Silver (Ag) is a minor constituent in the soil. Certain areas may see elevated levels of Ag due to mining activity or other anthropogenic inputs. Silver nanoparticles (AgNPs) are one of many metallic nanoparticles with widespread industrial use. Their indiscriminate usage inevitably leads to them entering waste streams. About 85% of AgNPs in wastewater accumulate in the biosolids, often reaching concentrations of about 1 mg/kg. These metal-polluted biosolids (sewage sludge) are often used as nutrient amendments for agricultural fields in the USA, making them a significant input of Ag. Low levels are not considered concerning and tend not to affect plant growth. At the root surface, AgNPs <40 nm in diameter can pass directly through the cell wall, and thus smaller NPs are taken up at a faster rate.

Ag preferentially binds to thiol groups, forming silver sulfide (Ag2S) nanoparticles. Silver may also form AgCl, metallic Ag (Ag0), and other species. Ag2S, the predominant form of silver in most soils, is poorly soluble in water. Biological reactions in the soil may produce free Ag+ ions. Ag+ ions are more mobile and bioavailable than AgNPs and Ag2S. High pH values and high cation exchange capacity create negatively charged binding sites for the positively charged Ag+, increasing Ag sorption to soil. Dissolved Ag can be complexed by humic and fulvic acids, significantly decreasing its bioavailability. In addition, soils with more organic matter, finer soil texture, and iron oxides adsorb more Ag than coarse, sandy soils. Ag mobility has decreased significantly in anaerobic conditions compared to aerobic.

There is no clear consensus on a healthy level of soil Ag. But the ability of the soil to adsorb free Ag is the primary determining factor of Ag bioavailability and toxicity. In plants, silver has been reported to have positive effects, attributed to its ability to exchange electrons with Fe and other essential ions, thereby improving biological redox conditions. However, other studies have found Ag to inhibit the growth of cucumber and wheat at levels as low as 10 mg/L and 0.5 mg/L, respectively. Plant susceptibility to silver varies from species to species as does plant capacity to accumulate silver, with some species being hyperaccumulators.

Silver toxicity can be of concern in sandy soils with little ability to adsorb Ag, especially for the soil microbiota. AgNPs are effective agents against fungi and Gram-negative and Gram-positive bacteria and have been used to treat and prevent various phytopathogens. However, they can have unintended effects on beneficial organisms. AgNPs have a large surface area, and they can bind to microbial cell membranes and disrupt essential enzymes. AgNPs vary in their toxicity to microorganisms, with smaller particles being the most toxic. Ag can disrupt the activity of numerous soil exo-enzymes, including urease and other enzymes used by nitrifying and denitrifying bacteria. Ag toxicity can potentially cause ecosystem-level disruptions to the nitrogen cycle, especially in poor soils.