Shorter Than Normal Plants? Your High Tunnel May Have a Salt Problem

If so, you may observe one of the first signs of a soluble salts problem in your high tunnel. High tunnel growers in Pennsylvania typically do not remove their plastic in the winter or relocate their tunnel annually, so fertilizer salts can begin to accumulate in the soils over time. As soluble salts increase in the soil, plant growth and yield will begin to decline.

A recent high tunnel study by Dr. Elsa Sanchez, Professor of Horticultural Systems Management at Penn State, and Penn State Extension educator Tom Ford found that the average soluble salt level detected in 27 high tunnel soils was 2.02 mS. As per the Penn State Agricultural Analytical Services Laboratory soil electrical conductivity values, 1.61–3.20 mS range are strongly saline. Unfortunately, only four of this study's 27 high tunnel growers reported soil soluble levels in the negligible range of <0.40 mS.

Elevated salt levels in the soil are the primary reason growers observe cropping problems in their high tunnels. Due to high salt levels, smaller plants will yield less fruit or be less productive. Tomato and pepper plants under stress due to elevated soluble salt levels may observe higher incidences of blossom end rot than plants grown in soils where the electrical conductivity is in the negligible range of <0.40 mS.

Fixing soluble salt problems in high tunnels can be relatively easy. Growers can leach the salts from the soil using clear water, remove the plastic cover to allow precipitation to naturally leach the soil, or move the high tunnel to a new location. Since labor is a major issue on vegetable farms, we typically encourage growers to use a prescriptive leach to reduce soluble salt levels in high tunnels before planting next year’s crop.

High tunnel growers should test their soils this winter and pay extra fees to measure the soil soluble salt and organic matter levels in the soil. The soluble salts and organic matter tests are extra tests that are not included in a standard soil test at Penn State. While pH, P, K, Ca, Mg, and CEC levels are important for most field-grown vegetables, soluble salts and organic matter level tests are critically important for high tunnel growers.

Organic matter, as it decomposes, releases additional nitrogen that can be utilized by the crops grown in the high tunnel. By knowing what your soil organic matter level is in your high tunnel, you may be able to reduce the amount of fertilizer that you will need to apply pre-plant. If you know what your soluble salts level is, you can determine first if you have a soluble salts problem and, if so, how much water you will need to apply to the soil to leach the salts from the soil effectively. Based on data from the California Fertilizer Association (1985), growers will need to apply 6 inches of water to reduce soil soluble salt levels by 50%, 12 inches of water to leach about 80% of the salts, and 24 inches to leach about 90% of the salts.

If your soil tests reveal that your soil soluble salt levels are .80 mS and you apply 6 inches of water, you should reduce the soil soluble levels to .40 mS. If your soluble salt levels are 2.02 mS, you will need to leach with 12 inches of water to drop the soil soluble salt levels to the negligible range (<0.40 mS).

The highest soluble salt reading recorded in the high tunnel study was 9.39 mS. To reduce the soluble salts levels in this high tunnel to the negligible range, the grower must apply >30 inches of water. Even when applied over several weeks, this amount of water thoroughly saturated the soils and forced the grower to plant later in the spring than he would like to. If a soluble salt problem exists, leach early so your soils can dry before you need to work the soils or plant the spring crop.