High fertilizer levels can be too much of a good thing, leading to excess growth, nutrient toxicity and potential runoff of nutrients into the environment. Conversely, low fertilizer levels can lead to nutrient deficiency symptoms. A basic goal for a nutrition program is to supply nutrients to the crop within an acceptable range for healthy and controlled growth.
One way to ensure that nutrients are being supplied at adequate levels is with a soil test. So long as your irrigation water has salt concentrations within an acceptable range and you use a balanced fertilizer containing both macro- and micronutrients that doesn’t contain a lot of useless salts (like sodium or chloride), then there is a good relationship between the nutritional status of the root medium, and the media electrical conductivity (EC) measured using common soil testing methods (Table 1). This last article of our series discusses how to manage media nutrient levels based on soil test EC values.
Like media-pH, nutrient management can be thought of as a balance (Figure 2). On one side of the balance are the fertilizer additions, like addition of salts contained in the irrigation water or fertilizer. The additions from slowly soluble fertilizer (i.e. gypsum or urea-formaldehyde) or controlled-release fertilizer are included on this side of the balance, but only after the salts have been released into the soil solution. One challenge with using materials that have limited solubility in container media (like compost or insoluble fertilizers) is the difficulty in predicting rate of decomposition and availability of these nutrients for plant growth.
On the other side of the balance are fertilizer losses from the soil. The most obvious reason for fertilizer salts to be removed from the soil is that they are taken up by roots for plant growth. The faster the crop is growing, the greater the amount of nutrients that are removed from the soil solution. Plants often require greater amounts of nutrients as growth rate increases during the middle of the crop, and less nutrients both when plants are very small or are mature and flowering. Some growers may increase the applied fertilizer concentration during this rapid growth phase to compensate for the increase in uptake, but keep the fertilizer concentration lower during the early and later stages of the crop when less uptake occurs.
Research has confirmed that healthy root and shoot growth and post-sales performance in a poinsettia crop resulted from a staged fertilizer program with lower initial and final water-soluble fertilizer rates (e.g. 75 to 150 ppm N) with higher fertilizer rates (e.g. 200 to 250 ppm N) during mid-season.
Leaching is an important type of fertilizer loss, because nutrients leaving the container can enter the environment. Leaching is also one type of fertilizer loss that is completely controlled by the grower. Many growers were taught to routinely leach their crop in order to prevent salts from building up in the root medium. While leaching can be a valuable tool for removing excess salts from the root medium, regular leaching is not necessary if the irrigation water and fertilizer contain few impurities such as sodium and chloride.
Other research at Michigan State University demonstrated the effect of leaching on determining the acceptable nitrogen concentration needed to grow a crop (Figure 3). In general, applying high fertilizer concentrations (400 ppm N) to a crop requires that you use high leaching rates (55 percent) or salt levels in the media will increase to unacceptable levels for growth. Low fertilizer concentrations (100 ppm N) can be used to grow most crops, but only if leaching levels are at or near zero. In fact, it was demonstrated that similar nutrient levels could be maintained in the root medium using a fertilizer concentration of 100 ppm N with zero leaching or 400 ppm N with 55 percent leaching.
What is the “best” fertilizer concentration? Based on our experience, we suggest the following as a starting point for fertilizer concentrations:
–50-75 ppm N for plugs
–50-150 ppm N for liners
–75-150 ppm N for bedding plant flats
–200-250 ppm N for pot plants and larger containers
It is important to remember that the “best” fertilizer concentration is the one that maintains media nutrient levels within an acceptable range in your greenhouse or nursery and provides controlled plant growth. You should make adjustments to these concentrations depending on your media, climate and watering (leaching) practices. Finally, the concentration you use should be based on weekly or biweekly media-EC tests and plant vigor. Fertilizing is not just a “calendar” activity that is always the same each season–use soil tests for consistent quality!
There are many subtle points to managing EC levels and interpreting tests. For example, plug growers often hold back shoot elongation by using low fertilizer rates (50 to 75 ppm or even lower) to produce compact plants. Media-EC in plug trays or bedding plant flats is often low even though plants are healthy because vigorous plants quickly take up all the nutrients from the small cells. Media-EC tests are therefore more useful in large containers.
If you do not fertilize with every watering, then you may need to increase fertilizer concentration to compensate for times you irrigate with clear water. To determine the overall average fertilizer concentration that you are applying, divide the fertilizer concentration by the fertigation plus the number of clear water application between fertigations. For example, if you fertigate once a week with 300 ppm nitrogen, and you apply clear water or a fungicide drench twice more during the week, the overall average fertilizer concentration will be approximately 100 ppm (300 ppm divided by three irrigations). In other words, applying 300 ppm N every third irrigation is similar to applying 100 ppm with every irrigation.
With slow-release fertilizers, use label incorporation rates, and supplement with additional water-soluble fertilizer or leaching as needed, based on soil test results.
In most cases, the media-EC can be used as an effective tool for managing nutrient levels within the pot. However, there are limitations on using media-EC as the only measure of the nutrient concentrations. EC is a generic measurement of all the salts contained in the solution, and so it is impossible to know, based on EC data alone, if one or more nutrients have become limiting.
For example, a high initial EC might mean that too much complete fertilizer was added to the medium before planting. Assuming that the nutrient level is not excessive, you would not need to apply much fertilizer early in the crop. However, a high media-EC may also mean that too much of a single fertilizer salt was added, and all other nutrients were extremely low. In this case, you would have to start fertilizing right away, perhaps with some leaching to remove the excess salt.
We recommend you use both on-site media EC testing, as well as laboratory analysis. On-site testing gives you the ability to do a large number of sampling inexpensively, but you still need the laboratory analysis to quantify individual nutrient concentrations and balances.