How do I represent the "-ed" in "witnessed" phonetically?

Question

How do I represent the -ed in witnessed (or any other similar word) phonetically? Is there a reference sheet somewhere for these? I notice that other endings do not get shown in most dictionaries either (for example -es in phrases)

I use Word web which has a pronunciation guide for the root of the word, but I can't find anything to write the -ed part phonetically. It does not seem to be on this Wikipedia page.

I would prefer the same format as Word web. Many people could probably figure this out for themselves (without a direct reference), but I do not trust myself.

Answer 1

I use the (paper-based) Cambridge English Pronouncing Dictionary (hardcover)
It gives you the pronunciation for inflections such as past tense, continuous form, plural. Also, people and places names.
The rule for past tense "ed" of verbs is:

1. "d" after voiced consonant or vowel (sagged)
2. "t" after voiceless consonant (cracked)
3. if the verb ends already in "t" or "d" (panted, faded) it is followed by ɪd or əd.

The plural parallels the past tense:

1. "z" after voiced (bags,pads)
2. "s" after voiceless (trucks,cats)
3. if the word ends up in "s", "z" or a fricative such as "sh","ge", "ch", it is followed by ɪz or əz. (houses,buzzes,crashes)

The reason for adding a vowel is the need for transparency; without it the sequences [s+s],[z+z], [t+t], [d+d] would not be heard well. Historically, -es, -ed were separate syllables in all instances(*); the vowel was lost only in cases where the transparency was not endangered. English Words, P.Stockwell,D.Minkova

_{(*)e.g."...and wordes tho/That hidden pris now wonder nyce and straunge/Us thinketh hem..." from G.Chaucer, Troilus and Chriseyde.}

The same principles hold for possessive: see the answers here. It is nice to see English pronunciation is not so random as thought to be.

Answer 2

-ed after voiceless s is pronounced /t/ in most past tenses and participles, like a regular t. The Oxford English Dictionary gives 'wɪtnɪst.

You could use the OED for common participles. Howjsay has the pronunciation of a great many inflected words in real sound.