"The soup is a little salty" - means the soup is saltier than the average. I heard this before so I guess it is correct. What is the part of speech of "a little"?
Can we say "the soup is little salty", meaning that the soup is not salty enough? Also, what is the part of speech of "little"?
The soup is a little salty.
The sentence is grammatical; you can use "a little" as an adverb to mean slightly or to a small degree in front of the adjective salty. This usage is a little formal. You can use "a bit" instead in informal spoken English.
On the other hand, you cannot use "little" with the adjective salty. You don't use it with an adjective except for some adjectives. A couple of examples are as follows:
He's little known as a journalist.
His voice was little more/better than a whisper. (The Free Dictionary)
Can we say "the soup is little salty", meaning that the soup is not salty enough?
No, we cannot because the adjective salty already means: tasting of, containing, or preserved with salt. Moreover, "little" and "bit" are normally used with the indefinite article with adjectives, their function is to "soften" a statement; e.g. a little hot, a bit spicy, and a little bit strong. Without the article, little, few, and bit, carry a more negative meaning; e.g. I have little money (I don't have much money) vs. I have a little money (I have 'some' money).
If I say a drink, or a snack is sugary, I am stating that it contains enough sugar for it to be noticeable.
A bit sugary suggests that I consider it to be on the sweet side.
Colloquially, native speakers also say something is "a little bit sweet".
Too much sugar means it is too sweet for my taste buds, some might describe it as sickly-sweet; therefore, the expressions a little sugary means something is on the sweet side, and overly sweet. Similarly, a little salty means it contains enough amount of excessive salt for it to be "spoilt".
"A little salty" could also be a gross understatement, or used in polite company. In thie former case, the speaker could be implying that the food is almost inedible, but only context will tell us for sure what the real situation is.
To suggest that food is lacking in salt, we can say it in several ways, using the NOUN salt
What do we call "a bit", "a little bit", etc? The British Council website tells us that they are known as mitigators, others say: hedging, or approximators.
Mitigators are the opposite of intensifiers ...
We use these words and phrases as mitigators:a bit - just a bit - a little - a little bit - just a little bit - rather - slightly
References
I believe "a little" is an adverb of extent in the same way that "a lot" is.
"The soup is little salty." does not sound correct to me (and I'm quite confident it is not grammatical). If I heard it, I would not know whether the speaker meant that the soup was too salty or wasn't salty enough.
A possible source of confusion might be what TRomano brought up in the comments. The expression "little known" is common and completely grammatical. This is because "known" is behaving as a noun in this context and "little" is modifying it as an adjective.
A seemingly valid way to think about it is that "a little" is an adverb form of "little".
