pH correction
To correct pH either acid (pH lower) or alkali (pH raise)
is added to the nutrient solution. The acid or alkali should be diluted before
addition to the solution to prevent a localized chemical reaction from occurring
as it enters the water. Any reaction here can gradually precipitate out vital
nutrients and alter the nutrient balance. It is normal to dilute to below 5% for
this purpose.
Normally the acids used are Phosphoric acid or Nitric acid. Note that besides correcting the pH these acids will add either Phosphorus or Nitrogen to the solution. These are both plant nutrients which is good but if large amounts are added then the P-N nutrient balance can be altered. Commercial growers will often mix the Nitric acid and Phosphoric acids together in a particular ratio to match the nutrient requirements. If you wish to do this Grower Technology can advise you on suitable ratios. Note that working with acids and alkalis is very dangerous and suitable precautions and protective equipment (especially face shield and gloves) must always be used.
Recently I was contacted by a grower who had moved to a different area. He was perplexed by the fact that previously he had used about 2Ltrs per day of acid to drop his pH from about 6.5pH to 5.8pH. Now, suddenly, he needs over 5Ltrs to achieve the same result. Also, he noticed that when adding acid the pH would rapidly fall but then would soon climb back up again. The difference is that in his new location the water is harder and contains more bicarbonates. As compared with his previous soft water the bicarbonates take much more acid to break them down and this also takes time. You will notice a similar effect if you switch between rain water and bore water. The bore water will usually take a lot more acid to lower its pH.
In situations where the water is very hard it is often wise to "pre-treat" the water in a large standing tank prior to using it. Here it can be dosed with acid and because of the long standing time the chemical reaction has plenty of time to neutralize the bicarbonates and gas off the CO2 resulting from the reaction. The Autogrow Mini pH controller is often used in this application.
We have available systems to pre-treat irrigation water supply tanks, contact us for details
Monitoring the root zone
Many growers growing in media are overly focused on the irrigation water EC and pH whereas it is much more important to consider is the EC, pH and moisture content of the root zone. By measuring the root-zone variables the irrigation EC, pH and quantity can be adjusted to suit.
Interestingly, the root zone (and run-off) EC is affected by both the irrigation EC and the irrigation quantity. If irrigation quantity is reduced then salts build up in the root zone and the EC rises. Normally the run-off EC can be expected to be as much as about 1ms/cm (10CF) greater than the irrigation EC but if it rises much more than this it can severely stress the crop and even cause it to wilt.
With tomatoes it is usual to allow the media to dry down overnight. There is great debate from various experts and media type specialists to what this should be. In the morning you will find that frequent irrigation’s are needed to get run-off to occur by about 10:30 or 11:00am, again experts vary on opinion on this. The initial run-off will have a very high EC and it important to normalize this within about an hour. During the rest of the afternoon the EC should fall a little or stay constant as should the moisture level in the media.
This is all very important but is difficult and time consuming to monitor in person. Many growers have some ‘buckets’ at various locations and tend to monitor a ‘days worth’ and look to averages, but this does not optimize crop potential.
There are a number of automatic systems on the market that assist with these measurements and data logging tasks. Autogrow manufacture a very cost effective MINDER system that monitors the irrigation quantity, EC and pH, the media moisture level and temperature and also the run-off quantity, EC and pH. It also measures solar energy and from this calculates the irrigation to light ratio. This system has alarms on all key variables and logs data graphically on a PC. For the modern grower it is a vital piece of equipment that has not been available until now.