Every fact in the three soil lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Where the facts are simplified or less certain: the make-up of soil (about 45% minerals, 5% organic matter, 25% air and 25% water) is for a 'typical' soil [1][2]; real soils vary a lot, and the air and water shares change after every rain. The FAO figure of about 1000 years for 1 cm of topsoil [3] is a rough global average: Britannica explains that young soils can add a centimetre of horizon in less than ten years while old, strongly weathered soils need several hundred years [4], so we present the FAO figure as an order of magnitude. 'More living things in a tablespoon of healthy soil than people on Earth' is stated by the Ontario ministry without an exact count [7]. The particle density of 2.65 g/cm³ is only an assumed value, used when it has not been measured [9]. In the classroom experiments, a sieved or loosened sample does not keep the structure it had in the ground, so measured densities and drainage times are only estimates for the real field soil.
All sources
[1] Soil basics
University of Illinois Extension · Reliability: High
Type: University extension service (education page)
A typical soil is about 50% solids and 50% pore space; a typical mineral soil is 45% minerals, 5% organic matter, 25% air and 25% water. Most soils form from weathered rock (parent material) over hundreds to thousands of years. Particle sizes: sand 2–0.02 mm, silt 0.02–0.002 mm, clay under 0.002 mm. Sand has larger pores, drains quickly and holds fewer nutrients; clay is flat and plate-like, packs tightly, holds water well and can drain poorly. An ideal loam is about 40% sand, 40% silt and 20% clay. Particles cluster into aggregates (peds), shaped by freeze-thaw, earthworms and roots. Horizons: O (leaves and decomposed plants), A (topsoil, main layer for roots, usually darker), B (subsoil, minerals and clay leached from A, harder to dig), C (weathered parent material above bedrock). Without life, soil would be inert regolith like that on rocky moons.
Soil is the outer loose layer that covers Earth's surface. Its components: inorganic mineral matter (about 40–45% of the volume), organic matter (about 5%), and water and air (about 50%). Soil forms by biological, physical and chemical processes; mineral soils form by weathering of bedrock. Five factors: parent material, climate, topography, biological factors and time. Humus is the organic material of soil, from decaying organisms and plant and animal matter; it improves soil structure and supplies plants with water and minerals. Horizons O, A, B and C, with bedrock beneath. Plant secretions promote microorganisms around roots, in the rhizosphere.
Food and Agriculture Organization of the United Nations (FAO) · Reliability: High
Type: UN agency (feature article)
It is estimated that 95% of our food is directly or indirectly produced on our soils. It takes about 1000 years to form 1 cm of topsoil, so soil is effectively non-renewable within a human lifetime. One third of our soils are moderately or highly degraded, due to erosion, loss of organic carbon, salinization, compaction, acidification and pollution. Most well-known antibiotics, including penicillin, originated from soil microorganisms.
Soils in an early weathering stage need less than a decade to develop a centimetre of horizon thickness; intermediate-stage soils less than a century per centimetre; advanced-stage soils several hundred years per centimetre. Humus in the A horizon rises steadily in soils younger than about 10,000 years. Soil-forming factors: parent material and topography (site-related), climate and organisms (inputs), and time.
USDA Natural Resources Conservation Service · Reliability: High
Type: National soil agency (education page)
Soil scientists use five soil-forming factors: parent material, climate, topography, biological factors and time. The National Cooperative Soil Survey identifies and maps over 20,000 kinds of soil in the United States. Soil scientists study the upper few metres of the Earth's crust.
Royal Horticultural Society (RHS) · Reliability: High
Type: Horticultural charity and scientific society (advice page)
Britain has nearly 30 earthworm species, about 16 of them likely in gardens. Earthworms eat decaying plant material and are important to soil structure and fertility. Some live in permanent burrows and pull fallen leaves into their tunnels; some come out at night to feed on dead plant material on the surface. Casts are made of soil and undigested material. Compost worms live in compost heaps.
Ontario Ministry of Agriculture, Food and Agribusiness · Reliability: High
Type: Government agriculture ministry (fact page)
There are more organisms in one tablespoon of healthy soil than there are people on Earth (no exact count given). Fungi, actinomycetes and algae decompose organic matter; fungi help make nutrients available to plants. After roots, fungi make up the largest amount of living material in soil. Nitrogen-fixing bacteria, associated with soybeans, peas, clover and alfalfa, are an important part of the soil's nitrogen cycle. Earthworms increase the soil's nutrient pool.
American Geosciences Institute · Reliability: High
Type: Geoscience federation (classroom lesson)
Classroom method: pour a set amount of water into equal volumes of different soils in identical containers and time how long it takes to soak in; a jar of soil and water shaken and left to settle separates into layers that can be measured. Sand drains quickly because large spaces exist between grains; silt holds more water than sand but still drains relatively well; clay drains very slowly because of tiny spaces between particles.
Oklahoma State University Extension · Reliability: High
Type: University extension service (fact sheet)
Bulk density = mass of dry soil solids / total volume (g/cm³). Porosity f = 1 − (bulk density / particle density). If particle density has not been measured, about 2.65 g/cm³ is often assumed. In 47 Oklahoma fields the average bulk density to 30 cm was 1.45 g/cm³ (range 1.23–1.67). Bulk densities that restrict root growth: sandy soils above 1.80, silty above 1.65, clayey above 1.47 g/cm³.