For a general run down of pH - What is it? Why is it important? How is it measured? I suggest you read through Soil pH first.

Water pH also plays an important role in plant health and foliar performances. Simplistically, low or high pH water can have the capacity to burn crops - the ideal pH level for crops is between 6.0 - 8.0. But possibly the most important issue is its impact on the performance of agricultural additives.

A little know secret (though the manufacturers seem to be aware of it) is the impact water pH has on the efficiency of many agricultural chemical inputs - for example, fungicides, pesticides and herbicides. Most chemicals work best at a particular pH, and as pH moves away from their ideal, chemical breakdown occurs rapidly (see attachment).  Pesticide Breakdown Rates 15/05/2008,12:03 195.50 Kb

A wonderful example of this was discovered in the US where growers were finding they needed up to 7 litres of glyphosate to kill weeds that a few years previously only required 1. Knowing that glyphosate works best in pH 5, their agronomist suggested lowering their naturally alkaline spray tank water to below 6 - the farmers were back to using 1 litre immediately!

Note: poor performance is not always a pH issue, though it is an easy one to test for straight up. It could also be the result of mineral interference in the water source (e.g. high cation levels) or dust, or weed resistance build up. 

A simple and cheap way to reduce spray tank water pH is to add citric acid. The total amount required differs for different water sources so the recommendation is to run a calibration test in a small tank yourself. Simply add the citric acid till the water pH reaches the desired range. Note that some water can have a buffering capacity, so it is a good idea to let the water sit for another 1/2 hour-1 hour and re-test the pH. If it has risen, add more citric acid until it stabilises. Add up how much citric acid was used in total, and apply that dilution to all your spray tanks.

It has also been observed that if water pH is lowered further you can achieve an additional acid shock - which might be ideal in the case of knockdown herbicides! 

As an example of likely citric acid rates, one grower found that in a 1000 litre tank:

500gms citric acid dropped the water pH from 7.2 to 3.4

1000gms citric acid dropped the water pH from 7.2 to 2.7

1500gms citric acid dropped the water pH from 7.2 to 2.2

Nutri-Tech Solutions sell a range of easy to use pH meters.