by Sanne Kure-Jensen
“A high tunnel is an irrigated desert,” opened Bruce Hoskins of the University of Maine. He continued by describing a common problem faced by high tunnel growers of salt and nutrient build-up in upper soil layers. Hoskins presented several ways to minimize build-ups and to mitigate problem sites in his workshop “High Tunnel Soil Management Updates” at the 2017 New England Vegetable & Fruit Conference.
Typical high tunnel growers only irrigate crop root zones. Plants transpire and soil surfaces evaporate moisture into the air. Nutrients and salts accumulate in significant and potentially toxic levels in the top one to two inches of soil. Soil electrical conductivity (EC or soil salinity) typically decreases with soil depth. EC affects crop yields, plant nutrient availability and soil microorganism activity. Excess salts impede plant growth by affecting their soil-water balance.
Hoskins recommended pushing salts down by watering deeply and/or remixing soils between crops. Growers should leave high tunnel frames open to allow rain to penetrate soils several weeks whenever replacing plastic covers. Ensure 4” to 8” rain or irrigation water has a chance to penetrate high tunnel soils before installing new high tunnel covers.
Another approach is to dilute nutrient or salt build-ups. Hoskins suggested growers supplement high tunnel soils with up to one-third (by volume) peat or field soils. He also recommended thorough mixing of high tunnels soils at least once a year. Beware adding too much compost as this can raise phosphorus (P) levels.
An important factor affecting soil pH in high tunnels is the alkalinity or dissolved lime content of irrigation water. Hoskins advised growers to test their water source — and not just for water quality compliance with Food Safety Modernization Act (FSMA).
Hard water can contain fast acting lime. Hoskins gave an example of alkalinity (lime equivalence) at 275 parts per million (ppm). This is equivalent to adding a quarter pound of lime per 100 gallons of irrigation water or approximately 75 pounds of lime per 1000 square feet over a full season. This rate can raise high tunnel soils pH 0.5 to 0.7 each year. High alkalinity irrigation water can also be a big problem for bench production.
According to Hoskins, surface water sources like ponds, streams or shallow/dug wells are superior to well water and usually low in alkalinity. Growers irrigating with well water may need to acidify their soils with yellow sulfur (15 pounds/1000 square feet per 0.5 drop in pH needed). Alternately, growers could inject an acidifier like sulfuric or nitric acid into their irrigation water. Certified organic growers may use citric acid.
“Tomatoes are luxury consumers of potassium,” said Hoskins. Many high tunnels are low in potassium (K), especially after a tomato crop. Even when growers add potassium before or during the growing season, tomato plants will use all or nearly all of the available potassium. Typical symptoms of low potassium soils include bronzing of lower leaves and yellow or green shoulder on tomatoes.
To maximize yields, fruit quality and plant health, Hoskins recommended supplementing soils with a fine granulation of potassium sulfate or K-Mag (langbenite) bringing levels to 300-450 ppm (equivalent to 600-900 lbs./acre). Hoskins said organic growers have no restriction on the amount of added potassium, only the source of the potassium.
Achieve ideal plant health and yields by meeting optimal potassium (K) and nitrogen (N) requirements and nutrient balances.
- Corn needs 70 percent as much K as N
- Apples need 70 percent as much K as N
- Squash need 80 percent as much K as N
- Peppers need equal parts K as N
- Potatoes need 110 percent as much K as N
- Tomatoes need 110-150 percent as much K as N
Dry zones in high tunnel beds are a common problem. Hoskins recommended growers use fine granulation amendments and use three to four drip lines per bed to minimize dry areas, improve nutrient release, and reduce salt buildups. Alternatively, potassium amendments can be completely dissolved in water before application to ensure consistent application, plant availability and prevent areas of excess.
To calculate amendments needed based on soil test recommendations, add up all bed areas. Apply amendments only to the bed areas; skip the paths.
When collecting samples for soil testing, growers should be sure to sample a full 6” of soil and send a blended sample for testing. If the ground is frozen, wait until soils have thawed enough to take a full depth sample. Shallow samples will give false results often showing nutrient deficiencies and excess salts.
Unlike rain falling evenly on field soils, indoor soils are rarely deeply flushed. Hoskins recommended growers thoroughly saturate high tunnel soils at least once a month. Keeping soils moist will help make all nutrients available to soil life and plant roots.
Send questions to Bruce Hoskins, Assistant Scientist at the University of Maine at firstname.lastname@example.org or call 207-581-2945.