ORGANIC VEGETABLE PRODUCTION: MANAGING NUTRIENTS AND PESTS
Vern Grubinger
Vegetable and Berry Specialist
University of Vermont Extension

Discussions of organic farming often focus on which practices are allowed or prohibited by organic certification standards. However, the fundamental goal of organic farming is not the perfection of a set of rules, but the implementation of agricultural systems that are ecologically stable and reasonably productive without toxic interventions. This requires working with, rather than against, natural processes, even though they may not be fully understood.

Simply substituting organically-allowed fertilizers and pesticides for conventional materials does not create a more ecologically stable farming system. The ‘substitution approach' usually raises production costs and reduces marketable yield since organic inputs often cost more and don't work as quickly as their synthetic counterparts. To succeed with organic farming you've got to ‘buy into' a farming system that values long term yield stability and pest avoidance over maximum short term production. The attributes of a such a farming system include: plant diversity, healthy soil, reliance on cultural practices for pest control, and minimal use of least-toxic external inputs. Essential organic vegetable production practices include: crop rotation, use of green manures and compost, pest prevention, and mechanical weed control.

Crop rotation is at the core of ecological stability on vegetable farms. Yet most vegetable growers, including many organic growers, do not practice rigorous crop rotation. Instead, they tend to follow a minimal rotation with the goal of moving plant families around the farm so that a field rests for a few years between related crops. In most cases this is done using a flexible ‘seat of the pants' plan rather than a systematic one, so the rules get bent as needed to accommodate production constraints like weather, labor and equipment. In addition, rotation is mostly among the cash crops - little land is ever taken out of crop production.

Maximizing the rotation effect - which benefits soil fertility as well as insect, disease and weed control - is essential to successful organic vegetable production over the long-term. Cover crops and cash crops must be intentionally integrated into the rotation, ideally in equal proportions. At a minimum, I'd suggest that a quarter of a farm's tillable land should be ‘resting' from vegetable production at any given time if an organic system is to succeed over the long-term. Putting together a clear and workable rotation plan that suits the farm's market, climate, and available equipment and labor is essential to ‘going organic'. Obviously, a rotation plan must have flexibility in terms of which cash crops and cover crops get planted in a given year, and sequences will be refined and improved over time, but the basic rotation should never ignored.

Soil fertility management on organic farms is sometimes defined by the expression ‘feed the soil, not the plant'. The implied goal is to build up reserves of nutrients in the soil and to establish a system of nutrient cycling. That is done in part by adding organic residues to the soil on an ongoing basis so that their decomposition releases nutrients sufficient to feed the crops while enhancing soil structure at the same time.

Rather than fertilizing crops with bagged nutrients and adding residues to the soil as an afterthought, the organic approach is to supply as much of the crop's needs as possible with residues, and make up the shortfall, if any, with purchased fertilizers. In some cases, organic crops will need significant amounts of bagged fertilizer in order to optimize yield and quality. For example, on light textured soils with low organic matter levels, during the transition to an organic system, or when organic residues are not available in sufficient quantities to meet crop nutrient needs. Over time, the addition of fertilizer inputs on organic farms tends to decrease if a rigorous soil-building program is adhered to.

Organic residues are those that contain carbon: cover crops or green manures, animal manures, crop residues, and off-farm residues such as yard waste, aquatic weeds, food by-products, etc. Compost is produced from a mixture of these ingredients. Ideally, an organic vegetable farm utilizes a variety of cover crops as well as a variety of other residues that have been composted to stabilize nutrients and minimize weed seeds and pathogens. Which residues are utilized depends of the cost of acquisition and handling, and the nutrient needs of the crops. Organic farms often have considerable expenses associated managing organic residues.

As with conventional management, managing the mineral nutrition of crops is critical to successful organic management. Organic farms should soil test regularly, lime to the proper pH, and broadcast phosphorus, potassium and magnesium as required. A variety of cost-effective materials are generally allowed under organic standards. Limestone is a source of calcium and magnesium, gypsum can be used to supply calcium, sul-po-mag supplies potassium and magnesium, mined potassium sulfate supplies potassium. Correcting a severe phosphorus deficiency can be costly since rock phosphate is the material most likely to be applied in large quantities, although this is often a one-time expense.

When soil organic matter and/or mineralization is low, it may be necessary to sidedress organic nitrogen fertilizer. This can also be expensive, since approved materials such as dried blood, alfalfa meal, or pelletized chicken manure cost more than synthetic sources. However, properly timed applications of small quantities are often sufficient to supplement the release of nitrogen from soil organic matter and added residues. Trace elements such as copper, iron and zinc are generally permitted to be applied as chelates if needed, and boron can be applied as borax or solubor.

Weed management is one of the most challenging aspects of organic vegetable production, even with rigorous crop rotation. If dairy manures is used to provide fertility, weed pressure can be particularly intense. In addition to cultural practices that limit weed pressure, such as composting of manures, use of black plastic mulch, and cover cropping with smother crops, organic farmers must be equipped to mechanically cultivate weeds. Failure to cultivate effectively results in reduced crop yield, increased hand labor costs, or both. Just as a single herbicide or two cannot provide effective weed control, one or two cultivation implements will rarely provide satisfactory weed control. Experienced organic  farmers rely on a variety of cultivation tools to cope with various weed and crop combinations.
 

Mechanical weed control has several stages. Pre-plant weed control, following primary tillage, is often accomplished with disk harrows, field cultivators or flex-tine weeders. Post-planting but pre-emergence cultivation is conducted with flex-tine weeders or rotary hoes. With small-seeded crops, hand-held or tractor-mounted flame weeders are often used to create a ‘stale seed bed'. Post-emergence cultivation tools address one of three zones: in-row, between-row, or tire-tracks. The growth habit of the crops and weeds in a field influence how aggressive a tool can be used, in other words, how much soil can, or must, be moved to bury weeds in the row. Post-emergence cultivation is facilitated by use of a cultivating or a high clearance tractor. Appropriate selection and placement of shanks with attached tools is necessary to achieve precision cultivation.

In-row cultivation tools for removing small weeds, but not the crop, include: finger weeders that work around crop stems, side knives or beet hoes that work very close to the crop, wiggle hoes that are ‘steered' around plants in the row,  and tine weeders that go over the top of the entire soil surface in a ‘blind' cultivation. In-row weed control can also be accomplished with tools such as hilling disks, rolling cultivators, or sweeps that throw soil from between the row into the crop row, burying weeds. Other between-row tools include basket weeders and shovels, that don't move much soil into the row, which is desirable when cultivating crops such as lettuce.

A sequence of cultivation equipment use is necessary for many crops. For example, on sweet corn, before and after planting and/or shortly after emergence there may be several cultivations with a flex-tine weeder. As the young crop grows, sweeps, hilling disks, or rolling cultivators, angled to pull soil away from the row, control small weeds between the rows. Once the crop is large enough, those tools are used again, but angled to throw soil into the row. That may add up to 5 or 6 passes to maintain good weed control - an expensive proposition, but there are several ways to keep costs down. First, planting straight, evenly-spaced rows enhances the speed of cultivation. Second, multiple-row cultivation equipment increases efficiency. Third, not every block will need all the passes, depending on timing of planting and degree of weed pressure. And fourth, use of other practices that limit weed pressure and weed seed bank build-up will limit the need for cultivation over time.

Insect Management using organic methods is a lot like IPM (Integrated Pest Management), with an emphasis on cultural practices and without the option of using synthetic insecticides. At the core of organic IPM are: crop rotation to minimize overwintering pest problems and diversification to provide beneficial insect habitat. Other practices include timing of plantings to avoid peak insect pressure, trap crops to divert or concentrate pests away from cash crops, and exclusion of pests with floating row covers. The latter technique can be used, on a rather large scale, to prevent damage by flea beetles, cabbage ‘worms' and other foliage-eating pests. Monitoring and scouting for insect pests are critical in order to determine when cultural practices alone are not enough to provide sufficient insect control, and application of an organic pesticide is necessary.

The arsenal of organic insecticides is growing, and the historical and undesirable reliance of organic growers on non-selective botanicals such as rotenone and pyrethrum is lessening. Newer botanicals such as azadiractin (neem extract), biologicals such as B.t., myco-insecticides such as Beavaria bassiana, and other materials including insecticidal soap, kaolin (clay) and a variety of plant-based repellents such as garlic and hot pepper sprays are available.

Except for materials that are widely used in conventional farming, like B.t , there is a paucity of  information about the efficacy of organic insecticides, and how to optimize their effectiveness. There is also very little information about economic and action thresholds appropriate for organic farming.

Disease management on organic farms relies on a combination of preventative techniques. Along with rotation, sanitation through the prompt removal of disease plants and the incorporation of crop residues is important. Practices that enhance prompt drying of foliage, such as wide row spacing and drip versus overhead irrigation, are desirable. Promoting drainage of soil though raised beds and subsoiling is helpful on heavy soils. Use of resistant varieties is obviously desirable, too. Fungicides that may be allowed organically include many copper and sulfur compounds, and some newer materials including potassium bicarbonate, hydrogen dioxide, and a variety of biological fungicides containing species of Trichoderma, Bacillus, Gliocladium, Streptomyces and other beneficial microbes. Some of these are effective and affordable, like the T-22 strain of Trichoderma, and are widely used on conventional farms, too.

The economics of organic vegetable production is difficult to assess fairly, since the value of certain indirect benefits, such as an improvement in soil structure, or a reduction in risks associated with pesticide use, are hard to quantify and are never reflected in enterprise budgets. In addition, the price premium that growers may receive for producing a wholesale commodity organically is about as volatile as vegetable prices in general. Two things are for sure: 1) poor quality organic vegetables are not likely to garner a premium, and 2) providing consistently-high product quality and service is the way to get a fair and consistent price for what you grow.

 References:
Coleman, Elliot. 1995. The New Organic Grower. Chelsea Green Publishing Co., White River  Junction VT

Grubinger, Vernon P. 1999. Sustainable Vegetable Production from Start-Up to Market. Natural  Resource and Agricultural Engineering Service, Ithaca NY
 

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