Suffixes for verbification: -ify, -icise, -ificate

Short version:

According to my sources, those suffixes share roughly the same set of meanings. The choice seems to be motivated phonologically and/or etymologically.

-ify attaches to:

• Monosyllabic words
• Words stressed on the final syllable
• Words stressed on the penultimate syllable followed by a syllable ending in unstressed /ɪ/

It differs from -ize in that it may be used derogatorily in some cases, as in preachify, Frenchify, etc. Like -ate, it is a Latinate suffix, and formations outside the neo-classical lexicon are “often facetious or pejorative” (via A Comprehensive Grammar of the English Language). According to corpus findings in Longman Grammar of Spoken and Written English, the suffix -ify is the least frequently occurring one out of the three verbifying suffixes with which we are concerned here (-ize, -ate, and -ify). The frequency orders of these suffixes between registers (academic, conversational, fiction, etc) are the same. Meanings (for a more elaborate list of meanings, see -ize):

• make ~ (mainly with adjective bases)
• make into ~ (mainly with noun bases, especially with technical words)

-ificate is either back-formation of -ification or the combination of -ify and -ate, the latter of which is likely to be added to words of Latin origin or modelled on Latin word-formation. It's not a suffix in its own right. -Ate is primarily attached to noun bases, but in rare cases, it can be attached to verb bases as well, and, according to A Comprehensive Grammar, it is especially productive in scientific English (chlorinate, delaminate, etc).

-ize is the most popular suffix for verb-formation in Present-day English (particularly common in academic prose), which makes mostly intransitive verbs, and which can be used to produce words with the following set of meanings:

• provide with ~
• make ~ (generally with adjective bases)
• become ~
• perform ~
• act like ~
• make into ~

It attaches primarily to bases ending in an unstressed syllable. Sometimes, -ize is interchangeable with -ify, though distributed across different styles (syllabize — syllabify). Different specialized meanings for the same base is also possible: liquidize — liquidate — liquefy.

From the Longman Dictionary of Contemporary English:

-ify
1. to make something be in a particular state or condition
2. to fill someone with a particular feeling
3. to do something in a silly or annoying way
4. to make something or someone be like or typical of a person or group

-ise
1. to make something have more of a particular quality
2. to change something to something else, or be changed to something else
3. to speak or think in the way mentioned
4. to put into a particular place

-ate
1. to make something have a particular quality

Some (long) quotations of sources used:

As @Billare noted, there is lots of superfluous information for the sake of producing those quotes verbatim without distorting whatever the authors intended to convey. You may not want to read this all.

Ingo Plag, Word-Formation in English:

-ify
This suffix attaches to three kinds of base word: to monosyllabic words, to words stressed on the final syllable, and to words stressed on the penult followed by a final syllable ending in unstressed /ɪ/. Neologisms usually do not show stress shift, but some older forms do (húmid—humídify, sólid—solídify). These restrictions have the effect that -ify is in (almost) complementary distribution with the suffix -ize. The only, but systematic, exception to the complementarity of -ize/-ify can be observed with the said base words ending in /ɪ/, which take -ify under loss of that segment (as in nazify), or take -ize (with no accompanying segmental changes apart from optional glide insertion, as in toddy[j]ize). Semantically, -ify shows the same range of meanings as -ize and the two suffixes could therefore be considered phonologically conditioned allomorphs [emphasis mine — Vitaly].

-ificate does not seem to me a genuine suffix in its own right, so I am quoting what that book has to say about (related) -ion instead. It doesn't say anything about -ificate or any possible version thereof that comes to mind. It's either back-formation of -ication or -ify + -ate.

-ion
This Latinate suffix has three allomorphs: when attached to a verb in -ify, the verbal suffix and -ion surface together as -ification (personification). When attached to a verb ending in -ate, we find -ion (accompanied by a change of the base-final consonant from [t] to [ʃ], hyphenation), and we find the allomorph -ation in all other cases (starvation, colonization).
-ize
Both -ize and -ify are polysemous suffixes, which can express a whole range of related concepts such as locative, ornative, causative/factitive, resultative, inchoative, performative, similative. Locatives can be paraphrased as ‘put into X,’ as in computerize, hospitalize, tubify. Patinatize, fluoridize, youthify are ornative examples (‘provide with X’), randomize, functionalize, humidify are causative (‘make (more) X’), carbonize, itemize, trustify and nazify are resultative (‘make into X’), aerosolize and mucify are inchoative (‘become X’), anthropologize and speechify are performative (‘perform X’), cannibalize, vampirize can be analyzed as similative (‘act like X’).
The suffix -ize attaches primarily to bases ending in an unstressed syllable and the resulting derivatives show rather complex patterns of base allomorphy. For example, bases are systematically truncated (i.e. they lose the rime of the final syllable) if they are vowel-final and end in two unstressed syllables (cf. truncated vowel-final mémory—mémorize, vs. non-truncated consonant-final hóspital—hóspitalize). Furthermore, polysyllabic derivatives in -ize are not allowed to have identical onsets in the two last syllables, if these are unstressed. In the pertinent cases truncationis used as a repair strategy, as in feminine—feminize and emphasis—emphasize.

The Oxford English Dictionary:

-ise
Suffix of ns., repr. Old French -ise, properly:—Latin -ītia, but also, in words of learned formation, put for Latin -icia, -itia, -icium, -itium, as in Latin justitia, judicium, servitium, Old French justise, juise, servise. Hence it became a living suffix, forming abstract nouns of quality, state, or function, as in couard-ise, friand-ise, gaillard-ise, marchand-ise. In the words from Latin, -ise was subsequently changed in French to -ice, as in justice, service, in which form the suffix mostly appears in English, as in justice, service, cowardice; but -ise is found in franchise, merchandise, the obsolete or archaic niggardise, quaintise, riotise, truandise, valiantise, warrantise, and in such barely-naturalized words as galliardise, gourmandise, paliardise; also, in exercise, French exercice, Latin exercitium. Native formations on the same type are inconvenientise, sluggardise.

-ify
The usual form of -fy suffix when combined with English roots, as acidify v., Frenchify v., personify v., etc. Etymology: In loanwords (directly or indirectly) from classical Latin with this ending the -i- belongs to the stem of the preceding element. Formations on English words (in some cases ultimately of Latin origin) are found from at least the 16th cent., e.g. gracify v., firmify v., foolify v., neatify v.

-fy
<…skipped a text wall of somewhat unrelated text wall text…> It is now used as the regular rendering of -ficāre in new words adopted from Latin or formed on assumable Latin types, and is also freely added to English adjs. and ns. to form vbs., mostly somewhat jocular or trivial, with the senses: ‘to make a specified thing’, as speechify; ‘to assimilate to the character of something’ (chiefly in pa. pple., as countrified); ‘to invest with certain attributes’, as Frenchify. (A large proportion of these vbs are from ns. and adjs. ending in -y or -ey, the suffix then having the form -fy instead of the usual -ify. An early example is beautify, but the analogy on which this word was formed is not clear.) In a few cases the suffix has been quite irregularly added to vb. stems, but the words are either obsolete, as dedify, hindrify, ornify, or merely jocular or illiterate, as argufy. The noun of action related to vbs. in -ify normally ends in -ification, though, by confusion of suffix, petrifaction is used in English where French has more correctly pétrification. The words in which -fy represents Latin -facĕre having their corresponding nouns of action ending in -faction.

The Cambridge Grammar of the English Language:

-ate
This suffix has never been widely used in the creation of English adjectives. Such words as celibate, fortunate, intricate are loans, not English formations. There are numerous learned words using classical bases (corporate, degenerate, geminate), but the clear English formations are based on nouns ending in -ion: affectionate, extortionate, passionate.
The vast majority of verbs in -ate, if not direct loans from Latin, were based on Latin forms rather than on English ones. Although we may recognise the -ate in such words as alleviate, equate, locate, etc., the bases are bound and can be found only in other words borrowed from Latin or founded firmly on Latin models. Even those words which appear to have English bases are frequently, from a historical perspective, either back-formations from nouns in -ation, (orientate, vaccinate) or based on Latin (captivate, domesticate). Nevertheless, there are some genuinely English formations mainly from noun bases, such as hydrogenate, hyphenate, orchestrate, or (with adjective base) activate. This suffix has also been used in the production of mock-learned words such as absquatulate and discombobulate. In addition, there are a very few verbs where -ate occurs with a verb base, as in fixate or prolongate; again, however, such verbs probably did not arise by the affixation of -ate to the bases fix and prolong, but by back-formation from the nouns fixation and prolongation.

-ify Like -ate, this suffix appears mainly with Latinate bases, even if the words were coined in English rather than Latin. With adjective bases it usually means “make ~”, as in humidify, purify, simplify. Such verbs as falsify and justify have gained other meanings through lexicalisation. With noun bases, the meaning is generally "make into ~", especially with technical words, such as mummify and personify. Other meanings can, however, be found in this set—for example, in beautify, classify, glorify. In some words the suffix has a clearly derogatory flavour, as in countrify, Frenchify, speechify, preachify. The last of these, perhaps modelled directly on speechify, is very unusual in being formed from a verb base (but cf. also the relatively recent scarify, “make scared”). The -ify suffix has never been used to make many words, but is still productive: witness such neologisms as yuppify. A form -fy is recognisable in a number of loans, such as liquefy, rarefy, satisfy, stupefy (all of which form nouns in -faction), but the form used in English word-formation is always -ify (replacing -y in cases like beautify).

-ise, -ize These are variant spellings of a single suffix; in BrE both are widely used, AmE has -ize, while AusE and NZE increasingly prefer -ise. The variation does not apply to words ending in an ise or ize that is not a suffix but part of the base; there are a considerable number of such words in ise (e.g. advertise, advise, circumcise, comprise, despise, exercise, improvise, surprise, televise, etc.), but very few in ize (capsize, size).
This is the most productive suffix for forming verbs in Present-day English; relatively recent examples include colourise, computerise, walkmanise. So productive is it, indeed, that prescriptive criticism is levelled against what some perceive as the unnecessary proliferation of -ise verbs.
Most -ise verbs are transitive, but we also find intransitives such as deputise, philosophise, theorise. With adjective bases the meaning is typically “make ~”: equalise, italicise, legalise, liquidise, urbanise. Often, however, there are more specialised meanings, as in penalise, nationalise, visualise. With noun bases, there is no single generalised meaning: compare anthologise, burglarise, computerise, hospitalise, idolise, itemise, pasteurise, scrutinise, standardise, terrorise, etc. Noun bases bases drop final -y, as in apologise, colonise, economise; there are also cases where -ise attaches to a bound base (or a bound form of one) and can be seen as replacing -ic: dramatise, systematise, hypnotise. The -ise suffix is in competition with other verbalising processes, and with some bases we find different formations with the same meaning (legitimise/*legitimate*, syllabise/*syllabify*) or with contrasted meanings (equalise, equal, equate).
This suffix is one of those that does not behave consistently as a Class I or Class II suffix. It is like a Class I suffix that regularly comes before -ation (as in marginalisation) and causes the base-final /n/ of solemn to be pronounced (contrast solemnly, with Class II -ly), but it is like a Class II suffix in that it can follow -er (as in containerise) and is normally stress-neutral (compare masculinise and masculinity, with Class I -ity).