Nitrogen (N), a limiting nutrient in all agricultural systems, can be difficult to manage in certified organic vegetable production. Organic agriculture relies heavily on fertilizers and soil amendments from off-farm sources such as fish emulsion, blood meal, feather meal, alfalfa (Medicago sativa) meal, compost, and animal manures (Gaskell et al., 2006) for soil fertility and crop nutrients. The use of supplemental fertilizers is so common that the marketplace is now inundated with variations of these organic N options, but there is little research comparing their true impacts on plant growth and quality (Hartz and Johnstone, 2006).
Organic N fertilizers are usually found in two forms, solid and liquid. Solid fertilizers are often incorporated into the soil before planting, while liquid fertilizers are generally applied post planting and are frequently applied season-long through irrigation systems. All these organic materials are rich in slow-releasing organic N, and the mineralization rate is difficult to predict when planning to meet crop uptake needs. In a 2006 study by Hartz and Johnstone, fish powder, blood meal, and feather meal were all found to have very high levels of organic N (93% to 99% of total N was in organic form). These fertilizer types rely on soil microbes to convert organic N into inorganic N forms such as ammonium (NH4+) and nitrate (NO3−) before plant uptake (Gaskell and Smith, 2007). Therefore, organic fertilizers are less predictable than conventional fertilizers, and while there is an abundance of literature on manure and compost use in the field, little research has been done comparing supplemental organic N fertilizers.
Most organic fertilizers are produced off-farm and distributed widely throughout the United States. Fish emulsions are produced as a byproduct of the seafood industry and often traded internationally (Soares et al., 1973). The fish hydrolysate used in this study (Neptune’s Harvest) is produced in Gloucester, MA and shipped in a concentrated liquid form (it journeyed ≈1900 miles to the location of this study). Because these liquid fish fertilizers are affordable, relatively fast acting, and easy to apply, farmers are encouraged to rely on outside sources for the additional N needed during the growing season and to pay the extra cost of shipping heavy liquids around the United States. However, there are new fertilizer options being developed that allow farmers to produce N on-farm. This study includes one such fertilizer, cyanobacteria-based, to compare with off-farm sources.
Cyanobacteria (formerly known as blue-green algae) have been documented as a source of N in rice (Oryza sativa) paddies as early as 1973 (Roger and Kulasooriya, 1980), but they were presumably present and provided N to the paddies for centuries before. On-farm production of cyano-fertilizer cuts out the costs associated with relying on off-farm sources; and by using a biological process outside of the soil, the farmer can control application timing and N rates. A greenhouse study by Sukor (2013) found N availability in solid cyanobacteria to be 6% greater than compost, but 9% less available than fish emulsion when used on clayey soils, with similar results on sandy soils. Sukor (2013) also showed that cyano-fertilizer increased lettuce (Lactuca sativa) yields by 58% when compared with composted manure. Research is underway to improve N fixation efficiency and economic feasibility of cyano-fertilizer.
Kale (Brassica oleracea and Brassica napus) is one of the most widely consumed vegetables in the United States along with others in the species B. oleracea [cauliflower, broccoli, cabbage (Thomson et al., 2007)]. In 1993, Dangler and Wood found optimal yield increases in kale cultivars at N rates of 112 kg·ha−1 when grown in the Norfolk-Orangeburg loamy sand of Alabama. These rates produced yields of 9.9 to 14.9 t·ha−1, depending on in-row spacing. In terms of human nutrition, these cruciferous vegetables are often high in iron (Fe) and zinc (Zn), micronutrients important in reducing oxidative stress, which is highly correlated with chronic disease reduction (Tomey et al., 2007). These micronutrients also act as cofactors for antioxidant enzymes long known for their importance in human health. Zn plays an important role in blood coagulation and in protecting DNA from modifications, thus decreasing the risk of cancer (Messias et al., 2015) and has also been found to increase plant tolerance to adverse environments (Cakmak, 2008). While there are many studies focusing on Zn composition in cereal grains, there is a growing interest in leafy greens as a substantial dietary source of Zn (Broadley et al., 2010). Deficiencies in micronutrients such as Fe can cause anemia, a state of low red blood cells, which limits the movement of oxygen within the body. Thirty-three percent of women of reproductive age suffer from anemia worldwide (Food and Agriculture Organization of the United Nations, 2017). The accumulation of these micronutrients in edible plant tissue can vary based on the type and quantity of fertilizer applied (Cakmak, 2008). Nitrogen fertilizers have shown positive impacts on a plant’s ability to accumulate Fe and Zn; increased N rates can elevate Fe and Zn concentration in leaf tissue (Aciksoz et al., 2011). As consumer interest in nutritionally dense foods increases, kale may be an important crop to which to turn.
The objective of this study was to compare the effects of different organic fertilizers on yield and nutrient content of three cultivars of kale. Kale was chosen for its long growing season (frost tolerance) and continuous need for a steady supply of N for plant growth.
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