Manures for Organic Crop Production

Manures for Organic Crop Production SOIL SYSTEMS GUIDE Abstract: Livestock manures are an important resource in sustainable and organic crop production. This publication addresses the problems and challenges of using both raw and composted manures and discusses some of the solutions. It also deals with guano, a similar material. By George Kuepper Table of Contents NCAT Agriculture Specialist Introduction ..................................................... 1 March 2003 Raw Manure Use: Problems & Solutions ...... .2 Contamination ................................................. 2 Produce Quality Concerns ............................... 3 Introduction Fertility Imbalances ......................................... 3 Livestock manure is traditionally a key fertilizer in organic and sustainable soil manage- Weed Problems................................................ 4 ment. It is most effectively used in combination Pollution .......................................................... 4 with other sustainable practices. These include crop rotation, cover cropping, green manuring, Composted Manures ........................................ 5 liming, and the addition of other natural or bio- About Guano ................................................... 6 logically friendly fertilizers and amendments. In organic production, manure is commonly Field-applying Manures & Composts ............. 7 applied to the field in either a raw (fresh or dried) Summary ......................................................... 9 or composted state. This publication addresses the advantages and constraints of using manure References ...................................................... 9 in either form, but with particular focus on raw Recommended Resources.......................... 10 manure; it does not discuss the specific circumstances and challenges associated with handling and applying slurry manure. There are clear restrictions on the use of raw manure in organic farming. These restrictions are de- tailed in the National Or- ganic Program (NOP) Regulations, which consti- tute the federal standard for organic production. Details will be discussed later in this publication. ATTRA is the national sustainable agriculture information service operated by the National Center for Appropriate Technology, through a grant from the Rural Business-Cooperative Service, U.S. Department of Agriculture. These organizations do not recommend or endorse products, companies, or individuals. NCAT has offices in Fayetteville, Arkansas (P.O. Box 3657, Fayetteville, AR 72702), Butte, Montana, and Davis, California. sible, avoid manuring after planting. Fall spread- ing is advised. •Do not use dog, cat, or pig manures (fresh Raw Manure Use: or composted). These species share many para- sites with humans. Problems & Solutions •Wash all produce from manured fields thor- oughly before use. Persons especially suscep- Raw manure is an excellent resource for or- tible to food-borne illnesses (children, the eld- ganic crop production. It supplies nutrients and erly, those with compromised immune systems, organic matter, stimulating the biological pro- etc.) should avoid uncooked produce. cesses in the soil that help to build fertility. Still, In February 2000, the issue of manure use on a number of cautions and restrictions are in or- organic farms was highlighted on the television der, based on concerns about produce quality, news program 20/20. The segment suggested that food contamination, soil fertility imbalances, fertilization with livestock manures made or- weed problems, and pollution hazards. ganic foods more dangerous than other food products in the marketplace (4). The show’s pro- ducers arranged for a sampling of various or- Contamination ganic and nonorganic vegetables from store shelves and tested for the presence of E. coli. The Some manures may contain contaminants samples of both organic and nonorganic produce such as residual hormones, antibiotics, pesticides, were generally free of serious contamination. disease organisms, and other undesirable sub- The exceptions were bagged sprouts and stances. Since many of these can be eliminated mesclun salad mix. Of these, more E. coli con- through high- tamination was temperature observed on the aerobic organic samples. It composting, was largely on this this practice is basis that the news recommended program chal- where low lev- lenged organic els of organic farming. contaminants The attack was may be present. embarrassing to Caution is ad- the organic indus- vised, however, try and forced its as research has membership to un- demonstrated dertake a lot of that Salmonella “damage control,” and E. coli bacte- despite the fact ria appear to that the allegations survive the composting process much better than were contrived and based on poor science. The previously thought (1). The possibility of trans- sampling was not statistically significant (i.e., the mitting human diseases discourages the use of same sampling done today might produce the fresh manures (and even some composts) as pre- opposite result). The show failed to point out plant or sidedress fertilizers on vegetable crops— that the specific test used does not distinguish especially crops that are commonly eaten raw (2). between pathogenic and benign forms of E. coli. Washington State University (3) suggests that Also ignored was the obvious fact that conven- growers: tional farmers use manure, too! Furthermore, the •Apply animal manures at least 60 days prior reporter failed to disclose the vested interests of to harvest of any vegetable that will be eaten the individual bringing the charges (Dennis without cooking. (Note: The NOP’s specific re- Avery of the Hudson Institute—a “think tank” quirements on the timing of manure applications heavily funded by conventional agriculture in- are discussed later in this publication.) If pos- terests), presenting him instead as a former offi- PAGE 2 //MANURES FOR ORGANIC CROP PRODUCTION cial with the Agriculture Department (5). John Stossel—the journalist responsible for the 20/20 Fertility Imbalances report—subsequently issued an apology and a correction (6). Raw manure use has often been associated Unlike conventional farmers, who have only with imbalances in soil fertility. There are sev- safety guidelines regarding manure use, certified eral causal factors: organic farmers must follow stringent protocols. •Manure is often rich in specific nutrients like Raw manure may NOT be applied to food crops phosphate or potash. While these nutrients are within 120 days of harvest where edible portions of great benefit to crops, repeated applications have soil contact (i.e., most vegetables, strawber- of manure can result in their building to detri- ries, etc.); it may NOT be applied to food crops mental levels. A good example is the overuse of within 90 days of harvest where edible portions broiler litter in the mid-South, which has put do not have soil contact (i.e., grain crops, most excessive phosphate in the soil and polluted sur- tree fruits). Such restrictions do not apply to feed face waters. Nutrient excesses also “tie up” other and fiber crops (7). minerals. Excessive phosphate interferes with Organic substances are not the only contami- plant uptake of both copper and zinc; excessive nants found in livestock manures. Heavy met- potash can restrict boron, manganese, and even als can be a problem, especially where industrial- magnesium (9). scale production systems are used. Concerns •Continual manure use tends to acidify soil. over heavy-metal and other chemical contami- As manure breaks down it releases various or- nation have dogged the use of poultry litter as ganic acids that assist in making soil minerals an organically acceptable fertilizer in Arkansas, available—a benefit of manure that is often un- where it’s readily and cheaply available (8). This recognized. Over time, however, this process matter is discussed in more detail under depletes the soil of calcium and causes pH levels “Composted Manures.” Heavy-metal contami- to fall below the optimum for most crops. Ma- nation is also a concern with composted sewage nures do supply some calcium, but not enough sludge (biosolids)—a major reason for its being to counterbalance the tendency toward increased prohibited from certified organic production. acidity (9). Possible exceptions include caged- Under federal organic standards, certifiers layer manure (when oyster-shell or similar cal- may require testing of manure or compost if there cium supplements are fed) and manure from is reason to suspect high levels of contamination. dairy operations where barn lime is used. •When fresh manure containing large amounts of nitrogen and salts is applied to a crop, Produce Quality Concerns it can have the same effects as excessive applications of soluble commercial fertilizers—it can It has long been acknowledged that improper burn seedling roots, reduce immunity to pests, use of raw manure can adversely affect the qual- and shorten produce shelf life. Excessive salin- ity of raw vegetable crops such as potatoes, cu- ity is often associated with heavy applications of cumbers, squash, turnips, cauliflower, cabbage, feedlot manure in regions where little leaching broccoli, and kale. As it breaks down in the soil, naturally occurs—as in most western states (10). manure releases chemical compounds such as For example, growers in southwestern states like skatole, indole, and other phenols. When ab- Arizona are advised to apply gypsum and leach sorbed by

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