The Trial of a Potato

High yields on small acreage are a vote for organic potato production throughout the world.

By Dave Wilson

Rodale Institute interns and volunteers from left to right: Bart Renner, Jane Dahms, Alison Grantham, Lauren Tracey, Kaitlin Harrigan.

Potatoes are a crucial, though sometimes labor-intensive agronomic crop in many parts of the world. Their high calorie and nutrient density makes them a valuable staple crop for populations who must rely on “backyard” harvests for nutrition and self-sufficiency.

Potatoes have been grown in the Andes for 8,000 years. The Spanish took the potatoes to Europe in the 16th century, and they have since been grown by farmers throughout the world.

Trials over the past year at the Rodale Institute have illustrated that when properly fertilized, watered, and cared for, even a small acreage can yield a high-volume crop. Choosing the appropriate variety and maturity range of seed is important, but once growers figure out how to coax a successful harvest from their patch of ground, the potato never fails to satisfy. It can be prepared in endless innovative and traditional recipes, and fits well into a wide variety of cuisines.

The potato comes in over 1,000 varieties, classified by shape and skin color. Some may be very specific in their adaptation to regions, and others grow well in broad variety of regions. Varieties vary greatly in disease, insect and nematode resistance.

Potatoes generally come in three maturity ranges. Early maturity potatoes are ready to harvest within 65-95 days; mid-season takes from 95-115 days; and full season anywhere from 115-135 days.

Many on the eating end of the potato’s life cycle classify the tubers by use. “New potatoes” refer to those harvested early, before they are mature and the skins are set, or to freshly dug fall potatoes. These are best eaten boiled. “General purpose” potatoes are versatile and can be boiled, fried, baked and used in casseroles, salads or soups. “Baking” potatoes are, as the name implies, best right out of the oven, with butter, sour cream, cheese or bacon. Russet Burbank is a well-known variety of baking potato.

Scientists, on the other hand, like to categorize potatoes by their molecular composition. Potatoes high in amylase, a more soluble compound, will have a floury, mealy consistency when cooked. Those with a higher amylopectin content, by contrast, hold up better to cooking, and show greater integrity, since amylopectin is less soluble. These potatoes are better for use in soups and salads.

Diversity

You can plan a potato polyculture in a variety of ways. By planting different varieties and maturity ranges in staggered plantings, you may spread out the time and labor needed for managing the crop. Diversity in the potato crop is also the best defense against pests and diseases, since a variety of genes means a variety of degrees of susceptibility. Longer-day potatoes (later maturing varieties) often keep longer in storage than early varieties, which many be an important consideration as well.

Certified seed

When planting, it’s best to choose certified seed tubers (not to be confused with certified organic seed). Certified seed guarantees to the grower that the seed meets standards for minimal disease incidence. Certified seed is generally taken from the highest-yielding seedstocks, maintained under carefully controlled conditions and isolated from disease.

The Rodale Institute potato plots had garlic growing in them previously in 2007. Then after the garlic was harvested (July 07) compost was applied to the plots and incorporated.

Certified seed was originally developed to decrease the spread of a variety of diseases. It is usually bred in northern states, because lower growing temperatures favor the expression of viral disease symptoms, which allows growers to identify and weed out infected plants. Lower temperatures are also ideal because they support fewer insect populations to spread diseases, making pathogens more easily isolated.

Bona fide state certified seed potatoes will have a blue certified seed tag with the certification information on it from the state in which the potatoes are grown. This tag certifies that the seed tubers were produced using state certified seed and the production field and tubers were inspected. The cultivar name, grade size, class, crop year, grower, shipper and the state certification agency are listed on the tag.

In addition to looking for a blue state certification seed tag, certified organic growers must also make sure the seed tubers have USDA certified organic seal to ensure they have been grown under organic conditions.

Rotation

The potato is a member of the nightshade (solanaceae) family, which includes tomatoes, peppers, tobacco and eggplants, sharing with them several pests and diseases. The Colorado potato beetle, for example, is partial to both potato and eggplant. Avoid diseases such as Verticillium by rotating out of solanaceous family crops. It is important to space potato plantings in the same soil at least three years apart. Fungal spores and water molds, pathogens which rot the lower stem and root, can remain dormant in the soil for many years.

A traditional rotation may include corn, potatoes, a small grain and a clover. In the South, potatoes planted in early spring can follow an early fall-planted cover crop – preferably a legume – such as crimson clover, vetch, winter peas or lupines. Since potatoes like a low pH, lupines may be ideal in many respects, tolerating acidity while still producing nitrogen.

In warmer climates, late summer and fall-planted potatoes frequently follow a summer legume, such as velvet bean, lespedeza, cowpea, soybean or crotalaria. Potatoes following a sod crop or old pasture often run into wireworm problems, but this risk may be reduced with a buffer year of cover crops before planting potatoes.

Both potato plants and tubers use large quantities of soil nitrogen and potassium, both of which tend to be deficient and in need of continual replenishment in normal agricultural soils. Compost is a good source of fertility. Manure works well too, but keep in mind that organic standards dictate that “raw” or “fresh” (uncomposted) manure must be applied at least 120 days before harvest. Applying too much manure may, however, lead to scab problems with potatoes. To avoid scab problems presented by raw manure or compost, it is best to plow it under 120 days in advance of planting. Composted or well-rotted manure is the best option, when available.

Legume cover crops and compost can each provide fertility for the potato crop while contributing the critical component of organic matter to the soil as well. Organic matter is important to improving soil structure. Microbes that feed on organic matter secrete glue-like substances that help bind together the particles in coarse, sandy soils and breaks up finely textured, heavy clay soil for better water infiltration.

Daikon radishes were planted as the cover crop in the Rodale Institute research plots in August 2007 before the potatoes. The radishes winter killed and then in the spring of 2008 the plots were roto-vated and the potatoes were planted.

In addition to legumes, daikon radishes are a surprisingly useful cover crop. Planted in the fall, they compete with winter annual weeds, contributing their allelopathic effect to prevent many weed seeds even from germinating. The long, dense root vegetable opens up compacted soil and scavenges and recycles free nitrate nitrogen that might otherwise leach through the soil.

For potassium-deficient soils, an application of organically-approved potassium sulfate may do the trick, and the sulfur content is helpful in offsetting the alkalinity of a compost application.

Potatoes will also leave soil that needs to be replenished of these nutrients, but they will leave a nice seedbed with loose soil requiring little or no cultivation.

Planting

When planting, it is best to use the whole potato, since the skin protects it from diseases and decay, leading to poor emergence and stand. For this reason, small, whole potatoes are ideal for planting. Plant into moist soil, about 3-4 inches deep. A shallow covering of soil is best to prevent sprout disease and achieve quicker emergence.

Hilling is important to make space for tuber development, and help to reduce the greening of tubers from exposure to sunlight. Hills with a wide base of 14-16 inches, with flat or gently rounded tops are ideal. Many small-scale farmers hill by hand with a rake or hoe, but it can be done mechanically as well.

In our small research plots, where weeding and hilling was done by hand, potatoes were planted in rows spaced 24 inches apart. This is an ideal layout, since the plants form a weed-shading canopy as they grow. Potatoes grown on a large scale, however, are often planted at a 36-inch row spacing to accommodate mechanical hilling and cultivation.

Tubers begin to form at around five to seven weeks after planting (plants will be about 6-8 inches at this point). The botany of tubers is somewhat unique from other root vegetables since they are actually more of a submerged stem than a root. They are the enlarged tips of stolons which arise from the vegetative underground stems of potato plants. The first stolons develop close to the seed piece, while later stolons develop closer to the soil surface.

Water

Potatoes tend to be shallow-rooted and therefore less able to withstand water stress. There are critical windows during the lifecycle when the plants must be watered, so potatoes planted on a large scale will very likely require irrigation, while small plots can be hand-watered. The plants generally need one inch of rainfall per week over the period when they are actively growing. The greatest use of water is after the potatoes bloom, since tubers grow quite rapidly at this point. Most of the dry matter accumulation in the tuber starts at 50 days after emergence. A uniform supply of soil moisture is essential for the production of well-formed potatoes.

It is important not to water potatoes late in the day or at night with overhead irrigation, since many fungal diseases, such as late blight, thrive in cool, wet conditions. Overhead irrigation should be done early in the day to allow the leaves to dry out before evening. Drip irrigation also helps to keep the leaves dry.

Pests

Leaf-hoppers, evident by the “hopper-burn,” or the drying and yellowing of leaf margins they cause, are the foremost predator of potato plants at our location in Kutztown, Pennsylvania. Hopper-burn can be lethal to the plant because it drastically reduces the live leaf area capable of performing photosynthesis. Organic insecticidal soaps and oil sprays, followed by a switch to pyrethrums as populations increase, can be a good solution to an infestation.

Colorado potato beetles, another major threat, were not bad this year, and were easily controlled in small plots by hand-killing of eggs and larvae. Straw mulch is a good way to avoid the potato beetle, since they don’t like straw and it blocks their easy access to the soil. Straw mulch also helps to conserve soil moisture. Spinosad, biological pest control product, works on the Colorado potato beetle in its larval stages, but not for the egg and adult stages.

Late blight, another common concern for potato growers, can be dealt with organically primarily by using copper-based materials, applied often and thoroughly. Bacillus subtillis, or a compost tea mixture (or some combination of these) can be used for suppression of the fungus that causes late blight. In small plots, a cover of agricultural fabric (called Remay or Agribon) also decreases plants’ exposure to fungal spores. Make sure to keep moisture on leaves to a minimum.

The Rodale Harvest

The research plots where our potatoes were grown supported a garlic crop over the winter of 2006-2007. After the garlic was harvested, 150 lbs of compost was applied to each plot, incorporated by rotovator and planted in Daikon radishes in August 2007 as a fall cover crop. The radishes helped to break up the soil and recycle free nitrate nitrogen that might otherwise leach.

Harvest time at the Rodale Instutute.

The Rodale Institute’s replicated research plots, each 9 x 25 feet, or 225 square feet, were planted intensively to maximize yields. The two varieties we compared were Dark Red Norland and Superior. The plots yielded with the following results.

Dark Red Norland, an early variety, was harvested July 28, 2008, and averaged 166 lbs of potatoes per plot. Of these,

• 76 lbs were greater than 2 ¼ inches in diameter
• 61 lbs were between 1 7/8” and 2 ¼” in diameter
• 29 lbs were less than 1 7/8” in diameter

Superior, a mid-season variety, was harvested on August 22, 2008 and averaged 203 lbs per plot. Of these,

• 144 lbs were greater than 2 ¼” in diameter
• 45 lbs were between 1 7/8” and 2 ¼” in diameter
• 14 lbs were less than 1 7/8” in diameter

Although Dark Red Norland was not quite as high-yielding, it is a useful variety in many respects because of its short growing season. It can be ready for harvest as early as 70 days after planting, and can fit into a rotation in a variety of places. Good time slots include both an early planting and a planting in late summer with a harvest before frost. The later planting may also help the grower avoid some warm-season pests and diseases.

A small-scale, locally-suited crop to feed the world

Our potato trials over the past year convinced us that it is indeed possible to reap a substantial organic potato harvest from a small acreage. From small backyard gardens to massive farms, the potato is an integral part of the global food system, a staple crop for people across the globe and the number one non-grain food commodity crop in the world. Its value and consumption continues to grow, particularly in developing countries, where its ease of cultivation and high energy content make it a valuable cash crop and source of self-reliance. In this sense, the potato is a “food security” crop, particularly vital for low-income farmers and vulnerable consumers.

The 2008 International Year of the Potato project notes on its website that “the potato produces more nutritious food more quickly, on less land, and in harsher climates, than any other crop – up to 85 percent of the plant is edible human food, compared to around 50 percent in cereals.” Potatoes are rich in carbohydrates and have the highest protein content of any crop in the root and tuber family. They are also high in Vitamin C and potassium.

Potatoes respond quickly to a low level of inputs and yield well. Breeders and researchers are now working to find integrated management systems to prevent late blight, one of the potato’s most serious and costly threats, and to find varieties with traits such as better drought resistance. As world hunger rises, developing viable and diverse potato crops may be one answer among many to helping people feed themselves and enriching local economies.

Dave Wilson is Research Agronomist at the Rodale Institute, Kutztown, Pennsylvania.