Admixture is a minor ingredient, a smaller part of mixture, added to mixture
Mixture is irrelevant of ingredients proportions, and is one whole mixture.
In your quote, the author wants to stress that every single component that was added, plays its role in the final impression. To make such an emphasis, 'admixture' works better.
Technical example from Federal Highway Administration:
(I am giving a technical context because in this case 'admixture' is the only acceptable term, and is absolutely different from 'mixture')
Chemical admixtures are added to concrete in very small amounts mainly
for the entrainment of air, reduction of water or cement content,
plasticization of fresh concrete mixtures, or control of setting time.
Seven types of chemical admixtures are specified in ASTM C 494, and
AASHTO M 194, depending on their purpose or purposes in PCC. Air
entraining admixtures are specified in ASTM C 260 and AASHTO M 154.
General and physical requirements for each type of admixture are
included in the specifications.
An article on Yourdictionary.com packed with lots of other context examples (follow the link).
Most dictionaries do not explain the difference, I found only two that mention that admixture as 'a minor ingredient':
The Oxford Pocket Dictionary of Current English (2009)
ad·mix·ture / adˈmikschər/ something mixed with something else,
Collins dictionary article is good for giving synonyms and clearly splits admixture in two meanings, 'mixture' and 'component':
- mixture, combination, compound, blend, fusion, alloy, medley, amalgamation, meld, intermixture
- His heart beat with an admixture of aversion and thrill.
- ingredient, element, component, constituent
Example from the question: (mixture or admixture)
The 'Natura' section features a series of sixteen prose poems,
redolent in many ways of Hill's Mercian Hymns in their admixtures of
natural and urban landscape, dramatised autobiography and historical
reference, fomal and colloquial styles.
In this case replacing 'admixtures' with 'mixtures' would not be a grammar mistake but it would shift the emphases. The author refers to different components of the Mercian Hymns, and enumerates them. But, if 'mixtures' were used, the emphasis would shift towards hymns themselves rather than components. The description would become less poetic.