sweeten

verb12

1. •
   To make sweet to the taste.
2. •
   To become sweet.
3. •
   To make more attractive; said of offers in negotiations.

   • to sweeten the deal by increasing the price offered
4. •
   To make (more) pleasant or to the mind or feelings.
5. •
   To make less painful or laborious; to relieve.

   • Our God to bless out home delights, / And sweeten every secret tear:— [...] — 1827
6. •
   To make mild or kind; to soften.
7. •
   To make warm and fertile.

   • to dry and sweeten soils
8. •
   To raise the pH of (a soil) by adding alkali.

   • [T]hey had prepared the garden carefully, plowing and sweetening the dirt with fireplace ashes and manure from the barn[.] — 1997
9. •
   To make pure and healthful by destroying noxious matter.

   • to sweeten rooms or apartments that have been infected
10. •
    To restore to purity; to free from taint.
11. •
    To supplement (a composition) with additional instruments, especially strings.

    • In most popular music the bowed strings usually play long, sustained, sweeping parts, and are sometimes added to a vocal track later in a process known as sweetening. — 2011
12. •
    To soften to the eye; to make delicate.

    • Correggio has made his memory immortal by the strength he has given to his figures, and by sweetening his lights and shadows, and melting them into each other. — 1695