Did Common High Tunnel Production Issues Impact Your Yields?

Growers who observe reductions in high tunnel yield should initiate a troubleshooting program to determine why the previous year’s crops underperformed.

Soil testing is the backbone of any high tunnel production system. When growers experience a decline in crop yield, it is critically important to evaluate the soil to see if any nutritional deficiencies or excesses could adversely impact your crops. A standard soil test will inform growers about the nutritional content of their soils. Still, when analyzing high tunnel soils, a grower should also pay for two additional tests: percentage organic matter and soluble salts.

Growers often ignore soil organic matter levels in high tunnel systems. Yet, excess organic matter can provide an overabundance of nitrogen to the high tunnel crop unless accounted for in the overall fertility program. Soil organic matter levels in high tunnels tend to increase over time because many growers annually apply manure-based compost in the tunnel to reduce fertilizer costs. While the utilization of manure-based compost is an acceptable practice, over-application of this nutrient-enriched source can create nutrient imbalances that can cause both yield and produce quality issues.

Soil organic matter levels typically range from 2–5%. For each percent of organic matter over 2%, twenty pounds of nitrogen will be available for crop use. One of the high tunnel operations that participated in a research study with Dr. Elsa Sanchez and me a few years ago had a soil organic matter level of 7.3%. Based on this soil test result, the organic matter in this high tunnel would provide 106 pounds of nitrogen per acre to the high tunnel crop. If the grower does not consider this when developing their high tunnel fertility program, excess nitrogen will be supplied to the crop.

Elevated soil-soluble salts are the second most commonly observed production problem associated with high tunnels. Soil soluble salt levels can increase as a direct result of over-fertilization, overapplication of manure-based compost, and poor water quality. When elevated soluble salts are an issue, growers must determine the source of the problem and then implement the appropriate steps to mitigate the issue.

Penn State Extension’s Agricultural Analytical Laboratory uses a 1:2 soil-to-water dilution to determine soil-soluble salt levels. Soils testing <0.40 mmhos/cm are considered to have negligible salinity. High tunnel soils analyzed in this study had an electrical conductivity (EC) range from 0.14 to 9.39 mmhos/cm. Growers can expect to observe some reduction in yield on salt-sensitive crops when the EC exceeds 0.40 mmhos/cm. The following table summarizes the impact on vegetables at various EC levels. As the soil EC levels (soluble salts) increase, growers will notice that their plants may seem shorter than normal and that overall yield may be reduced.

Plant Response to Soil Salinity Levels 1:2 method Adapted from Ag Analytical by Dr. Elsa Sanchez

If a grower discovers that the EC or soluble salt levels are elevated, they can mitigate the injury risk by removing the polyethylene cover from the high tunnel structure over the winter to allow natural precipitation to leach the salts from the soil, leach prescriptively with clear water, or relocate the high tunnel to another location. When confronted with these options most growers opt to conduct a prescriptive leach with clear water (EC < 0.6 mmhos/cm) to lower soil soluble salt levels.

If you are going to consider a prescriptive leach, use the guidelines from the California Fertilizer Association (1985) as a starting point. As per these guidelines, it will take 6 inches of water to leach about 50% of the salt from the soil, 12 inches of water to leach about 80% of the salt, and about 24 inches of water to leach about 90% of the salt from the soil. Prior to considering a prescriptive leach a grower should determine the EC or soluble salt level of their high tunnel soils first and then determine the amount of water in inches that will be needed to leach the salts from the soil.

Most growers find that if they use oscillating sprinklers in the high tunnel that they can lay out a network of small catch basins on the high tunnel floor. The sprinklers are turned on and when the targeted amount of water is caught in the basins the leaching process is completed. The grower should check the efficacy of their leaching by submitting additional soil samples for analysis to an accredited soil testing laboratory to determine the post-leach EC or soluble salt level. If the post-leach soluble salt level is still higher than desired the grower may need to leach their soil again.

Plant parasitic nematodes are also becoming a significant problem in high tunnels. Plant parasitic nematodes can cause direct feeding injury on the roots of crops resulting in reductions in yield, virus transmission to the growing crop, and the potential introduction of fungal pathogens into the injured root tissues. Growers should consider conducting periodic nematode assays in their high tunnels to determine if plant parasitic nematodes are present, the level of infestation, and the types of plant parasitic nematodes that are present.

Nematode management in high tunnels is very challenging, but there are several bionematicides that may be used to reduce the nematode populations to levels that will not cause plant injury or crop loss. Penn State has recently opened a new nematology lab on campus. If you wish to conduct a nematode assay in your high tunnel please reach out to the Penn State lab.

High tunnel production affords growers the opportunity to produce crops 3–4 weeks earlier than field-grown crops while also offering growers the opportunity to extend the harvest season in the fall and winter. When confronted with unexpected reductions in yield growers should conduct some basic analysis in their high tunnel to determine why yields were reduced and how the issues can be mitigated.