The world may already have passed peak phosphorus, back in 1989
Phosphorus is the nutrient soils most often run short of, and plants cannot divide cells or grow seedlings without it. It comes from limited deposits of rock phosphate, and some estimates put America's production peak in 1988 and the world's a year later. Farming's future partly rests on a rock that is running out.
A fertile soil supplies water and essential nutrients in the right amounts and proportions, and contains nothing toxic, such as the dissolved iron that can poison crops. Helpful traits include enough depth for roots, good drainage for air, although rice tolerates waterlogging, a topsoil rich in organic matter, a pH between about 5.5 and 7.0 for most plants, and a thriving community of beneficial microbes.
Beneath the surface, nutrients cycle constantly. Microbes decompose dead plants and dung, releasing inorganic nutrients in a process called mineralization, but they also compete with crops for the same nitrogen, phosphorus and potassium and lock some away in their own bodies, which is known as immobilization. Lightning and bacteria fix nitrogen from the air, while other bacteria in waterlogged ground return it to the atmosphere. Potassium and many trace nutrients cling electrostatically to negatively charged clay and humus through cation exchange.
Fertilizer labels reflect those needs. A bag marked 10-10-15 contains 10 percent nitrogen, 10 percent available phosphorus and 15 percent soluble potassium, and a fourth number may give sulfur. Inorganic fertilizers are concentrated and immediately usable by plants, but leaning on them too heavily upsets the soil's nutrient balance, erodes organic matter and lets soluble nitrate wash into waterways; slow-release products reduce those losses. Heavy phosphorus use has also driven pollution and eutrophication. One study found compost made from food waste improved soil more than manure-based compost.
Depletion follows when what fertility needs is taken and not replaced, through over-tillage, salinization or mining the nutrient bank. Colonial New England offers a case study. The writer William Cronon described settlers letting herds roam, skipping manure, burning undergrowth and clearing forest, which brought floods, compaction and erosion, while Indigenous farming knowledge was ignored. In the Amazon, by contrast, pre-Columbian communities deliberately created dark earth, soils still sought out today for their richness.
Source: Soil fertility
