Focus on fertilizer

Photos by Matt McClellan

In part one, we covered mineral nutrition, including tests you can perform on your water, amendments and placement. In part two, we will cover fertilizer formulation, longevity and rate.

Formulation

The 3-1-2 ratio of macronutrient fertilizers for woody crops has been the standard in production. For example, an 18-6-12 has a ratio of 3 nitrogen (N) to 1 P2O5 to 2 K2O when using conventional oxides of phosphorus (P) and potassium (K). This is equivalent to 3.0 nitrogen to 0.4 phosphorus and 1.7 potassium ratio. The nitrogen component comprises various ratios of urea, ammonium, or nitrate; with urea typically being the least expensive nitrogen source. Regardless of source, most of the nitrogen applied will become nitrate (NO3) via enzymatic or microbial processes when available to plants in the substrate solution or pore water. USDA-ARS researcher Jake Shreckhise and I demonstrated that a ratio of 3.0 nitrogen to 0.3 phosphorus to 1.7 potassium can safely be used to further reduce phosphorus (an environmental concern) by 25%, reducing the risk of contamination of surface waterways or groundwater aquifers near or underneath production sites. The adage that more phosphorus is good has been disproven repeatedly.

Longevity

The overarching aim of a fertility program is to use a fertilizer product that can be applied when potting and last until the plant is shifted up or sent to the retail center. The worry about fertilizer lag (i.e., delayed initial nutrient release after application) is overblown, in our opinion, except for a few crops (e.g., hydrangea and crape myrtle), of which one should be included in your trials. Remember, your liner can be fertigated prior to transplant, additionally some liners have incorporated fertilizer that is transplanted with the cutting; carrying it from the “shock” of being transplanted into production as measured in days or weeks. Thus, you should be trying fertilizer longevities (months) that work with your production scheme. Success should be measured in the finished products of an equivalent size plant that may require less pruning while producing a more uniform crop.

Rate

Rate equals cost. A simple but crucial test is to evaluate your existing and potential new products across label rates to see how low you can go when finishing with an equivalent crop. You might happily find that you can reduce the rate of your fertilizer by 25% to 50%; reducing the quantity of fertilizer needed per year, lowering handling costs and increasing the amount of fertilizer used by your plant.

Monitoring is knowing

I think an inexpensive and simple $60 to $100 Bluetooth temperature button-like dataloggers should be used in everyone’s nursery to monitor substrate temperature, the dominant factor effecting mineral nutrient release from your controlled-release fertilizer. Substrate temperature is currently not predictable because of variation in light from sun or cloud cover, irrigation scheduling, container color, air temperature, etc. that are unique to each individual operation. Recent data collected by my colleagues and I have been eye-opening regarding maximum temperatures observed, continued duration of heat into the night, and effect of irrigation and container color on overall substrate temperature. The resulting average and maximum temperatures will aid you in selecting the right longevity fertilizer.

The preference to monitor substrate solution or pore water is to use non-destructive methods versus saturated media extracts that require plant be harvested. More about this in future articles. Regardless, the pour-thru method described by Drs. Whipker or LeBude for young plants and greenhouse crops or nursery crops, respectively, is the most utilized method to routinely measure electrical conductivity (and pH), a proxy for nitrogen and subsequent macronutrient availability. Great “how to” and results interpretation publications can be found online at North Carolina State University via a simple google search. Many moisture sensors can also provide bulk electrical conductivity, considering the substrate and fertilizer itself, and therefore is hard to translate into mineral nutrient availability for the crop unless specific calibrations are performed, and values are correlated with your prevailing methodology. A main consideration in substrate solution testing is the overall trend of nutrient content; such trends will alert you if the nutrient supply is insufficient or excessive.

The last indicator, and most important, is the crop. While responses might lag – overall plant health and growth are the most important indicators. Perceived low electrical conductivity and pH may not always result in slower production and could possibly reduce labor costs specifically pruning and shaping. Routine measures of substrate solution EC and pH, coupled with substrate temperature while observing growth and crop health will give you the needed information to set targets for a given crop; especially those finnicky ones that are giving you headaches.

In summary

Time optimizing your fertilizer program is time well spent. Find a place in your nursery that is representative of your production system, turn off the fertigation, eliminate amendments that easily wash through the substrate (water soluble nitrogen, superphosphate, sulfur, etc.), place the appropriate longevity controlled-release fertilizer for your production scheme (e.g., subdress) and patiently wait to see where you end up at the end of the production cycle. Once optimized, stay diligent in revisiting your fertilizers if the substrate, water source, irrigation scheduling or fertilizer formulation are altered. Happy testing!

James “Jim” Owen Jr., USDA ARS, and Alex Niemiera, Virginia Tech, have been working together over the last decade to understand and optimize nitrogen and phosphorus fertility. Forrest Brown is currently a doctoral candidate in the School of Plant and Environmental Sciences at Virginia Tech.