ORGANIC GREENHOUSE VEGETABLE PRODUCTION

Introduction 

Although several Extension bulletins are available on greenhouse vegetable production, few of these concentrate on organic production methods. This publication presents an overview of greenhouse production systems and profiles several farmers raising organic vegetables in greenhouses. It is hoped this will give new growers ideas of how to set up their systems, and provide more experienced farmers with examples of alternative methods of production.

 The term greenhouse means different things to different people. A greenhouse used to be a the structure formed of glass, with a heating (and usually cooling) system that was used year-round, but especially in winter. Then came houses built of thermoplastic (Plexiglas and others), followed by Quonsets covered with plastic, which may or may not be heated, have one or two layers, and be used year-round or for only a few months every year. The type of greenhouse you have will largely be determined by your crop and your capital and, to a lesser extent, by your management intensity and your market strategy. 

For the purposes of this publication, a greenhouse can be any of the above. Another ATTRA publication, Season Extension Techniques for Market Gardeners, contains further information on protected shelter structures such as cold frames, high tunnels or “hoop houses,” and low tunnels.

The Greenhouse Vegetable Industry

 The U.S. greenhouse vegetable industry is a mixture of small, family-run operations in the 2,500 to 10,000 square foot range and a small number of large, multi-acre facilities 10 acres or more in size. The larger greenhouses often use waste heat from a power plant or other source of cogeneration (1). 

Current U.S. production estimates are somewhere around 800 acres (2). In comparison, Mexico has about 450 acres, Canada has about 1,600 acres, and Holland has over 11,000 acres (3, 4). In the latter part of the 1990s, Canadian greenhouse vegetable production grew at a rate of 20% a year. How has Canada been able to generate this huge growth? “Significant new greenhouse vegetable production technology that was transferred to commercial producers have been primarily responsible for dramatic yield increases over the last 7–8 years, estimated at 100–120% for tomatoes and 70–80% for cucumbers” (3). Canada is strongly supporting its greenhouse growers, both with research and with investment dollars. Their research facilities at Harrow is recognized as topnotch the world over. Although most Canadian greenhouse vegetables are not produced organically, there has been an emphasis of late to use IPM strategies, rather than pesticides, to accommodate the growing market of consumers who want pesticide-free produce. Most of the organic produce imported into the U.S. is now coming from Mexico.

 Tomatoes are the leading greenhouse vegetable crop, followed by European cucumbers, lettuce, peppers, and culinary herbs such as basil, sage, and rosemary. See the ATTRA publications on these specific vegetables for more information. In addition, growers aiming at niche markets raise specialty crops, greens, and Oriental vegetables.

How Can Small Producers Compete?

With so much competition from Canada, Mexico, and overseas, how can small farmers realize a profit raising greenhouse vegetables? One issue of increasing importance to consumers is vegetables grown with minimum pesticides. The public has also become educated on the values of locally grown produce: it’s fresher, it tastes better, and it may even be less expensive since there are fewer shipping costs involved. Also, money paid to a local farmer is re-invested in the local community and helps to keep that economy strong. 

Year-round production is key to maintaining the greenhouse’s profitability. However, this does not necessarily mean that growers should be producing the same crop year-round. (Winter tomatoes bring more money than do summer ones.) Another option would be to raise a crop other than vegetables, like bedding plants for early spring sales or poinsettias for Christmas. The grower may decide that the most cost-efficient way to use his or her greenhouse during the summer is to shut it up to solarize the soil and “cook” insects (and their eggs) that are present.

 Small growers must find niche markets. It is pointless to try to compete with mass merchandisers like Wal-Mart because the small grower will always lose. What are some niche markets for organic greenhouse vegetable producers? Some of the general niches have already been mentioned: consumers are looking for organic, locally grown, early-season produce. Whatever the niche market, it is important for growers to realize that the nature of niche markets is for them to disappear after a while. Oversupply or lowered demand will create lower prices. The market will change to favor one product and disfavor another. This may happen when mass merchandisers enter the market, when the popular press promotes a particular vegetable, or when new medical evidence points to increased or decreased health benefits from certain vegetables.

Solar Greenhouses 

Greenhouses can be designed to take advantage of solar radiation and cut fuel expenses. Solar greenhouses are popular with small-scale growers. These are super-insulated greenhouses designed to collect and retain solar energy. The technology associated with solar greenhouses is rather detailed. In addition, the literature on solar greenhouses is quite large. To help growers identify some of the best resources on this topic, ATTRA compiled the Solar Greenhouses Resource List. One recent publication that features organic vegetable production in a solar greenhouse is Anna Edey’s book Solviva: How to Grow $500,000 on One Acre and Peace on Earth. Solviva is Edey’s award-winning solar-powered and animal heated greenhouse on Martha’s Vineyard. The book discusses greenhouse design, function, construction, and management. Ms. Edey includes numerous energy-efficient designs like water walls and grows tubes. She also tells how much everything costs, which is invaluable for market gardeners.

Composting Greenhouses 

Heating greenhouses with waste heat generated by compost is a second option that takes advantage of local resources and integrates different farm activities. In a composting greenhouse, heat and carbon dioxide are generated from manure-based compost contained in a special chamber attached to one side of the greenhouse.

 Compost-heated greenhouses gained a lot of attention from work undertaken at The New Alchemy Institute at Falmouth, Massachusetts. The New Alchemy Institute was one of the premier appropriate technology centers that operated in the 1970s and 80s. The Institute published widely on ecology, wind energy, solar energy, shelters, solar greenhouses, integrated pest management in greenhouses, organic farming, and sustainable agriculture. 

Though the technology to implement compost-heated greenhouses exist, they are seldom done on a commercial scale. ATTRA can provide more information on this topic on request.

Animal-Heated Greenhouses

Small animals like chickens and rabbits produce heat and carbon dioxide in addition to products like eggs and meat. A few growers have taken advantage of this fact and integrate animals with greenhouses as a source of heat. However, it can be a challenge to keep livestock in a greenhouse—the higher temperature and the humidity of a greenhouse are generally not healthy for animals. 

Anna Edey, mentioned above, uses an “earthling” to filter out the toxic ammonia gas from the rabbit and chicken manure she uses in her Solviva greenhouse. In addition, she keeps her chickens in a poultry room attached to the greenhouse where temperatures do not fluctuate from about 70°F.

Organic Greenhouse Production 

As defined by the USDA in 1980 (6), organic farming is a system that excludes the use of synthetic fertilizers, pesticides, and growth regulators. Organic farmers rely heavily on crop rotations, crop residues, animal manures, legumes, green manures, organic wastes, and mineral-bearing rocks to feed the soil and supply plant nutrients. Insects, weeds, and other pests are managed by mechanical cultivation, and cultural, biological and rational controls. 

Organic certification emerged as a marketing tool during the 1970s and 80s to ensure foods produced organically met specified standards of production. The Organic Foods Production Act, a section of the 1990 Farm Bill, enabled the USDA to develop a national program of universal standards, certification accreditation, and food labeling. Early in 1998, the USDA

Fertility

 Although the process is more complicated, it is possible to obtain adequate nutrients from organic sources, but it takes more care and creative management. No single fertilizer will provide all of the essential elements required, but a combination of organic products can be devised.

 Organic fertilizers have not been well researched in greenhouse vegetable production. However, a 1999 study performed at The University of Kentucky analyzed several products for the levels of nutrients they supplied. The researchers were attempting to prove that organic fertilizers could supply nutrients at the same level as synthetic fertilizers. Products derived from algae (Again, a liquid, and Maxicrop, a powder), bat guano, and fish waste (Greenall Fish Emulsion, a liquid, and Mermaid’s Fish Powder) demonstrated nutrient levels comparable to conventional, synthetic fertilizers used for greenhouse plant production (7). The report concluded that these organic fertilizers could not be used as a concentrate for injector systems, but they would be suitable in a capillary mat sub-irrigation system. For information on how to obtain these products, see the Resources section.

FERTILIZER AND THEIR USE

INTRODUCTION

ORGANIC FERTILIZER OVERVIEW

All plants need certain mineral nutrients to survive. These minerals occur naturally in the soil and are taken up from the soil by the roots of the plants. Most soils usually have enough of these minerals to keep plants healthy. However, some nutrients are gradually used up by the plants, or are washed out of the soil, and need to be replaced to maintain optimal growth and appearance. The most common mineral nutrients that need replacing are Nitrogen (N), Phosphorus (P) and Potassium (K).

 Fertilizers are manufactured mixtures of chemical products that contain N, P, K, and other necessary nutrients. They are spread over the soil to re-supply the soil with the proper amount of these nutrients. The three numbers on the front of the fertilizer bag represent the percentage by weight of N, P and K in that particular mixture. These numbers are used to calculate how much of a particular fertilizer to apply at one time.

EXAMPLE -

A typical lawn fertilizer may have the numbers 25-5-10. This means that a 100-pound bag of this fertilizer contains 25% (100 x 25%) total nitrogen, or 25 pounds of N, 5% (5 pounds) phosphorus compounds and 10% (10 pounds) potassium compounds. The three numbers are the “analysis” and are used to figure out how much fertilizer to apply. 

Nitrogen is the most important mineral nutrient for healthy plant growth and the one that plants use the most. The total nitrogen in fertilizers can be supplied as nitrate compounds, ammonium compounds, or urea. Usually, a combination of ammonium and urea are used. Nitrogen keeps the plants green and promotes lush leaf and shoot growth. It is important to keep enough nitrogen in the soil, but do not add too much. Thatch is the layer of brown stems and runners that build up between the soil surface and the green leafy part of the grass. As thatch gets thicker it will give a spongy feel to the lawn and as it continues to become thicker it will eventually cause the mower to sink into the grass and cut lower into the brown stemmy underlayer. This is called “scalping” and causes unsightly brown spots on the grass. Scalping is a problem in highly maintained turf grasses where it is much more visible and unsightly, and where it causes temporary injury to the lawn surface. 

What kind of fertilizer will I use?

HDOT will eventually replace all slow-release fertilizers with the better and advanced technology ultra slow-release formulations; however, until then, you will be using mostly the traditional slow-release N formulations. 

The ultra slow-release fertilizers are specially formulated to slowly release nutrients over a period of one year. Unlike the current slow-release fertilizers, these ultra slow-release formulations can be spread once a year in much higher amounts without danger of burning the plants.

 They also do not require that the plants be watered immediately after application, so irrigation systems may not be needed in areas that are fertilized. The Engineer will supply you with this new ultra slow-release fertilizers formulated for HDOT for use on turfgrasses, trees, shrubs and palms.

Where do I apply the fertilizer?

• Fertilizing areas you are contracted to maintain will depend on the expected quality of maintenance for that area. The Engineer will determine this and include it in the specifications of the bidding process.

•  Most rural areas will not need fertilization. The only rural areas needing fertilization are interchanges with turfgrass

• . Bare soil or sparsely covered slopes subject to erosion should not be fertilized.

•   Areas most likely to need regular fertilization are high visibility locations with good turfgrass cover and trees and shrubs requiring extra care. Most are combinations of mixed grasses, some broadleaf ground cover, such as wedelia, and various trees and shrubs. Fertilizer application may be required once a year.

•   A few locations in or near main city highways have a higher level of maintenance expectations. Most of these areas are medians with St. Augustine grass, naupaka hedges and monkeypod trees or shower trees.

•   A few locations may have other plantings, such as ‘El Toro’ zoysia lawn grass or other species of trees and shrubs that require more specialized care. These high visibility locations must be fertilized once a year, using the special ultra slow-release fertilizers. It is desirable, but not mandatory, that areas getting fertilization have a working irrigation system. 

How do I get the fertilizer for the job?

• The State will supply you with the fertilizer for the job. You must get approval in writing from the HDOT before using fertilizers. 

• This approval should be a memo including the time, location, amount of fertilizer issued and the equipment to be used to apply the fertilizer. This memo should be submitted at least two weeks before applying the fertilizer. The Engineer will tell you where to pick up the fertilizer and the spreader.

•  The type of fertilizer you get will depend on the kind of plants and the location of your maintenance area. In lawn areas, you will be given the ultra slow-release turf type fertilizer higher in nitrogen; for trees and shrubs a more balanced ultra slow-release formulation containing equal amounts of N, P and K, and a special ultra-slow formulation for palms, which is higher in micronutrients and magnesium.

•  Within two weeks after the fertilizing, you have to submit a record of the area you fertilized, the date applied and the amount used. This is covered in more detail in  “Reporting.” 

 How do I determine how much fertilizer to use?

The amount of fertilizer needed for grass, trees, shrubs and palms is based on the amount of nitrogen (N) needed to maintain normal, healthy growth without burning the plants. This amount of fertilizer, given in pounds of nitrogen per 1,000 sq. ft., is called the recommended rate which depends upon the percentage of total nitrogen and also the type of nitrogen formulation. To determine the amount of fertilizer you need for a job you, will need to know the recommended rate. This is usually listed on the fertilizer label.

 Grass

      Typical recommended rates:

• Soluble quick-release fertilizers, such as ammonium sulfate, are one pound N per 1,000 sq. ft.
• Slow-release N products, such as sulfur-coated urea, are 1½ pounds N per 1,000 sq. ft.
• The new ultra slow-release fertilizers supplied by HDOT can be as high as 4 to 5 pounds of N per 1,000 sq. ft. without danger of burning the grass.

Trees and shrubs

      Typical recommended rates:

• Quick-release fertilizers are no more than 2 pounds N per 1,000 sq. ft. of the root zone.
• Slow-release formulations are up to 3 to 4 pounds N per 1,000 sq. ft. of root zone area.
• Ultra slow-release fertilizers can go as high as 7 to 8 pounds of N per 1,000 sq. ft. of the root zone. The ultra slow fertilizers will allow for one application a year. This will slowly release nutrients over the entire 12 months for all plants.

Soil fertility and its improvement

Fertile and productive soils are vital components of stable societies because they ensure the growth of plants needed for food, fiber, animal feed and forage, industrial products, energy and for an aesthetically pleasing environment. 

Soil fertility integrates the basic principles of soil biology, soil chemistry, and soil physics to develop the practices needed to manage nutrients in a profitable and environmentally sound manner. Soils differ widely in their ability to meet the nutrient requirements of plants; most have only moderate natural soil fertility. To achieve production objectives, more nutrients are usually required than can be supplied by the soil. 

High crop yields mean greater depletion of soil nutrient supplies, which eventually must be balanced by increased nutrient input to maintain the fertile soils needed by our societies. Thus a hallmark of high-intensity agriculture is its dependence on mineral fertilizers to restore soil fertility, and in the broader context of soil productivity, soil fertility regulates the supply of nutrients inherently available in soils or applied as manures and fertilizers to plants. 

Soils with high natural fertility can produce substantial crop yields even without added fertilizer but can produce even higher yields with an additional supply of the critical nutrients. Good soil fertility provides the basis for successful farming and should not be neglected. 

There are a number of ways of making use of soil fertility in farming: 

• nutrient mining–farming without any added fertilizer (e.g., in shifting cultivation); 

• utilization of as many components of soil fertility as possible without compensation and yet without negative yield effects (e.g., by applying only moderate amounts of fertilizer N and P); 

• maintenance and improvement of soil fertility to assure consistent high yields (e.g., by compensating for losses due to removal and by soil amendments to improve fertility). The large differences in fertility between different soil types and sub-types must be taken into account. Some soil characteristics important to nutrient management may be grouped geographically and general recommendations may be summarized as follows:

 Soils of the humid tropics

• partly very acid (liming is required, generally to pH 5.5 or above);

 • often low in available P or liable to P-fixation (use of fertilizer P is therefore often essential, combined if necessary with liming);

 • in very humid areas, often low in available K, Mg and S (therefore there are high fertilizer requirements for these nutrients); 

• often low sorption or storage capacity for nutrients (so fertilizer application should be split between several dressings); 

• often low in available N, although the decomposable organic matter is rapidly mineralized.

 Soils of the sub-tropics 

• water shortage (without irrigation, fertilizer use must be suitably adapted to efficient water use);

 • N is often the main critical nutrient, due to the low humus content; 

• widespread P deficiency, especially in sandy soils;

 • neutral soil reaction (therefore often a shortage of available Fe and Zn); 

• a generally good supply of S, Mn, and B;

 • risk of salinity due to lack of leaching of salts from the root zone. 

Soils of humid temperate zones 

• widespread soil acidity which requires liming; 

• partly obstacles to root growth (e.g., hard layers in subsoil); 

 • often insufficient aeration (poor natural drainage of heavy soils);

 • generally a shortage of available N and often of P, K, Mg

• low nutrient reserves in sandy soils, also only a little storage and therefore considerable leaching with water surplus;

• partial fixation of P and Mo (due to natural soil acidity) and Cu (in organic soils); • climatic cold stress retarding nutrient uptake