GET CULTURED: Fertilizer and media management during El Niño

by Don Merhaut

During the strong El Niño event that is predicted in California for the winter of 2015–2016, there is a risk of leaching of fertilizer from containerized plants and the production areas that are exposed to the weather. Most varieties of woody ornamental plants will be dormant or growing very slowly. In fact, even many of the California native plants will have limited growth until early spring, so most plants will have minimal fertilizer needs. In this article, I will discuss methods to maintain the minimal fertility requirements of container-grown ornamentals that are located outdoors. In addition, I will provide some housekeeping items to prevent fertilizer runoff from nursery production areas. I am assuming that once the rains begin, it should be almost continuous from January through March, as this was the weather pattern, especially for Southern California, during the last El Niño in 1998.

Brief Summary of Management Practices

Fertilizer type. Use polymer-coated fertilizers rather than liquid fertilizers to provide nutrients and to minimize potential for nutrient runoff. Most, if not all, polymer-coated controlled-release fertilizers (CRFs) release nutrients as temperature increases. Even though temperatures during this El Niño winter are predicted to be warmer than usual, fertilizer release from CRFs will be minimal. Once the warmer growing season commences, nutrient release from these products will increase to meet the needs of the plants.

Fertilizer use. If your region is experiencing warm days and plant growth is still occurring on select plant species, liquid fertilization may be suitable. However, take into consideration your growing region — what part of the state you are located in. If the rainy season has begun in your area, do not add liquid fertilizer at this point; fertilizer runoff will likely occur.

Media storage. If polymer-coated CRFs have been blended into the media, during warm days, higher media temperatures could cause rapid release of nutrients from the fertilizer prills, even in storage, if the media is moist. If uncoated granular fertilizers were blended into the media, the rate of fertilizer dissolving and the potential for nutrient leaching will be even greater. This is one reason why media containing pesticides and fertilizers should be stored on a concrete pad (fig. 1). The storage area should be covered — ideally under a water-proof structure — to minimize dispersal of media by wind and rain. Tarps can also be used over pallets but keep in mind that this may increase media temperatures.

Cultural practices. If your production palette contains winter-growing species, such as many aloes, some other succulents and camellias, consider moving these plants into a hoophouse or greenhouse. Preventing rainfall on these crops will provide better control of water and fertilizer to meet the specific needs for the crop. In addition, some of these plant species are flowering during the winter, so flower and foliar diseases may be easier to control or prevent if plants are inside waterproof structures.

Irrigation collecting and recycling facilities. If irrigation runoff is collected and recycled, be prepared for excess runoff from rain events. All drainage areas ditches and gutters should be free of debris. Areas susceptible to debris flows should be inspected regularly during the rainy season.

Production beds. All production beds should be prepared so that enough slope is provided for excess water runoff. Weed cloth and gravel should be in place to prevent plants from sitting in mud. Mud and sitting water may also encourage weeds, diseases and insects. Proper bed design and construction will also minimize mud from clogging drainage channels and accumulating in collection ponds.

Record keeping. Monitoring and keeping records of nutrient concentrations in leachates and media should be part of the crop production program. By routinely monitoring parameters like electrical conductivity (EC) or total dissolved solids (TDS) media and leachate, one can get an estimate of nutrient concentrations in containers. With proper record keeping, changes in fertility and adjusting for fertilizer needs of a crop can optimize fertilizer management and mitigate nutrient loss from cropping systems.

Discussion of Fertilizer and Media Use

What follows are more detailed descriptions of fertilizer and media use and how water from rain or irrigation influences fertilize use and runoff in containers. Challenges and opportunities are also discussed.

Planting media. There are two ways nutrients from fertilizers are held in containers: absorption, and adsorption. Absorption is the physical property of the media, where the nutrients in solution are absorbed into the medium pores, much like water is held in a sponge. Organic matter such as peat moss, coir, compost and bark are good sponges and therefore provide a high water-holding capacity to media. This will allow fertilizer that is in solution to remain in the containers through the absorptive properties; however, this also means this fertilizer solution can easily leach from containers if excess water comes from irrigation or rain events. Adsorption is a chemical property, where the charged nutrients are bound to charged sites on the media, much like iron shavings are held onto a magnet. Unfortunately, the adsorptive properties of almost all artificial media are limited. There are two properties of adsorption, cation exchange capacity (CEC) and anion exchange capacity (AEC).

Organic media in containers is not stable; it will slowly decompose. This is sometimes referred to as “shrinkage,” and is especially evident when incompletely composted products are used as a substrate. As microorganisms and insects consume and breakdown the organic matter of dead plant tissues — compost, coir, peat, dead roots etc. — some nutrients are released and will be available for plant uptake. The amount of nutrients from this process is very limited, especially during the cooler winter months when microorganisms are not as active; however, it is a pool of nutrients that should be mentioned since we are discussing nutrient availability from artificial media.

Fertilizer types. There are two types of fertilizers commonly used in container production: (1) liquid fertilizers, which are dissolved in water and applied through the irrigation system, and (2) solid fertilizers, which are blended into the media or top-dressed onto containers.

During rainy periods, liquid fertilizers leach through containers. The majority of the dissolved nutrients will not bind to the media because the media has low CEC and AEC, as described in the previous section. Since chelates are not charged compounds, micronutrients applied in a chelated form will also easily leach from containers.

Polymer-coated controlled-release fertilizers (CRF) slowly release nutrients, despite water availability. Most of these products increase release rates as temperature increases (fig. 3). Please refer to product labels for ideal soil and media temperatures for optimal use and longevity of nutrient availability of fertilizer. Even though forecasts are predicting warmer and wetter conditions this winter, limited nutrient release from most of these products is likely to occur, unless a media temperature above the specific CRF’s ideal release temperature (typically 70°F to 78°F) is sustained over a period of time. The limited release of the fertilizer during winter months will conveniently coincide with the limited needs of the plants during this period.

The release of nutrients from slow-release fertilizers are dependent on several factors, not just a single factor like media temperature, and for this reason nutrient release is less predictable than from controlled-release fertilizers. For example, sulfur-coated fertilizers slowly release nutrients based on the breakdown rate of the sulfur coating. Since breakdown of the sulfur coating can be both a chemical and biological process, nutrient release from these products may also be dictated by microbial activity, which is slower during the cooler winter months.

Non-coated granular fertilizers break down as moisture is available, the rate of which is determined by media temperature. Rate of fertilizer release is also influenced by particle size and solubility of the compounds used in the fertilizer: as temperature increases and particle size decreases, the rate of the fertilizer dissolving and nutrients being released will increase.

Manure and plant-based fertilizer products release nutrients based on nutrient solubility in the product, particle size, microbial activity and degree of decomposition of the product. Numerous types of fertilizers are available that are derived from organic materials and the nutrient release characteristics of many of these products have not been tested thoroughly. Therefore, if these products are being used, it is recommended to routinely test and keep records of the leachate and/or media for electrical conductivity (EC) or total dissolved solids (TDS).

In closing, with proper use of fertilizers and making some modifications to cultural programs, any negative effects on fertility in containerized crops should be minimal. In fact, in areas where lower quality secondary water sources are used, the rains from this winter should help reduce salt accumulation in our production systems.

Don Merhaut is a UC Cooperative Extension Specialist for Nursery and Floriculture Crops, Department of Botany and Plant Sciences, UC Riverside.