Take charge of your media

Starter charge is the amount of fertilizer incorporated into the potting media at the time of manufacture to provide for the crop prior to fertilization by the grower. The intent behind adding this charge is to provide some nutrition for the crop during the plant establishment phase of the crop cycle. The charge can be variable by media, and generally provides nutrition for the first 10 to 14 days of the growing cycle. The number of irrigations for a crop in the first 14 days of the growing cycle will dictate the amount of starter charge that remains in the medium. If the crop gets a heavy watering immediately after planting, then a good bit of this “starter charge” will be leached from the containers, packs or flats. In a particular growing situation, an experienced grower will have a good idea as to when this charge needs to be supplemented with additional fertilizer. The growing medium should be tested if a grower is not familiar with the medium, or has changed media from the product that is used regularly. Both commercial and extension laboratories are capable of doing this testing, and it is good idea to have that initial test data on hand when managing a fertility program on a given crop.

In-house testing should also be part of a fertility management program, and can be done by measuring pH and electrical conductivity (EC) with a pH/EC meter. The pour-through sampling procedure or 2:1(water : media) sampling procedure will provide this useful information. The procedure, combined with an initial media test will be a good foundation for managing the crop fertility. As the EC starts to drop in the containers, additional fertility should be provided to bring the media fertility level back up to the target levels for the crop. Developing root systems will mine the starter charge from the medium as the root system expands into the medium volume in the container, and combined with the irrigation frequency may rapidly diminish the nutritional level in that medium. This is why frequent monitoring of the pH and EC are so important to proper crop fertility management.

The effects of time

Media that has been stored for a considerable period will have lost some of its starter charge. Microorganisms will consume the fertilizer elements, and can also deplete wetting agents incorporated in the medium. These nutrient losses will be more rapid in warm conditions than in cool conditions. Media products begin to age after blending and formulation, so the product’s age from time of manufacturing is an important consideration for starter charge.

Generally eight to nine months after the time of manufacture is the extent of the reasonable storage time before use in the summer and 10 to 12 months in the colder periods. If a grower uses media that has been stored for a longer period, changes in the fertility management should be implemented to compensate for this aging.

Makeup and custom blends

The nature of the starter charge will generally be undisclosed, as such information is typically viewed as proprietary by the manufacturing company. The laboratory test will provide the levels of nutrients in the medium that are available, but not the source product that was used to provide those elements. Many choices of nutrient sources are available for the manufacturer, and these decisions are generally based on economics, feasibility of incorporation in the medium, nutrient source quality, and nutrient supply longevity. Growers who understand the medium used for a given crop may decide to feed right at (or after) planting, or wait for 10 to 14 days to start feeding additionally. While this is a personal decision, adjusting the media fertility level will provide additional fertility for the crop, and make up for leaching or other nutrient losses that have occurred.

For smaller greenhouse and nursery operations, bagged media products are often used, but with some smaller growers, as well as larger operations, custom blends are often the product of choice. A custom blend can allow a grower to work with the manufacturer to attain a better-suited product for the crop and the growing situation. Keeping good records will also help, by providing information about the changes in the medium, and the crop timing and quality. Most manufacturers work tirelessly to select ingredients, and maintain high levels of quality control in their handling and processing of those ingredients through the storage, mixing and shipping phases of their operations. Starter charges are a part of that quality control process, as it will not serve a manufacturer well if the starter charge has radically changed by the time it is used at the greenhouse or nursery.

Table 1 presents the results of two common bagged media for professional growers, and illustrates the differences in starter charge contributions to a growing situation. Not only are the pH values quite different, but the individual elements vary from product to product as well. Product “A” has a much greater total nutrient starter charge than product “B,” indicating that with product “B,” supplemental fertility should be applied rather soon. The nitrogen level in product “B” is probably insufficient for the crop, while product “A” could be satisfactory in nitrogen for 14 days. Potassium (K) levels are good for now in both products, but the phosphorous (P) level is low in product “B.” While the calcium (Ca) level in product “A” is over the normal range, Ca is used by the plant for cell wall growth. And so the level will drop as the root system expands and grows. Magnesium (Mg) is used by the plant as the central ion in the chlorophyll molecule, and also to unwind the DNA in a cell for cell multiplication. This slightly high level will drop as the plant grows, and should be in the normal range in a week or two. Micronutrients are rather limited in both media, and this indicates that supplemental fertility would be a good plan fairly soon.

A water sample should be taken periodically for a greenhouse operation, and with certain water supplies the elements in that sample could change over a growing season. Samples collected in spring will show lower levels of some elements compared to summer samples when the water table has gone down in a region. Testing the water used in production is an important aspect of managing crop quality, and it is often the case that these nutrient levels in the water as well as the pH will change. Balancing water contained nutrients, EC, and pH along with fertility inputs, as well as media pH, nutrient status and EC will give a better result in crop quality overall. It is important to test all three of these aspects involved in plant culture.

Sinclair is an extension educator at Penn State University specializing in floriculture, propagation, breeding and nutrition.