Monitoring fertilization in container plants

Testing both irrigation water and leachate (drainage) solution for several parameters is an essential part of regular monitoring of fertilization and irrigation in container plants. These parameters should include at least EC, pH, and % leachate. Simple field test kits are also available for other parameters, such as nitrates, chlorides etc. For more comprehensive analysis, water samples should be sent to the laboratory. The main goal of field testing the leachate solution is to quickly identify trends, indicating dynamics in plant requirements and salt content in the container. A single test result is not enough to get a clear picture, and a periodic testing is an important tool for that purpose.

During irrigation, leachate solution content is dynamic. For example, here are EC values tested in the same leachate solution at different times during irrigation. EC of irrigation water was 1.0 ds/m:

Interpretation of leachate test results depends entirely on the percentage of leachate out of the irrigation water amount! The same result (EC or any nutrient level) measured in different percentage of leachate solution can get completely different interpretations. 

Let’s consider the following case:

What will be the correct interpretation of these results? The fact is that just by looking at the results – it is impossible to say, unless the percentage of leachate is known. If the leachate percentage is low (e.g. 10%) – the result in the leachate can mean that irrigation amount is insufficient and that the amount of irrigation water has to be increased. If the percentage of leachate is high (e.g. 70%) – the same results can mean that fertilization level is too high and has to be decreased. The same rule applies for all elements in the leachate solution. Note that test results of nutrients are always expressed as concentration, and it is worthwhile to consider also the amounts in order to correctly interpret results. 

For example:

• A concentration of 100 ppm Nitrogen in 500ml leachate solution means 50mg (because 1ppm=1mg per liter). 
• The same concentration in 200ml water translates into 20mg.

Thus, the result “100 ppm” may not be used “as is”, but should sometimes be converted into the actual amount of the element tested. In order to effectively compare leachate solutions content, it is important to consistently use leachate samples that reach same percentage of irrigation water.

EC monitoring

The EC of the water indicates the total concentration of salts in it. Comparing the EC of irrigation water to EC of leachate solution collected from containers is a valuable practice. Testing the EC of irrigation water confirms that the actual EC resembles the planned EC. Testing the leachate solution gives indication of adequacy of irrigation water amounts as well as fertility levels. When the EC of the leachate solution is noticeably higher than the EC of the irrigation, it usually indicates salinity buildup in the container, either due to over-fertilization or due to too low irrigation water amounts. When the EC of the leachate solution is much lower than the EC of the irrigation water, it may indicate nutrient deficiencies. Commonly acceptable difference between EC of leachate solution and irrigation water is 0.5-1 dc/m, depending on crop type.

pH monitoring

The pH of the irrigation water and leachate should be tested as soon as possible after irrigation is over. Waiting too long might result in an incorrect reading, since CO2 from air dissolves in water, creating bicarbonates and increasing pH levels. It is very important to understand that pH of irrigation water is rarely the actual pH that the plant “sees”. There are various reactions taking place near the roots, influenced by nutrient concentrations in the growing media and in the irrigation water. As a result, pH in the root zone changes frequently.  Nevertheless, the pH of the leachate solution can give a rough, yet accessible, indication of the status in the media, and is therefore widely used. When the pH of the leachate drops below an optimal level, or even to a harmful level (<5.0), ratios between nutrients in the irrigation water should be assessed, mainly ammonia/nitrate ratio. A higher ammonia/nitrate ratio usually results in a lower pH in the root zone as well as in the leachate solution. Other causes of low pH may be excess of Potassium, calcium or magnesium.

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