Crop metal contents

It is to be expected that a relationship exists between the metal content of soils and the metal content of the crop. Table 11.2 gives data on the maximum metal contents observed in soils (taken from Table 11.1) and the maximum metal contents determined for various crops, including corn, wheat and rice flours, apples, potatoes, broccoli and kale. A plot of maximum metal contents (mg/kg) in soil and crops respectively, shows the relationship between these parameters (Fig. 11.1). Metal contents in crops in the range 0.01 -1000 mg/kg increase with increasing soil metal content in the range 1-100 000 mg/kg. 

Figure 11.1. Relationship between metal content of soil and metal content of crops grown in soil (from author s own files)...

Use has also been made of the metal content of crop plants in the assessment of contaminated soils. Kabata-Pendias et al. (1993) suggest that legumes are promising as bioindicators of metal pollution since they have in general a relatively higher tolerance to and uptake of metal than monocotyledons. Kovacs et al. (1993) have... 

Smith, J. L., Summers, G., and Wong, R. (2010). Nutrient and heavy metal content of edible seaweeds in New Zealand. N. Z. J. Crop Hort. 38,19-28. 

Agricultural uses for ammonium thiosulfate take advantage of both the sulfur and ammonium content by blending with other nitrogen fertilizers such as urea (71). Some foHar-spray fertilizers contain ammonium thiosulfate together with other metal micronutrients (72,73). Ammonium thiosulfate or mixtures with ammonium nitrate can also be used as desiccants and defoHants ia crop-beariag plants such as cotton (qv), soybean, alfalfa, rice, and peppers (74,75). 

The uptake of heavy metals from soils is also determined by their contents in soil and by plant species. If these crops are used for anaerobic digestion one has to consider that there is a remarkable risk of an accumulation of heavy metals in soil. In the case of combustion, most heavy metals can be removed by filtering the ash and it can be used safely as fertiliser. Thus cultivation and combustion of short rotation coppice is a smart scheme of removing heavy metals from contaminated soils. 

The plant availability of some nutrients may be limited in both types of residues. Moreover, in certain cases the contents of some heavy metals (Cd, Pb, Cu, Zn, Ni, Cr and Hg) may exceed the legal thresholds (BioAbfV 2002). Nevertheless, the use of energy crop residues as fertiliser significantly reduces the need for mineral fertiliser. 

Table 5.10 Average and span of the content of some environmentally relevant heavy metals in selected energy crop species (Ocker et al. (1984), von Steiger and Baccini (1990), KTBL (1990), Stadelmann and Frossard (1992), Wolfensberger and Dinkel (1997), Heinzer et al. (2000), Hartmann and Kaltschmitt (2002), Wittke (2002), Scholz (2004), FNR (2004), FNR (2005), KTBL (2006), Rohricht and Kiesewalter (2007), Stolzenburg (2008))...

Table 4. Content of metals in forage crops from Udlybaev-Chalil Ni-Cu-Co bio geochemical provinces of South Ural sub-region of biosphere, ppm by dry weight.

This sludge contains organic substances (Table 16.3) also heavy metals. The possible deleterious effects to humans and animals of these applications to land are discussed above. Clearly, to avoid adverse effects on crops, animals and humans, control must be exerted on both the organic contaminant content of the applied sewage and its application rate and frequency. 

Ammonia is used in the production of several chemicals to make nylon adipic acid, hexa-methylene diamine, and caprolactam. It is used to treat metals in annealing, nitriding, and descaling. Ammonia is an excellent fungicide that is used to treat citric fruit. It is also used to increase the nitrogen content of crops used as feed for livestock. Ammonia dissolves readily in water to produce aqueous ammonia or ammonium hydroxide NH + 2 < > NH4+(a(i) + OH (ahydroxide ions shows that ammonia acts as a base in aqueous solution. Concentrated aqueous solutions contain 35% ammonia. Household ammonia cleaners contain between 5% and 10% ammonia. 

The assessment of plant-available soil contents can frequently be achieved and validated by field experiments for nutritionally essential elements, and, for a few potentially toxic elements such as chromium, nickel and molybdenum, at the moderately elevated concentrations that can occur in agricultural situations. The validation of extraction methods, devised for agricultural and nutritional purposes, is much less easy to achieve when they are applied to heavy metals and other potentially toxic elements, especially at the higher concentrations obtained in industrially contaminated land. This is not surprising in view of the fact that for some heavy metals, for example lead, there is an effective root barrier, in many food crop plants, to their uptake and much of the metal enters plants not from the root but by deposition from the atmosphere on to leaves. In these circumstances little direct correlation would be expected between soil extractable contents and plant contents. For heavy metals and other potentially toxic elements, therefore, extraction methods are mainly of value for the assessment of the mobile and potentially mobile species rather than plant-available species. This assessment of mobile species contents may well, however, indicate the risk of plant availability in changing environmental conditions or changes in land use. 

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