It could be salty out there !

Ajay Nair

Assistant Professor, Department of Horticulture
Iowa State University

High tunnels are simple, plastic-covered, greenhouse-like structures that are passively ventilated and heated and the crops are grown directly in the soil. Over the last 4-5 years, an increasing number of fruit, vegetable, and flower growers have added high tunnels or are considering adding them to their farming operation to extend the season in the spring and fall. High tunnels have become an important tool for Iowa’s specialty crop producers to extend the growing season, increase production of quality crops, and increase profitability.

The environment without rainfall, limited space, and potential climate control in a high tunnel calls for unique set of crop management skills and serious attention towards management of soils. Both chemical and biological properties of soil, associated with continuous crop production inside high tunnels, can substantially change over short course of time. An important issue most growers are reporting is the buildup of excessive salts inside high tunnels. This is due to continuous application of synthetic fertilizers, animal manures, or manure-based compost in high tunnels. Intensive use of water soluble fertilizers of N, P, and K which are salts of respective nutrients significantly contributes to the salinity issue. Composts and manures high in salts can substantially increase soil salinity especially when they are applied in large amounts. The salt is often visible as a white crust on top of the soil (Figure 1). The issue salt buildup is compounded by the lack of precipitation inside high tunnels which prevents leaching of salts.  A build-up of salts over time leads to poor seed germination, causes salt injury including burnt leaf margins, stunted plants, reduced plant vigor, and reduces crop yield. Nutrient uptake and other metabolic processes can also be disturbed by soil salinity.

Fig. 1 Salt crust on soil aggregates inside high tunnels

One way for growers to keep track of salt levels is to regularly test their soils. Soil testing at least once a year will provide data on soil nutrient levels for proper nutrient management and help monitor soil attributes such as pH and salinity. Salt concentration in soil is estimated by testing the electrolytic conductivity (EC) of the soil. Soils with higher salt concentration have increased EC as compared to pure water which does not conduct electricity that well. There is lot of information available on ranges of soil EC that are detrimental to plant growth and cause plant injury; however, information on the method applied to measure EC is equally important to know. Depending upon method used, EC values change and so do the interpretation of those values. Three methods are generally used to measure soil EC.  The saturation paste extract method (SPE) is the most commonly used laboratory procedure for determining conductivity. In this method the soil sample is saturated with distilled water and mixed to a paste consistency. After letting the paste stand for one hour, the electrical conductivity of water extracted from the paste is measured using electrodes. A variant of this method involves measuring conductivity from a 1:1 or 1:2 soil-water mixture. In this case EC is measured after 15-20 minutes of shaking. The latter methods take less time but often are not as well related to the soil solution as is the SPE method. Electrolytic values from the 1:1 extracts are typically lower than those of SPE extracts due to increased dilution. Despite the differences in results between the two methods, many soil salinity samples are analyzed using a 1:1 extract because of reduced monetary and time investments. A conversion factor is used to convert EC values between two methods; however, value of the conversion factor is influenced by soil texture and organic matter content of the soil. A general rule of thumb to convert EC value obtained from 1:1 method to SPE method is to multiply by 2. So, an EC value of 1.5 dS/m from 1:1 method is equal to 3.0 dS/m in the SP method. Most plant responses to salinity levels reported in literature are based on values obtained from the SPE method. Table 1 shows various levels of EC at which plants show salt stress symptoms.

Table 1. Electrolytic Conductivity values from Saturated Paste Extraction method and respective plant responses

Electrolytic conductivity (dS/m)	Plant Response
0 - 2	Optimum plant growth
3 – 4	Plants show initial stress symptoms; smaller leaves
5 – 7	Growth affected; smaller and distorted leaves; reduced yields
>8	Detrimental to plant growth; plant death; only salt tolerant plants can survive

Different plant species have various levels of salt tolerance levels (Figure 2). Plants with higher drought tolerance typically handle increased soil salt concentrations better than more drought susceptible plants. Table 2 below provides information on threshold values beyond which crops would show reduction in yields.

Fig. 2 Salt stress symptoms on pepper crop

Crop	EC (dS/m)
Beans	1.0
Broccoli	2.7
Cantaloupe	2.2
Carrot	1.0
Sweet corn	1.7
Cucumber	2.5
Lettuce	1.3
Onion	1.2
Pepper (bell)	1.3
Potato	1.7
Tomato	2.5
Watermelon	2.0

High tunnel growers should periodically check their soils for salinity issues and take steps to prevent high salt build up. Under high tunnels, investment per square foot is high and any crop damage or injury would significantly reduce profitability of the production enterprise. Below are some steps growers could take to prevent salt build up in their high tunnels

1.      Flush out salts periodically. This can be done by removing the cover, leaving the tunnel open, or flooding the tunnel with water to leach out salts. Flooding could be an expensive option, but leaving the tunnel sides open during the winter months is less expensive and doable. This would allow the snow to get in (certainly in a windy area for which Iowa is a strong contender). Alternatively when changing tunnel cover every 3-4 year, it could be left open for a period of time for the natural precipitation to get in

2.      Provide adequate drainage inside the high tunnels.

3.      Use of cover crops to improve soil organic matter and soil structure, reduce compaction, and improve drainage (installation of drainage tile pipes)

4.   Judicious use of water soluble fertilizers and compost.