Chemical fertilizers, particularly inorganic fertilizers, are a crucial input for crop production. Consequently, large quantities of inorganic fertilizers, including nitrogen (N), phosphorus (P), potassium (K), and compound/mixed fertilizers are routinely added to agricultural lands to supply adequate quantities of these macronutrients. For instance, it was estimated that in 2019, more than 220 million tons of commercial fertilizers and liming materials were applied worldwide, mostly to agricultural fields [21].

Among these fertilizers, P fertilizers contain the highest level of HM contaminants [4,22,23,24]. For example, superphosphate fertilizers can contain Cd, Co, Cu, Pb, Zn, Cr, and Ni as contaminants. In a study that assessed soil with and without P fertilizer amendments, the concentration of Zn was higher not only in the amended soil, but also in the plants grown in that soil [25]. Cadmium content in the soil has been shown to increase persistently due to the application of P fertilizers [12,23,26]. Cadmium is extremely toxic to plants because of its high solubility and mobility in soil solution. The concentration of Cd present as an impurity in several P fertilizers evaluated in a study is shown in Table 1.

In addition to P fertilizers, copper sulphate, iron sulphate, and zinc sulphate fertilizers can also contain HM contaminants, including Pb [22,27,28]. A study reported from greenhouse experiments that repeated application of chemical fertilizers significantly increased the accumulation of several HM elements in the soil (Table 2). Experiments carried out with soil samples collected from multiple locations in agricultural lands of peninsular Malaysia and Guangdong Province of China show that the concentrations of different HM elements (As, Cd, Co, Cr, Cu, Hg, Ni, Pb, Zn) were severalfold higher as compared to control soil samples (refer to Table S2). These HM elements have originated from suspected agricultural practices, including fertilizer applications. Sources of HM contamination in fertilizers include the raw materials used in the manufacture of inorganic fertilizers. For instance, phosphate rock, also known as phosphorite, is utilized in the production of P fertilizers [29,30]. Over 90% of potash extracted from mines is used in the manufacture of K fertilizer [31]. Based on the level of HM impurities, chemical fertilizers can be ranked as follows: P fertilizers ≥ compound fertilizers> K fertilizers> N fertilizer [32,33].

Pesticides play an important role in global agriculture. It has been estimated that without pesticides, the world’s food production could be reduced by close to ~40% [34]. Another study estimated a 78% loss of fruit production, 54% loss of vegetable production, and 32% loss of cereal production without pesticide use [35]. Pesticides such as insecticides, fungicides, rodenticides, nematicides, and herbicides are composed of either organic or inorganic compounds that are toxic to the targeted organisms. Analysis of these compounds shows that some of them contain HM elements either as active ingredients (Table 3) or as impurities in the formulations (Table 4).

Several fungicides and insecticides extensively used in the past in agricultural lands were shown to contain significant concentrations of HM elements in their active ingredients. Examples include Cu-containing fungicides such as copper sulphate (Bordeaux mixture, also referred to as Bordo^® mix) and copper oxychloride; Pb-containing insecticide such as lead arsenate; and Cu-containing insecticide such as copper acetoarsenite. The commonly found HM elements in the active ingredients of pesticide products include Cu, As, Pb, Hg, Cr, Zn, Al, Li, Ba, B, and Ti (titanium) [36,37].

On the other hand, HM elements can also be present in pesticide products as impurities. For example, certain pesticide products used for pest control in Japan contained Cd, Hg, As, Cu, Zn, and Pb as contaminants [17]. A chemical analysis of several pesticides, including 11 glyphosate-based herbicide formulations, by utilizing inductively coupled plasma/optical emission spectrometry (ICP-OES) detected As, Cr, Co, Pb, and Ni as contaminants [38]. A similar analysis of several insecticides using ICP mass spectrometry detected Zn, Cu, Cr, Co, Pb, and Tl (thallium) as contaminants [39]. It has been suggested that the HM elements contaminate pesticide products during the manufacturing process, while some of them are intentionally added as nano pesticides for increased efficacy [38,40].

Livestock manures are organic fertilizers composed predominantly of poultry, cattle, and pig manures. Application of these manures and the compost made from them to farmlands is a common practice in agricultural crop production. However, these manures and compost contain high concentrations of HM elements such as Cu, Zn, Cd, Ni, Cr, As, Pb, and Hg as contaminants [20,26,41,42]. A study conducted to determine the concentrations of HM elements in different livestock and poultry manures is presented in Table 5. The major sources of contamination of HM elements in the manures include the minerals supplied with the commercial feeds [41,42]. For example, supplementation of animal feeds with growth-promoting organic arsenical products was practiced for many years [43]. Some studies confirmed that Zn, Cu, As, and Cd were artificially added to commercial feeds to promote animal growth and improve disease resistance [44,45]. However, animals cannot digest these HM elements, and discharge them through manure [46]. Because HMs are non-degradable elements, they are also not broken down during the composting process [47]. Thus, long-term repeated applications of manures and compost can result in the buildup of HM elements to toxic levels in agricultural soil [48,49] and can affect crop health and productivity.

Sewage sludge originated from municipal and industrial wastes can be highly contaminated with HM elements such as As, Cd, Cr, Cu, Pb, Hg, Ni, Mo, Zn, and others. Long-term application of untreated sewage sludge in some developing countries has led to increased concentrations of HMs in the agricultural lands [14,20,50]. However, the biosolids generated from sewage sludge processing plants can be typically low in HM contamination and can contain organic materials rich in nutrients, and be used as fertilizers [51,52]. When applied to arable lands, processed sewage sludge can improve soil physical properties and crop productivity. Utilization of these byproducts for agricultural crop production is, therefore, a common practice in many countries. In the United States, about 3.0 million dry tons of biosolids are utilized annually for crop production [4]. The European community countries utilized >30% of processed sewage sludge as a fertilizer in agricultural lands [53]. Australia incorporates over 175K tons of dry biosolids into agricultural soil [54]. In the United States, federal regulations limit concentrations of major elements (e.g., As, Cd, Cu, Pb, Hg, Ni, Se, and Zn) commonly found in biosolids for land application (Table 6) [52]. Although biosolids produced from sewage sludge processing treatment are generally low in HM concentration, repeated applications of these products can result in the buildup of HM elements in agricultural soil and can negatively impact crop health and productivity.

The irrigation of agricultural lands with contaminated water from surface water bodies as well as groundwater sources is another route of HM contamination in agricultural soil. The above irrigation practices are more frequently followed in some developing countries [9,15,16,56,57]. A review of many related articles published in a span of over two decades (1994 to 2019) that determined the HM contamination in surface water bodies throughout the world showed that the average content of Cr, Mn, Co, Ni, As, and Cd exceeded the permissible limits as prescribed by the WHO and the United States EPA [9,20]. Studies conducted to determine HM concentrations in irrigation water in several locations of the Gazipur district of Bangladesh and the Gondar city of Ethiopia showed that in almost all tests, the concentration of HMs exceeded the FAO (Food and Agriculture Organization) prescribed admissible levels (Table 7).

The causes for HM contamination in surface water bodies are both natural and anthropogenic. The natural causes include, among others, atmospheric deposition, geological and biological weathering, and climatic change. The anthropogenic causes include, but are not limited to, discharge of HM-contaminated agricultural, municipal, domestic, and industrial wastes. HM elements such as Pb, Ni, Cr, Cd, As, Hg, Zn, Cu, and others from diverse sources are transported to surface water bodies and irrigation canals through runoff, and to underground aquifers through vertical leaching with percolating rainwater [16,59,60,61]. Thus, irrigation of croplands using HM-contaminated water not only affects the growth and productivity of crops [62], but can also threaten soil quality. It should, however, be noted that the extent of crop damage will depend on the pH of the irrigation water, redox potential, and water solubility of the contaminated HM elements.