Effects of conventional and organic fertilization on spinach (Spinacea oleracea L.) growth, yield, vitamin C and nitrate concentration during two successive seasons
Research highlights
▶ Organically grown spinach contains lower amounts of nitrate and higher amounts of vitamin C. ▶ Plant growth in single application of CM is as high as in mineral fertilizer. ▶ FM and CM are superior to BM application. ▶ Organic applications respond well in the autumn season than that of the winter.
Introduction
Leafy vegetables are an important part in the human diet (Anjana et al., 2007). Spinach (Spinacea oleracea L.) is the most important leafy vegetable and also an important source of minerals and is a leafy cool season vegetable that produces a rosette during the vegetative stage (Kansal et al., 1981) with a production area of 893,494 ha area and amount of 14,044,816 tons in the world in 2007 (Anonymous, 2008a).
Organically grown foods are perceived as better quality, healthier and more nutritious than conventional counterparts (Warman and Havard, 1997). The 1240 studies showed that organically grown fruits or vegetables contain more mineral and vitamins than conventionally grown ones (Worthington, 2001, Citak and Sonmez, 2010).
Spinach is reported to contain very high level nitrate, accumulating more than 2500 mg kg−1 fresh weight (FW) (Santamaria, 2006). The factors responsible for nitrate accumulation in plant are mainly nutritional, environmental and physiological (Anjana and Iqbal, 2007). Moreover, nitrate accumulation in vegetables is caused by the imbalance between nitrate absorption and reduction by plants (Wang et al., 2008). The European Commission regulation set the maximum acceptable nitrate concentration as 3000 and 2500 mg kg−1 on fresh weight basis for crops harvested from November 1st to March 31st and from April 1st to October 31st, respectively (Anonymous, 2005).
The acceptable daily intakes (ADI) of nitrites and nitrates recommended by WHO is 0–0.06 mg NO2 kg−1 and 0–3.7 mg NO3 kg−1 body weight (Anjana and Iqbal, 2007, Anonymous, 2008b). Therefore, assuming a 60 kg body weight, ingestion of only 100 g of fresh vegetables with a nitrate concentration of 2500 mg kg−1 fresh weights exceeds the acceptable daily intake for nitrate by approximately 13% (Anjana et al., 2007).
Management of the nitrate level in vegetables is crucial for some diseases including blue baby syndrome (Majumdar, 2003). European Food Safety Authority stated that nitrate is relatively non-toxic, but its metabolites and reaction products, e.g., nitrite, nitric oxide and N-nitroso compounds, raised concern because of implications for adverse health effects such as methaemoglobinaemia and carcinogenesis (Anonymous, 2008b). Ascorbic acid (vitamin C) can afford protection this conversion of nitrate to nitrite to nitrosamines, so it is of some interest that the higher levels of nitrate in non-organic crops have often been linked with lower ascorbic acid levels (Anonymous, 2001).
Spinach is low in calories and a good source of vitamin C, vitamin A and minerals especially iron (Toledo et al., 2003). Vitamin C is one of the most important nutritional quality factors in many horticultural crops and has many biological activities in the human body (Lee and Kader, 2000). Vitamin C is the most important vitamin in fruits and vegetables for human nutrition and more than 90% of the vitamin C in human diets is supplied by fruits and vegetables. Today, ascorbic acid is a well-known antioxidant and enzyme cofactor with many roles in human health (Conklin, 2004).
The goal of the present experiment was to elaborate three different kinds of organic manure and its different mixtures, which are used commonly in organic crop production, in comparison to conventional chemical fertilizer application in relation to the control plot, without any application. The response of spinach growth, yield, vitamin C and nitrate concentration to organic and conventional fertilization was assessed during two successive spinach growing seasons. Therefore, plausible applications that would be an alternative instead of utilizing chemical fertilizer in maintaining soil fertility are to be recommended.
Section snippets
Location and experimental design
The experiments were carried out during two successive seasons including autumn (from October 10 to November 15, 2007) and winter (from December 12, 2007 to February 10, 2008) at Akdeniz University, Faculty of Agriculture, Research Station located between 36°53′58″ N latitude and 30°38′10″ E longitude in Antalya, Turkey. The experimental area was not used for any agricultural activity before setting up an experiment. Farmyard manure (FM), chicken manure (CM) and blood meal (BM) was used as
Evaluating spinach growth and yield
The leaf number, plant height, and petiole length in relation to the applications were summarized in Table 3. In general, chemical fertilizer application gave the highest values for the measurements taken in both seasons in comparison to the control. In the winter season, unfavorable conditions led all parameters except leaf number per plant to be higher in the autumn season than that of the winter season. The cooler the temperature was, the more leaves were noticed which might be related to
Discussion
The effects of three kinds of organic manure on the measured parameters differed greatly among the applications. The main reason responsible for these variations are possibly the climatic conditions and, the mineralization rate of the manures. Furthermore, combination of organic and inorganic fertilizers significantly affected quality (Kansal et al., 1981). In general, in the autumn season, all applications had positive effects on the spinach growth, and organic manure applications were found
Acknowledgement
We express our appreciation to The Scientific Studies Management Unit of Akdeniz University, Antalya, Turkey for funding this study.
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