health and yields
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Judson Reid, Cornell Cooperative Extension
High tunnels are lower technology greenhouses that use the soil for crop production. High tunnels are an effective tool for farmers that desire to extend their growing season with the technology benefits of a greenhouse while using mineral soil to provide crop nutrition and
health. Being soil based, high tunnels can be used for certified organic production, whereas hydroponic greenhouses cannot. In the case of C. sativa, the extended growing season, and decreased field stress may lead to higher yields of floral biomass as well as higher concentrations of cannabinoids.
However, the unique growing conditions in high tunnels present challenges in soil management; excess tillage, fertility, and continuous cropping without rotation or precipitation results in a number of soil health and fertility issues that require intervention for long term viability.
Distinct from hydroponic greenhouses, the soil is the most important material resource in a high tunnel. To properly manage the soil take we must test the soil including:
- A complete nutrient analysis; macro, secondary, and micronutrients
- pH
- Organic matter
- Soluble salts or electrical conductivity (EC)
In New York State, soil management in high tunnels requires attention to limit calcium, phosphorus, and magnesium inputs from either conventional and organic nutrient sources. In our experience these nutrients reach excessive levels and then limit the uptake of other key nutrients such as potassium and manganese. Excess nutrient applications occur when general use fertilizers such as 20-20-20 are applied in pursuit of adequate nitrogen without attention to the levels of phosphorus and potassium.
In NYS high tunnels, nutrient management will focus on nitrogen and potassium, as they are extracted from the soils in higher levels than other elements; while maintaining pH levels low enough to permit the availability of secondary and micronutrients. Exact amounts of any crop nutrient will depend on soil test results. For conventional fertility, the entirety of crop nutrient budgets can be applied via drip irrigation. For organic high tunnels, bulk amendments such as alfalfa meal and composts are applied pretransplant and incorporated, often with a target of at least 50% of the total nitrogen budget. Soluble certified organic nutrients are available for fertigation to achieve the balance of the nutrient requirements. Nutrient injection can occur daily, or 3X/week, alternated with clear water.
Irrigation Water Quality
Irrigation water quality is of heightened importance in high tunnels when compared to both open field and hydroponic production. The soil-based nature of high tunnels and the absence of precipitation (compared to open field), allows any problematic aspects of irrigation water to accumulate in the root zone. Whereas in hydroponics a reset of root zone nutrients is possible through flushing the growing media, soils retain and buffer the presence of cations and salts in high
tunnels. These interfere with nutrient uptake and crop root health. Therefore, for appropriate high tunnel irrigation requires water analysis including:
- pH
- bicarbonates
- soluble salts
- other nutrients
The elevated levels of pH and calcium bicarbonate in most NYS irrigation wells indicates the injection of acids to high tunnel irrigation systems. This increases nutrient availability in the root zone in the short term and prevents pH elevation over the long term. Although sulfuric acid is the most common conventional acid, citric acid is the choice for organic growers. The introduction of acids (and other nutrients) into the irrigation system is possible with a proportional injector. The
amount injected will depend on the pH and calcium bicarbonate of the untreated irrigation water.
Since Cannabis grows best in a pH of 6-6.5, using 6.2 (or a slightly lower) target pH for the irrigation water can estimate acid injection. There are online tools to help with the calculation of acid injection, such as the ALC CALC app from University of New Hampshire.
We urge growers to reach out to the Harvest NY Emerging Crops educator, Daniela Vergara or High Tunnel Specialist Judson Reid with questions on high
tunnels soils, prior to beginning a nutrient or acidification program:
- Judson Reid jer11@cornell.edu
- Daniela Vergara dv255@cornell.edu
