by Harold Schrock, Staff Consultant at Advancing Eco Agriculture
Growing demand for local food and more frequent weather extremes have made greenhouses and high tunnels a larger part of the North American agricultural landscape. In our experience, the most profitable growers of high value crops do some, if not all, of their production indoors.
Indoor production has higher costs and may not be suited for all management types. Installation and site preparation costs vary from about 25 cents a square foot for a basic high tunnel to well over $2 a square foot for a versatile heated greenhouse. To make this overhead pay off, a grower must be able to raise more marketable product inside than outside. Inside crops must be of reasonably high value to make this work. And the grower must minimize mistakes or neglect that can cause significant loss.
Water quality and accumulation of elements
Indoor production requires an adequate source of high quality water. Without natural rainfall to leach them away, salts and carbonates found in well and spring water can accumulate. When evaluating indoor production, an irrigation water quality test is essential. If the primary water source proves problematic, consider if other alternatives are available. In areas of moderate or high rainfall, surface water from streams and ponds often is better for irrigation than water from wells. From a food safety aspect, surface water may be suitable only for underground irrigation. If no alternatives are available the substandard water can often be buffered with an acid. In extreme cases, a reverse osmosis system may be the most viable option for quality water.
Indoor growers must be careful with buffering acids. Normally phosphoric or citric acid is preferred. Don’t use too much sulfur in any form. Under cover, sulfur tends to build up, and can reach levels that make it hard to maintain adequate nitrogen and potassium in crops. Organic growers need to be particularly cautious about sulfur accumulation. Since most organic potassium sources (including HoloK™) contain some sulfur, organic growers should normally avoid indoor applications of straight sulfur.
Sodium and boron are other elements that may accumulate under cover. Fertilizer recommendations for outdoor production often contain too much boron for greenhouse use. Be especially cautious of boron-containing material when fertilizing bedding plants or in any non-soil situation. Some AEA formulas designed to maximize plant health in the field should not be used full strength indoors. Ask your consultant for an indoor program.
Site, drainage and soil considerations
When preparing a site for a high tunnel, and especially for a heated greenhouse, be proactive and eliminate potential problems before they occur. A well-drained site is essential. With most soil types and site conditions, you will need to install a tile or open ditch around the perimeter. While some ground moisture may save on irrigation needs, too wet can be devastating. Good drainage also helps prevent frost heave damage to greenhouse perimeters.
Of equal or greater importance than water management is soil condition. Since covered crops are the most productive, site them on the best soil available. If site excavation is necessary, try to make sure at least 12 inches of good topsoil is returned over the excavation zone. While it’s possible to use systems of limited soil volume, such as bags or narrow beds, in our experience the best production and profits come from growing in soil. Here’s why: In biological systems, soil is the plants’ digestive system. The larger the digestive system, the bigger the production potential.
Indoor blackberry production in California
Managing soil as a digestive system eliminates many problems that plague some industrial shortcut growing systems. When soil is used only to anchor the roots of plants that are being fed soluble nutrition, diseases are an inevitable result. Managing the soil with beneficial biology and biologically enhancing nutrition results in a disease suppressive environment. Depending on the quality of irrigation water and the soil type and condition when the structure is erected, it may be necessary to remove the plastic for a season to flush out accumulated salts. It will seldom be necessary to relocate structures to move away from a disease problem in the soil.
Managing soil as a biological system under cover shares many important aspects with outdoor management. Diverse cover crops and keeping soil under growing plants as much as possible is important in any soil-based growing environment. Covered greenhouses need to be irrigated in the off-season. For best results, never let the soil dry out completely. In addition, a living soil requires good gas exchange at the surface and should not be smothered, so avoid covering the soil with a large sheet of plastic. A much better option for heat-loving crops is using raised beds covered with 3- to 4-foot wide plastic. Hay or straw can be a good option for walkways and even around plants after heat accumulation is no longer a necessity. But it's not a good idea to set young plants out in a straw mulch at the beginning of an early growing season. Bare soil or narrow plastic mulch will warm much faster.
Some intensively managed greenhouse operations we have worked with removed the mulch plastic midseason, after the lower leaves are stripped away from the stems of tomatoes and cucumbers. By establishing tillage radishes and other cover crop species midsummer, these growers can get a viable cover crop as well as two cash crops per year: greens production in late winter, followed by tomatoes, peppers or cucumbers.
Test and adjust
Indoor production is a place where good monitoring pays off. It does not take a very large greenhouse operation to economically justify regular sap analysis. Without knowing where you are and what direction you’re traveling, it's very easy to miss the last 30 to 40 percent of the season’s production.
Monitoring needs to be paired with corrective action. Profitable greenhouse operators adjust their programs regularly based on in-season testing. Typically indoor growers fertigate 1 to 3 times a week and foliar feed every 7 to 10 days. When a nutrient is falling off, it’s usually better to increase the number of applications rather than the intensity of the dose. In a healthy system, ensuring adequate mineral nutrition is always a profitable choice.
AEA consultants consistently observe that the most profitable growers are not the lowest cost. Managing for higher production is the profitable choice as long as it does not compromise the biological health of the system. This is especially the case in greenhouse and high tunnel production. In systems with high overhead, it doesn’t make economic sense to skimp on nutrient monitoring and nutrition.