Let's learn some soil pH chemistry!

Jelly,  77 73  71 bytes

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

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

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How?

“ ... »Ḳµ,Ṛ;€"“¡D⁺“a&»j“¿<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&»           - compression of [" alkaline"," acidic"]
             "                   - zip with:
           ;€                    -   concatenate for €ach
                             ¤   - nilad followed by links as a nilad
                       “¿<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<“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]
   <                    - less than? (vectorises)
                        -   i.e.: pH < [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

PHP, 199 bytes

foreach([35,9.5,6,5,5,5,8,5,6,5.5]as$l)$p+=$argn*10>=$s+=$l;$p-=$argn==9;echo[Ultra,Extremely,"Very strongly",Strongly,Moderately,Slightly][$p>6?12-$p:$p],[" acidic",Neutral," alkaline"][1+($p<=>6)];

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C# (.NET Core), 236 bytes

p=>{var l=new[]{70,89,101,111,121,131,147,157,169,180,280};var a="Ultra,Extremely,Very strongly,Strongly,Moderately,Slighty, acidic,Neutral, alkaline".Split(',');int i=0;for(;p*20>=l[i];i++);return i==6?a[7]:i<6?a[i]+a[6]:a[12-i]+a[8];}

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This solution considers that p cannot be greater than 14.