High tunnel tomatoes. Photo: Tom Ford, Penn State
At this juncture in the study, Dr. Elsa Sanchez, Professor of Horticultural Systems Management at The Pennsylvania State University, and Thomas Ford, Penn State Extension Horticulture Educator, have reviewed surveys, soil test results, and water test results for over 30 growers statewide. While the analysis and work are far from over, a variety of recurring themes appear to be the predominant reasons for the observed cropping issues in high tunnels.
When first reviewing the surveys, subsequent conversations with the study participants found that only a small minority of the growers had their irrigation water tested. If they had undertaken any water testing, they were focusing on generic E. coli and food safety but had not specifically looked at their water chemistry. Water chemistry and specifically the overall quality of the water can impact plant nutrition, soil permeability, soil chemistry, and crop yield. While many growers noted cropping and yield issues, few took the time or resources to evaluate their farm water supply to see if it could be a source of, or a contributing factor in, their high tunnel troubles.
Early stages of potassium deficiency in cucumbers. Photo: Tom Ford, Penn State
The most common observed water quality issue in this study was high alkalinity. The typical grower applies 1-2 inches of water per week in their high tunnel during the growing season. Growers utilizing very alkaline irrigation water are essentially administering a dose of CaCO3 or lime to their soil in every irrigation cycle. If the grower is not injecting acid into their irrigation water to reduce their water alkalinity and pH, the pH of the soils in the high tunnel will increase over time. As the soil pH moves beyond 6.8 - 7.0 the grower may begin to observe the earliest stages of micronutrient deficiency (specifically Zinc, Iron, and Manganese). Growers who have tested their water and have determined that they have a water alkalinity issue can input their data in the University of New Hampshire website ALKCALC to determine the appropriate amount of acid to inject in their irrigation water to reach their target alkalinity level (usually 50-80 ppm).
Iron deficiency in watercress. Photo: Tom Ford, Penn State
Additional water quality issues observed on some participants' farms were elevated sodium and chloride levels. Many water testing laboratories do not routinely test for sodium or chloride unless specifically requested by the grower. While accumulated sodium and chloride in the soil can be toxic to some crops, increased soil sodium accumulation in high tunnel tomatoes can result in a scenario where sodium cations compete with potassium cations for uptake sites on roots, resulting in potassium deficiency and low fruit numbers. Typically, chloride is associated with elevated sodium levels and competes with the uptake of nitrate-nitrogen, resulting in lower yields.
Iron deficiency in tomatoes. Photo: Tom Ford, Penn State
Elevated levels of sodium and chloride in irrigation water cannot be removed efficiently or economically. Reverse osmosis is frequently employed if the grower has a minimal operation that uses a limited amount of water. On sizeable multi-acre vegetable operations, it is often more cost effective to either dilute the irrigation water with rainwater that has been caught in a cistern or to locate another water source. In some cases in Pennsylvania growers have had no other recourse, but to connect to a public water supply to preserve their livelihood. For additional information on water quality issues and high tunnel production, please contact your local commercial horticulture educator.