Advancing sustainable nitrogen management in strawberries through participatory research and education

Soil N change and variability throughout the crop cycle

First-year results showed 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 demonstrated 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.

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.

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 and nitrate leaching

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. 

There was strong evidence of nitrate leaching by winter rains and some evidence of leaching due to over irrigation (Figure 3). We also found some evidence that fertilizer beads were still in the soil as late as June, or 8 months after application (Figure 1). Differences in plant vigor were visually apparent at the beginning of spring only in Ranch 1 and by June differences were not discernible (Figure 4). In all ranches, residual nitrogen from the previous crop and nitrate in the irrigation water provided substantial contributions to the crop total nitrogen balance, contributing to a seasonal nitrogen availability over 300 lb/ac in ranches in the low pre‐plant plots of Ranches 1 and 2 (Table 1), while in Ranches 3 and 4 the total nitrogen available in the low pre‐plant treatment was below recommended seasonal crop demand (158 and 197 lb/ac respectively). In Ranch 2, nitrate in the irrigation water and estimated nitrogen release from soil mineralization were sufficient to match the rate of 1 lb/acre day of crop uptake.

*See larger size Tables and Figures in attached poster: RCDSCC_SARE Project_ASHSposter

Crop yield

The total amount of nitrogen applied at the four ranches in the study varied substantially, but all ranches obtained good commercial yields (above 60,000 lb of fruit per year, Table 1). Ranches 1, 3 and 4 reported higher yields associated with higher rate of pre‐plant fertilizer, while in Ranch 2 no differences were observed. In Ranch 1, yields were measured weekly in a strip plot, the yield was about 13% higher for the pre‐plant treatment than in the fertigation treatment (Figure 2), despite the pre‐plant plot received from fertilizer less nitrogen than the fertigation treatment (117 vs. 158 lbof nitrogen/acre). ANOVA indicated that these differences were statistically significant (Table 2). Irrigation management at this ranch was very close to crop evapotranspiration, while over‐irrigation occurred in other ranches (data not shown).

*See larger size Tables and Figures in attached poster: RCDSCC_SARE Project_ASHSposter

Discussion

The application of pre-plant fertilizer in strawberry production used to be very common in the northern district, but growers have been phasing it out and adopting nitrogen management based on fertigation instead. The drivers of this transition are lower costs of fertilizer and anecdotal reports that high soil nitrogen concentrations negatively affect fruit quality. Previous work from the UC cooperative extension suggest that most slow release fertilizer is released in the first six months of the crop when uptake is low and leaching potential by rain is high (Bolda, et al., 2011). However, in this study we found some evidence that fertilizer was still available as late as 8 months after application, and statistically significant increase in yield in one ranch in the pre‐plant treatment. However, this data came from a one‐bed strip (52 inch wide, about 300 ftlong) in the ranch and may not be representative of field conditions. Additionally, most of the difference in yield came from one rep, while the other reps in the pre‐plant plot had yield comparable to the fertigation  plot (Figure 2). In the other two ranches that showed lower yield in the low pre‐plant treatment, the total quantities of nitrogen applied were below recommended crop uptake. We also found strong evidence of nitrate leaching following rain events, suggesting that applying large quantities of pre‐plant nitrogen in fall may lead to low nitrogen use efficiency. The nitrogen balance at the ranches showed that in some cases the contribution from non‐fertilizer sources can provide a substantial quantity of nitrogen, that sometimes can even match the crop uptake. This was the case in Ranch 2 where no differences in yield were observed. Total nitrogen application in some cases exceeded 300 lb, that is concerning, given the strict regulatory environment for water quality in California. In these scenarios, fertigation seems a more dynamic tool to adapt nitrogen management during the season. In conclusion, this work suggested that slow‐release fertilizer may remain available for plant uptake during production season and in certain conditions plant vigor and commercial yield may increase. However, pre‐plant applications cannot be adjusted during the season and unforeseen rain events can lead to nitrate leaching, while the contribution from non‐fertilizer sources of nitrogen can be substantial. In‐season fertigation allows to apply nitrogen after the rains and offers flexibility to adjust nitrogen applications with economic benefits, environmental and regulatory relief.