Kelp is an uncountable noun.
How can we use kelp?
I ate a kelp.
I ate two kelps.
(Are the expressions correct? How can we count 'kelp'?))
I don't believe kelp is a countable noun, so your first and second sentences are correct.
Your third and fourth sentences are not correct, though. You'd simply say, "I ate kelp," or "I ate some kelp."
Here's a bit more information. Notice how the word is used in here:
http://en.wikipedia.org/wiki/Kelp
http://esl.about.com/od/grammarforbeginners/a/g_cucount.htm
The word "kelp" is indeed an uncountable noun, which means it does not have a plural form or can be used with numbers to indicate a specific count.
To use kelp correctly, you could say:
- "I ate some kelp" or "I ate a serving of kelp" to refer to an unspecified amount or a portion of kelp.
- "I ate a piece of kelp" if you are referring to a distinct piece or portion of kelp.
However, saying "I ate a kelp" or "I ate two kelps" would not be grammatically correct because it treats "kelp" as a countable noun when it is not.
If you want to express a specific amount of kelp, you can use measurement terms such as "a handful of kelp," "a cup of kelp," or "a kilogram of kelp," depending on the context and the unit of measurement you prefer to use.