 members of the research staff of Fisons 

 Horticulture, Inc. For more information, 

 call them at Fisons Analytical Laborato- 

 ries, Warwick, NY; (914) 986-6667. 

 (Gary Rota tori is this area's Fisons 

 representative. For information about 

 Fisons products, contact Gary at P.O. 

 Box 285, U5 Jackson Street. Jefferson, 

 MA 01522; 508-829-9726.) ^ 



February/March 1991 27 



