Mixing sand into clay soil improves its drainage.
Refuted
Don't add sand to clay to "loosen" it — in the amounts anyone actually adds, the clay fills the gaps and you get a denser, worse-draining mix. Work in compost and mulch instead; organic matter is what builds the structure that drains.
What is actually known
A modest amount of sand in clay does the opposite of what is intended. The fine clay fills the gaps between the sand grains, giving a dense, poorly-draining mix closer to concrete than to loam — to actually shift the texture you would need sand in huge volumes (over half by volume), which is impractical for a bed. The thing that genuinely opens clay is organic matter: compost and mulch build aggregate structure and the pore space that drains. (The container drainage-layer question, B-009, is a separate and more equivocal case; here the mechanism — sand filling clay's pore space — is well established.)
Evidence: not yet confirmed against its 1 source.
what this rests on - one pointer
University of Arkansas Cooperative Extension, 'Will adding sand help my clay soil?' (uaex.uada.edu, read 2026-07-14): 'If you mix sand into clay, the clay particles will fill in all the open spaces between the sand particles and often the clay will act as a glue sticking all particles together, ultimately resulting in a more dense soil'; 'The volume of sand that would need to be added to change the soil texture and characteristics of a clay soil is very high; 50% or more by volume in most cases'; organic matter 'can help improve soil structure resulting in improved drainage properties of all soils.'
When would that grow at your place? →
Cite this page
Milpa Gardens. "Mixing sand into clay soil improves its drainage." Evidence grade: Refuted. https://milpa.garden/beliefs/sand-clay-soil-improve-clay-drainage/
A garden-planning corpus: every claim states its evidence grade and links its sources on the page. Cite it with the grade, and add the date you read it.