**2. Heavy metals in fertilizers and waste materials**

Horticulture is the branch of agriculture dealing mainly with the cultivation of vegetables, medicinal plants, ornamental plants and fruit trees and bushes. Horticultural plants are an important part of the human diet. For this reason, attention is paid to factors affecting the quantity and quality of yield.

Crop yield depends on many factors including variety, control of diseases and insects, and weather conditions. However, the essential role is played by the physical and chemical properties of the soil or growing medium. To improve soil quality, farmers use organic and mineral fertilizers. Unfortunately, fertilizers can be contaminated by substances that can potentially pose a risk to human and animal health and the environment. In the case of mineral fertilizers, this problem concerns mainly cadmium compounds.

The presence of cadmium in topsoil is a consequence of the use of phosphate fertilizers contaminated with this element [1–4]. Cadmium uptake by plants depends on pH of soil or growing medium. Under acidic conditions, cadmium solubility increases. In these conditions, the adsorption of cadmium by soil colloids, hydrous oxides, and organic matter is very low. However, zinc can reduce cadmium's availability to plants, by inhibiting calcium uptake and preventing it from moving from the roots to the shoots of the plants [5]. Lime fertilizers, as well as waste materials rich in calcium and/or magnesium, can also be contaminated by heavy metals [6–8]. Moreover, in this case, the list of potentially toxic elements is much longer: Cd, Cr, Co, Cu, Pb, Mo, Ni, Zn, As and Hg [8–11]. Essential and beneficial elements can become toxic only at high concentrations. In many countries, the use of fertilizers or waste materials contaminated with heavy metals is limited by the introduction of a maximum permissible content of these elements. However, the rules of individual countries are not unified [4]. For example, in Poland, the maximum permissible concentrations of heavy metals in fertilizers are as follows:


Heavy metals may be introduced into the soil and substrates also with soil improvers or mulches. 'Soil improver' is defined as a material added to soil in situ whose main function is to maintain or improve its physical and/or chemical and/or biological properties, with the exception of liming materials. 'Mulch' means a type of soil improver used as protective covering placed around plants on the topsoil whose specific functions are to prevent the loss of moisture, control weed growth and reduce soil erosion. According to a regulation of the European Union, the maximum content of heavy metals in the final product or constituent may not exceed the values shown in **Table 1**.

plants through the release of toxic substances from living parts or dead plant tissues during their decomposition is called allelopathy. Understanding of the causes and consequences of

Horticulture is the branch of agriculture dealing mainly with the cultivation of vegetables, medicinal plants, ornamental plants and fruit trees and bushes. Horticultural plants are an important part of the human diet. For this reason, attention is paid to factors affecting the

Crop yield depends on many factors including variety, control of diseases and insects, and weather conditions. However, the essential role is played by the physical and chemical properties of the soil or growing medium. To improve soil quality, farmers use organic and mineral fertilizers. Unfortunately, fertilizers can be contaminated by substances that can potentially pose a risk to human and animal health and the environment. In the case of mineral

The presence of cadmium in topsoil is a consequence of the use of phosphate fertilizers contaminated with this element [1–4]. Cadmium uptake by plants depends on pH of soil or growing medium. Under acidic conditions, cadmium solubility increases. In these conditions, the adsorption of cadmium by soil colloids, hydrous oxides, and organic matter is very low. However, zinc can reduce cadmium's availability to plants, by inhibiting calcium uptake and preventing it from moving from the roots to the shoots of the plants [5]. Lime fertilizers, as well as waste materials rich in calcium and/or magnesium, can also be contaminated by heavy metals [6–8]. Moreover, in this case, the list of potentially toxic elements is much longer: Cd, Cr, Co, Cu, Pb, Mo, Ni, Zn, As and Hg [8–11]. Essential and beneficial elements can become toxic only at high concentrations. In many countries, the use of fertilizers or waste materials contaminated with heavy metals is limited by the introduction of a maximum permissible content of these elements. However, the rules of individual countries are not unified [4]. For example, in Poland, the maximum permissible concentrations of heavy metals in fertilizers

**•** in organic and organic mineral fertilizer (in 1 kg of dry weight of the fertilizer): Cr—100 mg,

**•** in agricultural lime containing magnesium (calculated per 1 kg of the sum CaO + MgO): Cd

**•** in other mineral fertilizers (in 1 kg of dry weight of the fertilizer): As—50 mg, Cd—50 mg,

Heavy metals may be introduced into the soil and substrates also with soil improvers or mulches. 'Soil improver' is defined as a material added to soil in situ whose main function is

risks outlined above determines for rational decision‐making in horticulture.

**2. Heavy metals in fertilizers and waste materials**

24 Soil Contamination - Current Consequences and Further Solutions

fertilizers, this problem concerns mainly cadmium compounds.

Cd—5 mg, Ni—60 mg, Pb—140 mg, Hg—2 mg,

**•** in agricultural lime (expressed per 1 kg CaO): Cd—8 mg, Pb—200 mg,

quantity and quality of yield.

are as follows:

—15 mg, Pb—600 mg, and

Pb—140 mg, Hg—2 mg [12].


**Table 1.** Heavy metals limits for soil improvers, mulch and organic constituents of growing media [13].

In organic and mineral growing media, the content of heavy metals in the final product may not exceed the values shown in **Table 2**.


**Table 2.** Heavy metal limits for growing media, including mineral growing media [13].

The source of heavy metals may also be sewage sludge from municipal sewage treatment plants used to fertilize soil or compost from sewage sludge. The use of these materials in the EU is subject to a number of strict requirements. The most important are the Water Framework Directive 2000/60/EC on water protection, Directive 91/271/EEC on urban waste water treatment, Directive 96/61/EC concerning integrated pollution prevention and control, Directive 99/31/EC on the landfill of waste and Directive 86/278/EEC on the use of sludge in agriculture [13–17]. The limit values for heavy metals in sludge or in composts are defined in national regulations. The regulatory framework prevents harmful effects on soil, vegetation, animals and humans [18–20].
