Soil fertility

Lore & Background

Soil fertility is the capacity of soil to sustain agricultural plant growth by providing essential nutrients, water, and a habitat free from toxic substances, such as excess ferrous iron. A fertile soil typically possesses sufficient depth for root development and water retention, good internal drainage for aeration, and a surface horizon rich in organic matter. Most soils have three major horizons: the surface horizon (A), the subsoil (B), and the substratum (C); some also feature an organic horizon (O) or an eluviated horizon (E) where minerals have been lost. The ideal soil pH for most plants ranges from 5.5 to 7.0, though some species tolerate more acidic or alkaline conditions. Fertility also depends on adequate concentrations of essential plant nutrients in available forms and a diverse community of microorganisms. In agricultural lands, maintaining fertility requires soil conservation practices, as erosion and degradation diminish soil quality. Historically, colonialism and slavery introduced harmful land practices—such as intensive burning and deforestation—that degraded soil quality globally. Conversely, indigenous communities in the Amazon created fertile dark earth, still valued today. Bioavailable phosphorus is the most commonly lacking nutrient, especially in humid tropical soils, followed by nitrogen and potassium; these three are always listed on commercial fertilizer analyses. Inorganic fertilizers provide nutrients immediately but can disrupt natural nutrient balance, reduce organic matter, and increase erosion risk. Soil fertility involves a constant nutrient cycle: microorganisms decompose organic matter, releasing inorganic nutrients (mineralization), while also competing for these nutrients (immobilization). Natural processes like lightning fix atmospheric nitrogen, and bacteria perform both nitrogen fixation and denitrification, the latter occurring mainly under anaerobic conditions. Cation exchange retains nutrient cations such as potassium in the soil.

Reader's Guide

Soil fertility is the capacity of soil to support agricultural plant growth by providing essential nutrients and water while lacking toxic substances that inhibit development. A fertile soil typically has sufficient depth for root growth, good internal drainage for aeration, adequate organic matter in the topsoil or O horizon, a pH range suitable for most plants, and a community of microorganisms that aid plant growth. Soil scientists identify master horizons using capital letters: O for organic surface material, A for the surface horizon, B for subsoil, C for substratum, and E for subsurface horizons where minerals have been lost through eluviation. Hard bedrock is designated R. Fertility is maintained through constant nutrient cycling between organic and inorganic forms, driven by microorganisms that decompose plant and animal waste—a process called mineralization. These nutrients can also be tied up in microbial biomass through immobilization. Natural nitrogen fixation occurs via lightning or free-living and symbiotic bacteria, while denitrification happens under anaerobic conditions. Bioavailable phosphorus is the most commonly lacking nutrient, especially in humid tropical soils, followed by nitrogen and potassium, which is why commercial fertilizers list these three elements. Inorganic fertilizers provide immediately available nutrients but can disrupt natural balances, reduce organic matter, and increase erosion risk. Soil fertility has been historically impacted by colonial practices such as intensive burning and deforestation, and modern intensive farming continues to degrade soil quality. In the Amazon, pre-Columbian peoples created fertile dark earths that remain highly productive. Soil conservation practices are essential to counter erosion and degradation.

Did You Know?

• Bioavailable phosphorus is the element most often lacking in soil, especially in humid tropical soils.
• Inorganic fertilizers are immediately bioavailable but can disrupt soil nutrient balance and cause water pollution.
• Soil depletion can result from over-tillage, overuse of nutrient inputs, and salinization.
• Colonial settlers in New England depleted soils by allowing herd animals to wander freely and burning forest undergrowth.