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

Are the Soluble Salt Levels in Your High Tunnel Limiting Yield?
Mar 25, 2021

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By Thomas Ford
While these structures have increased profitability on many farms, high tunnel growing systems are not without their pitfalls. If manure-based composts, mushroom substrates, or chemical fertilizers are utilized as nutrient sources, soil soluble salt levels can rise quietly in the background over time until crop yields are reduced.
Excess soluble salt levels in the soil create plant stress which in turn reduces 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 because of the elevated soil salinity levels resulting in a chemically induced drought. When plants are stressed by elevated soil soluble salt levels, a grower may notice a reduction in 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 recent high tunnel study conducted by Dr. Elsa Sanchez and the author, one Pennsylvania grower had received counsel from a variety of industry experts regarding the yield issues that they were seeing in their high tunnel tomato crop. Based on the counsel that they had received from their plant nutrition expert, the grower continued the practice of “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.
Table 1: Plant Response to Salinity Levels using a 1:2 dilution (1 soil to 2 parts distilled water)
mmhos/cmEffects on Plants
<0.40Negligible salinity; beans, carrots
0.40 to 0.80Very slightly saline; 25-50% decrease carrots, onions, peppers, lettuces
0.81 to 1.20Moderately saline; seedling injury possible; 25-50% decrease broccoli, potatoes
1.21 to 1.60Saline; tolerant beets
1.61 to 3.20Strongly saline
>3.20Very strongly saline
Adapted from Penn State Ag Analytical Lab
In a field setting, precipitation from snow and rain leach out nutrient salts naturally, but in high tunnel environments, the polyethylene 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 polyethylene covering for a period of time (a growing season) 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.
Table 2: Leaching Guidelines to Reduce Soluble Salt Levels in Mineral Soils
Percent reduction in soluble salt level desiredNumber of inches of water per acreGallons of water per acre needed to leach soluble salt levelsGallons of water per 1000 square feet to leach soluble salt levels
50%6 inches162,9243,740
80%12 inches325,8487,480
90%24 inches651,69614,960
Adapted from Guidelines from the California Fertilizer Association (1985)
When leaching the soils in a high tunnel it is best accomplished by 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 leach has been completed and the soils have dried for a few days, a follow-up soil soluble salts test should be conducted by an accredited soil testing laboratory to check the efficacy of the grower’s leaching efforts.
Elevated soluble salt levels in high tunnels are one of just 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 too 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 means to mitigate your high tunnel soluble salt levels.
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