Potassium management and soil testing in high tunnel tomato production

Potassium management and soil testing in high tunnel tomato production

Summary

High tunnels are inexpensive, greenhouse-like structures used to extend the growing season and to increase crop quality and yields without increasing cultivated land area. High tunnels experience reduced leaching from precipitation and higher fertilization rates, but also much higher crop yields than are seen in the field. As a result, high tunnel soils typically show high macronutrient and soluble salt levels when subjected to field soil testing methods. Over- and under-fertilization of high tunnel crops is commonly seen in New England, despite regular soil testing and fertility monitoring programs.

The methodology of high tunnel soil testing is inconsistent between states in the Northeast and there have been no previous studies comparing the efficacy of these methods in high tunnel environments. We propose to assess the correlations between these tests in high tunnel soils, focusing on field soil tests commonly used in New England (Mehlich-3 (M3) and Modified Morgan (MM)) as well as the Saturated Media Extract (SME) test. For these experiments, we propose to focus on Potassium (K), an important macronutrient in tomato yield and quality.

In addition, we propose to assess the relationship between soil K with both yield and yellow shoulder incidence. We will also test the hypothesis that the SME test in conjunction with the M3 or MM test can be used to improve our ability to predict soil nutrient availability for high tunnel tomatoes. The project will provide a framework for better understanding soil fertility in high tunnel tomato agroecosystems.

Objectives/Performance Targets

Objective 1. Characterize the relationship between soil potassium and yield response and yellow shoulder incidence for high tunnel tomatoes.

In 2014, several levels of K (0 through 900 lbs./acre) were applied in a randomized and replicated design in five high tunnels (three in Monmouth, ME, one in Durham, NH, and one in North Haverhill, NH). In 2015, the experiment was repeated in Maine and in North Haverhill with no additional K, and at a new experimental site in Durham that received a new range of K levels (0 to 750 lbs./acre). Throughout the growing season, soil K levels, plant tissue K and N levels, plant growth characteristics, yield and fruit quality were measured. Results are not yet complete because University of Maine’s Soil Analytical Lab (UMSAL) will complete the analysis of the 2015 end-of-season soil samples in early 2016. However, the existing 2014 and 2015 data have not shown strong correlations between soil K and yield or yellow shoulder response using bivariate correlations, nor have there been significant differences between seasonal yield or yellow shoulder between treatments using Tukey’s Honestly Significant Difference test. However, at certain sites, moderately positive correlations (r value of .25) with yield and moderately negative correlations (r value of -.27) with yellow shoulder suggest that soil K may affect yield and yellow shoulder, but it is not the only influential factor.

Objective 2. Determine the correlation strength between Mehlich 3, Modified Morgan, and SME testing methods in high tunnel soils for potassium.

Throughout each growing season at all locations, multiple soil tests were taken per site to be analyzed with the M3, MM, and SME testing methods. This objective has not been fully assessed because the UMSAL will complete the analysis of the 2015 end-of-season soil samples in early 2016. However, after analyzing all 2014 and part of the 2015 soil test data with bivariate correlation models, very strong correlations were seen between M3/MM, M3/SME, and MM/SME test’s ability to predict soil K (r values = .94 to .99).

 Objective 3. Determine if the use of the SME test in conjunction with the Mehlich 3 or Modified Morgan tests improves predictions of available soil K.

To assess the ability of combined soil testing methods to predict K availability, hypothetical K removal metrics (yield and petiole sap K) were assessed with univariate (one soil test per model), bivariate (two soil tests per model), and polynomial (two soil tests compound together) regressions. This objective has not been fully assessed because UMSAL will complete the analysis of the 2015 end-of-season soil samples in early 2016. While the strength of r² values in all models was weak (r² value < .18), the values did increase with model complexity. However, the model strength may be hindered, as the hypothetical soil K removal metrics may not have truly reflected soil K removal due to questionable methodology (petiole sap K).

Accomplishments/Milestones

The two field seasons of this project have been completed and data analysis is underway. In summary, tomatoes were grown in replicated designs in high tunnels throughout New Hampshire and Maine over two seasons. Project collaborators independently managed all sites according to the experimental design, and collected yield, quality, and soil data (analyzed by UMSAL). The project manager visited all sites multiple times throughout the year, and additional petiole sap nutrient level data was collected and analyzed by the manager using LAQUA Twin Potassium and Nitrate meters, and by the UMSAL (2014). Except for the 2015 end-of-season soil data awaiting analysis at UMSAL in early 2016, all of the data has been collected and analysis is underway to meet the objectives of the two-year project. At this point, conclusions cannot be made until all soil data has been synthesized by the UMSAL and further analysis is completed.

In early 2016, all of the project collaborators will meet to discuss the completion of the second year of the project. The weak correlations and regressions between the data, potential outreach opportunities, as well as how this experiment can be improved for further study of high tunnel tomato agroecosystems, will be on the agenda for discussion.

Impacts and Contributions/Outcomes

While both field seasons are now complete, conclusions cannot be made, as raw samples are still awaiting lab analysis. However, after the analysis of the 2014 and existing 2015 data, insight can be gained into the objectives.

Our first objective was to assess the interactions between soil K and high tunnel tomato yield and yellow shoulder incidence. The existing data shows weak to moderate correlations between soil K and yield and yellow shoulder presence, potentially illustrating that while soil K does influence these variables, other influential factors are present. More raw data, data analysis, and discussion are needed to determine whether experimental design and/or the site variability of soil potassium dynamics play a role in our results. While our results may not provide clear critical soil K levels for maximum yield or yellow shoulder mitigation, they highlight the complexity of these factors and can serve as a framework for future study of the issue.

 Our second objective was to assess correlation strength between M3, MM, and SME testing methods in high tunnel soils for potassium. Overall, correlations between all soil tests from the existing data were very strong. Our results may indicate that the varying soil testing methodologies yield more similar results than previously expected and may lead to more uniformity in our regional high tunnel soil testing for tomatoes.

 Our third objective was to assess whether the use of the SME test in conjunction with the M3 and MM soil tests increased the ability to predict available soil K. The existing data illustrates that while the prediction strength of available soil K based on hypothetical K uptake metrics is weak with all individual soil tests, the prediction strength does slightly increase when combining M3 or MM with SME. Our current results may indicate that the SME test may have a future role in soil K predictions in high tunnel agrosystems. However, more raw data, analysis, and discussion on improved hypothetical K removal metrics are necessary before any final conclusions can be made.

Collaborators: