The use of pre-plant controlled release fertilizers (CRF) -regardless of pre-plant soil nitrogen levels- has been a common practice for years among strawberry producers on the central coast. UCCE research has questioned the efficiency and the value of CRF use, by showing that CRF releases nitrogen at a faster rate than crop uptake (leading to nitrate loss) and that reducing pre-plant CRF application by 50% or more has no effect on fruit yield. This project engages partnering professionals and strawberry producers in a collaborative research and education effort seeking to build confidence in research findings, and increase adoption of more sustainable nitrogen fertilizer use practices reflecting current knowledge and understanding. The project addresses the question: If growers reduce Controlled Release Fertilizer application by 50% or more and instead use data driven fertigation practices is yield maintained? Project Partners will address this question in multiple production settings and varying acreages to: a) design and implement field trials to evaluate different levels of CRF application in commercial fields, and b) interpret and disseminate the results to other growers and professionals. The experience of collaboratively designing and implementing research trials will not only produce new knowledge, but it will help growers gain confidence and ownership of the scientific research process, while helping researchers better understand barriers to adoption of new management practices.
By the end of the two and a half year project period we expect to meet the following objectives:
- Improve understanding among partnering professionals and producers about the effectiveness and limitations of different levels of controlled release fertilizer (CRF) application on commercial strawberry fields on the California Central Coast, considering their associated impacts in terms of crop yield and potential water quality impairment.
- Through participatory field trials with 5 strawberry growers, improve producer confidence in research results and recommendations, as well as their own capacity to evaluate and communicate the effectiveness of nitrogen management practices.
- Through participatory education and outreach to 30-50 additional strawberry growers on the Central Coast, increase willingness to reduce CRF applications and try more efficient, data-driven nitrogen management practices on a test plot within their own operations.
In each ranch the grower selected an experimental block where the the rate of application of pre-plant nitrogen fertilizer was reduced by an amount ranging from 40% to 100% (i.e. no pre-plant nitrogen applied).
In the blocks where pre-plant nitrogen was reduced, Nitrogen will be applied starting in March
Educational & Outreach Activities
All participating growers recognized an opportunity for reducing fertilizer use costs without impacting yield
Soil nitrate monitoring was completed on four commercial strawberry ranches throughout the entire 2017-2018 crop cycle. At each ranch there were control and experimental blocks ranging from 1400 ft2 to 1 acre, and reductions of pre-plant applied nitrogen ranging from 40% to 100%. Soil N monitoring started in Sep-Oct 2017 when a baseline soil nitrate test was collected at all ranches, before the application of pre-plant slow release fertilizer. Subsequently, monthly soil nitrate tests were collected between Nov 2017 and Sep 2018 at each ranch, both in the experimental and control blocks.
Figure 1. Collecting soil samples from the experimental blocks
Figure 2. Interpreting soil N measurements with the nitrate quick test
Soil N change and variability throughout the crop cycle:
First-year results indicate higher concentration and variability of nitrate in the first foot of soil versus the second foot of soil, throughout the crop cycle, both in control (more N) and experimental (less N) treatment blocks. This is to be expected and generally a sign of efficient management. However, the results also demonstrate high concentrations, and in a few cases very large spikes, of soil N in the second foot of soil, which is below the root zone (i.e. inaccessible to the strawberry roots). This suggests that at least some of the applied N was not used by the plants, and there might be opportunities for improving N fertilization scheduling on all ranches. Finally, the results suggest that while control blocks (more N) showed larger spikes in soil N than experimental blocks, there were no significant differences between treatments in terms of soil N availability throughout the crop cycle. In other words, the use of higher application rates of slow-release pre-plant fertilizer did not result in more soil N available to the crop.
Figure 3. Change and variability of soil nitrate content (Lbs N/ac) in the first foot of soil (0-12″ soil depth). The graph on the left shows the control (more N) blocks of the four ranches. The graph on the right shows experimental (less N) blocks of the four ranches.
Figure 4. Change and variability of soil nitrate content (Lbs N/ac) in the second foot of soil (12-24″ soil depth). The graph on the left shows the control (more N) blocks of the four ranches. The graph on the right shows experimental (less N) blocks of the four ranches.
Residual soil N across treatments:
Comparing the baseline (prior to pre-plant fertilizer application) and the end-of-crop (residual) soil N concentrations between treatments across the four sites, there was a common trend where experimental treatments (less N) resulted in slightly lower soil N concentrations at the end of the crop than control treatments (more N), regardless of the baseline concentration. In one of the sites the difference was quite large (136 Lbs/ac). See table below.
Crop yield comparison:
Two of the participating ranches tracked crop yield separately on control and experimental treatments. One of them found no difference between the two treatments and the other one showed slightly higher yield in the higher N treatment (use of slow release pre-plant N fertilizer). Crop yield and fertilizer use data is still being collected, and will be more thoroughly reported during the next period.