Are the Soluble Salt Levels in Your High Tunnel Limiting Yield?

While these structures have increased profitability on many farms, high tunnel growing systems are not without their pitfalls. Applying too much fertilizer, inorganic or organic, in repeated applications without sufficient water for leaching can cause soil-soluble salt levels to rise quietly in the background over time until crop yields are reduced.

Excess soluble salt levels in the soil create plant stress, reducing the health and vitality of the crops being grown. When soluble salt levels in the soil are high, moisture inside the plant moves from the roots into the soil, resulting in a chemically induced drought. Symptoms of high soluble salts include reduced plant height, lower yields, blossom end rot, marginal necrosis on leaf tissues, and even plant death in the most extreme cases. 

Most growers routinely test their high tunnel soils, but few request that a soil soluble salt level be determined. Soil soluble salt level is an optional test from most soil testing laboratories. In a high tunnel study conducted by Dr. Elsa Sanchez, one Pennsylvania grower had received counsel from various industry experts regarding the yield issues they were seeing in their high tunnel tomato crop. Based on the counsel they had received from their plant nutrition expert, the grower continued "pushing" nutrients despite the fact that the major nutrient levels were already in the "excessive" range, according to his soil test report. This grower continued to struggle with their high tunnel tomatoes until they participated in the Penn State High Tunnel study. It was at this point that the Penn State Ag-Analytical Laboratory determined that their high tunnel soils had a soluble salt level of 2.27 m/S which is considered "strongly saline" by our lab. Under high levels of soil salinity, crop yields may be reduced by 50% or more despite an apparent over-abundant supply of nutrients in the soil.

Adapted from Penn State Ag Analytical Lab

In a field setting, precipitation from snow and rain leaches out nutrient salts naturally, but in high tunnel environments, the plastic cover will shed all precipitation, resulting in the gradual build-up of salts in the soil. When confronted with elevated soluble salts levels in the soil, a high tunnel grower may consider relocating their high tunnel structure to another location, or they can remove the covering for a period of time (over the winter, for example) to allow for the area's natural precipitation to leach the fertilizer salts from the soil.

An easier but less labor-intensive means to reduce the soluble salts level in the high tunnel is to actively leach the soluble salts from the soil through the prescriptive application of water. Guidelines from the California Fertilizer Association (1985) indicate that to reduce 50% of the salts from the top one foot of soil, it will take 6 inches of water, for an 80% reduction it will take approximately 12 inches of water, and for a 90% reduction it will take approximately 24 inches of water. In terms of water application rates on a per-gallon per acre basis or per 1000 ft² basis please see the table below.

Adapted from Guidelines from the California Fertilizer Association (1985)

Leaching the soil in a high tunnel is best accomplished using overhead watering rather than your trickle irrigation system. If a grower does not have a meter installed on their water line to measure the output in gallons, the simple placement of a few coffee cans on the high tunnel floor to collect the water dispensed from the overhead irrigation equipment can work quite well. If the goal for a grower is to reduce the soil soluble salt levels by 50% in their high tunnel, the grower would halt the leaching process when a total of 6 inches of water is collected in the strategically placed coffee cans. Once the leaching has been completed and the soils have dried for a few days, the grower should do a follow-up soil soluble salts test to check the success of the leaching efforts.

Elevated soluble salt levels in high tunnels are one of several yield-limiting issues observed by high tunnel growers across the U.S. Plant-parasitic nematodes and the presence of persistent soil pathogens like Fusarium spp. are becoming prevalent because of limited crop rotations and the almost continuous production of high-value horticultural crops like tomatoes. While leaching will not lower plant-parasitic nematode populations or reduce soil pathogen levels, it is the best way to mitigate soluble salt levels.

High Tunnel Soil Test Report series