How To: Test Irrigation Water Quality
As we try to grow a large variety of plants in areas where only the toughest survive, irrigation becomes a major player. Water shortages and poor water quality cause us to monitor what is in our irrigation water and adjust our practices accordingly. Recycled water, or gray water, is becoming a staple in our management practices. Water is considered less than perfect if it has a high concentration of sodium and other cations, as well as high amounts of bicarbonates and other anions (some of which are actually nutrients) are present in excess. The problem these contaminants cause is two-fold. Excessive salts in the soil water around the roots of your desirable plants can actually suck the water out of the roots and out into the soil solution, resulting in desiccated or dead roots and plants that suffer from water stress because they now have no roots to take in water. Secondly, some salts in the soil cause clay and some organic matter particles to separate from each other, float around and lodge themselves in soil pores, or the salt causes the clay particles to be just large enough to “line up” like soldiers instead of clumping together in balls. This line of particles is strong enough to prevent water from moving past it into the soil and roots below. Soil scientists refer to this as a breakdown of soil structure. With the breakdown of soil structure, water is not able to move through the soil; even though you have irrigated, the roots below stay dry.
Whatever the origin of your water, there are a few properties you should track: salinity or electrical conductivity; sodium; sodium adsorption ratio (SAR); anions such as chloride and carbonates; and the pH. Looks complicated, and in some ways it is, but they all actually interact to influence your soil.
Most testing laboratories will also provide nutrient information, but don't rely on that information for fertilization decisions. Nutrient analysis should come from a soil test or plant tissue test. Collect samples several times a year for the first few years to provide yourself with a benchmark for your water supply. Note any seasonal fluctuations or variations caused by depleting the irrigation pool.
To test water from lakes or ponds, collect the sample from below the surface of the water. Completely fill a clean plastic container and seal it tightly with the lid. Collect at least one pint of water for each sample. If you can't send the sample right away, refrigerate it. For well water, run an irrigation station for at least five minutes and then collect the water being delivered.
Find a lab that reports the information in a way that you can understand. To find your preferred lab, you can either send water samples to several labs and compare how they present their findings, or you can contact the labs, tell them you are shopping around some irrigation testing, and ask them to send you a sample report to evaluate before you send your water samples to them. Most labs have representatives or technicians who can explain the way the data is reported and what it might mean to you. Many state university Extension offices have labs. In addition to private labs, some fertilizer and equipment suppliers will also send off samples for their customers. If possible, chose a lab that specializes in water testing. Some general labs are set up for each type of testing but they're not always calibrated to your needs and may provide data that is not very accurate.
All municipal water departments are required to supply water quality data to their customers. If you can't locate your water district on the Internet, call and ask them to send a copy of their latest test data.
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