Let's learn some soil pH chemistry!
Jelly, &nbsp;77 73&nbsp; 71 bytes

“£RĿÐƭðṚ°ƲṂṾẒ=ʂXḣsịɠ&lt;»Ḳµ,Ṛ;€"“¡D⁺“a&amp;»j“¿&lt;z»W¤ṙ3
×20&lt;“FYeoy³ƓɗʋṆ‘Sị¢⁾. y


A monadic link taking the number and returning a list of characters; or a full program printing the result.

Try it online!

How?

“ ... »Ḳµ,Ṛ;€"“¡D⁺“a&amp;»j“¿&lt;z»W¤ṙ3 - Link 1, name list: no arguments
“ ... »                          - compression of "Ultra Extremely Very.strongly Strongly Moderately Slightly"
       Ḳ                         - split at spaces
        µ                        - monadic chain separation, call that adjectives
          Ṛ                      - reverse adjectives
         ,                       - pair these two lists
              “¡D⁺“a&amp;»           - compression of [" alkaline"," acidic"]
             "                   - zip with:
           ;€                    -   concatenate for €ach
                             ¤   - nilad followed by links as a nilad
                       “¿&lt;z»     -   compression of "Neutral"
                            W    -   wrap in a list
                      j          - join
                              ṙ3 - rotate left by 3: ["Strongly alkaline","Moderately alkaline","Slightly alkaline","Neutral","Slightly acidic","Moderately acidic","Strongly acidic","Very.strongly acidic","Extremely acidic","Ultra acidic","Ultra alkaline","Extremely alkaline","Very.strongly alkaline"]

×20&lt;“FYeoy³ƓɗʋṆ‘Sị¢⁾. y - Main link: number, pH
×20                     - multiply by 20
    “FYeqy³ƓɗʋṆ‘        - code-page indexes = [70,89,101,111,121,131,147,157,169,180]
   &lt;                    - less than? (vectorises)
                        -   i.e.: pH &lt; [3.5,4.45,5.05,5.55,6.05,6.55,7.35,7.85,8.45,9]
                S       - sum
                  ¢     - call last link (1) as a nilad
                 ị      - index into (1-indexed and modular)
                        - ...note that the sum is never 11 or 12, so "Ultra alkaline" and
                        -    "Extremely alkaline" wont be fetched, but that a sum of 0
                        -    fetches "Very.strongly alkaline", as required.
                   ⁾.   - literal list of characters ['.', ' ']
                      y - translate (replace any '.' with a ' ' i.e. for "Very.strongly")
                        - if running as a full program, implicit print