February 25, 2018

Determining Optimal Nitrogen Rates

As promised in my last post, today I’m going to explain the purpose of the UofMN low tunnel strawberry project for the 2015 and 2016 seasons: determining optimal nitrogen rates for annual, day-neutral strawberry production in the Upper Midwest.

Over the last 2 seasons we focused on which cultural practices and cultivars worked best in combination. From that research we found plastic mulch and plastic mulch with low tunnel practices to outperform straw mulch practices in almost every measurable aspect, including weekly yields, cumulative yields, berry weight and pest pressures. Thus, the straw mulch treatment was eliminated from this project, but we kept both plasticulture treatments, as they performed similarly. While none of the cultivars tested were poor performers across all parameters, we selected ‘Albion’ as the cultivar of interest in this project, as it appeared to be the most ‘well rounded’ cultivar over the past two years.

The variable of interest in this project is nitrogen (N) rate. Nitrogen is often the most limiting element for plant growth, and we want to see if variable fertilizer rates lead to differences in yield, berry weight or total soluble solid (sugars, essentially) content. There are 5 different N rates we are using: 0 lb N/acre/week, 2.5 lb N/acre/week, 5 lb N/acre/week, 7.5 lb N/acre/week and 5 lb N/acre/week ONLY UNTIL FIRST HARVEST, and then 0 lb N/acre/week for the rest of the season. This last treatment is to test certain literature implying that N application during harvest season can actually reduce cumulative yields. We are hoping these treatments can help us answer the questions: 1) which N rate is optimal for annual, day-neutral strawberry production in the Upper Midwest? 2) Are there noticeable effects of too much or too little N in the system? 3) Is there a point where additional N incorporation will not lead to higher yields?

Let’s take a moment to explain the experimental design of this project. The fields at each site are “cut” in half, with each half representing one of the two cultural practices we’re testing (plastic mulch and plastic mulch with low tunnels). Within each of these halves are 4 replications of each of the 5 N treatments, arranged randomly throughout the rows. This is a stripped down version of what us sciency folk like to call a ‘split-plot design’. Having multiple replications of our 5 N treatments and arranging them randomly allows us to minimize the “error” that might occur from slight differences in field conditions.

Because incorporating 5 different N rates through our drip tape system was proving to be a hassle and a half, we designed a different way to apply the N that we feel is easier and more accurate. A plastic, perforated test tube was placed in the soil 6″ from every plant, and we pour a weekly dose of that plant’s fertilizer treatment into the tube through a super-fancy PVC pipe funnel. I haven’t decided on an impressive sounding name for the device yet, but its on my to-do list. Suggestions are appreciated.

Perforated test tube next to strawberry plant (this one is less than 6" away, i know).

Perforated test tube next to strawberry plant (this one is less than 6″ away, i know).

Super fancy PVC pipe-funnel device

Super fancy PVC pipe-funnel device.

Now seems like a good time to talk about the differences between ‘strawberry production’ and ‘strawberry research’. Both endeavors are necessary and similar, and while their interests often align, sometimes they do not. For example, theres no way i would expect a strawberry producer to incorporate a randomized 5 treatment fertility test into their production, where each plant is fertilized manually to increase accuracy. Its just not practical. But its still a question worth investigating, and strawberry research is trying to answer it in order to help producers grow more effectively. Also, our fertility source is inorganic chilean nitrate, 16-0-0, dissolved in solution. It’s the only inorganic source of N approved for use on organic farms, but most organic growers I know prefer other fertility sources, such as fish emulsion or feather meal. The reason we chose chilean nitrate for this project is because it doesn’t have potassium or phosporus in it, and thus if we notice any differences in yield, berry weight or sugar content we can be more confident that N alone is the reason for the difference, and not P, K or an interaction of these elements. P and K would be considered ‘confounding factors’ in this instance, and in strawberry research we’re looking to eliminate as many of those as possible. A producer on the other hand isnt as concerned with confounding factors, probably wants P and K anyways, and thus would be more likely to use another source for their fertility.

We’re already starting to notice slight differences in our treatments which may be due to N. In the picture below, the plant marked with the yellow flag is receiving 7.5 lbs N/acre/week, while the plant to the right is receiving no supplemental N. While the plant size difference is becoming noticeable, only time will tell if this will translate to yield, weight or sugar differences as well. Our next post will probably cover our first harvest, which we are hoping will occur in a few weeks!

Possible treatment differences

Possible treatment differences.