Depending on how many clerics you include in the army, and how many of them are 5th-level or higher, Create Food & Water may or may not be useful enough. At 3rd-level it'll support 9 soldiers a day, with each casting. Most 5th-level clerics will only have 13 or 14 Wisdom (remember 16s, 17s, and 18s are exceptional individuals, not average fellas), so they'll only get 1 CF&W per day. Now, if you have 1 5th-level cleric per 8 other soldiers, you could sustain the army indefinately, if the army has no horses or pack animals to support. However, while this may be useful financially for the army's nation or whatever, it does almost nothing to help the army in actual combat; a well-fed but undergunned or undermanned army is not much use unless they're super-skilled or something.
*5th-level cleric*
0-level: Cure Minor Wounds x2, Purify Food & Drink x3
1st-level: Magic Weapon (D), Bless x2, Bane
2nd-level: Spiritual Weapon (D), Sound Burst, Sound Burst
3rd-level: Magic Vestment (D), Summon Monster III
Summon 1d3 lemures with SM3, and they'll mow down enemy troops with impunity thanks to their damage reduction. Most grunts won't have magic weapons, and most grunts don't have 16+ Str, Power Attack, and a greatsword (or even just one of those).
*3rd-level cleric*
0-level: Cure Minor Wounds x2, Purify Food & Drink x2
1st-level: Magic Weapon (D), Bless, Bane
2nd-level: Spiritual Weapon (D), Sound Burst
Many of the low-level spells aren't so useful in mass combat. Darkness et al requires an object and then you'd have to chuck the object at the enemy, despite the darkness encompassing you at first, plus the enemy formation could just move out of the way on the next round, ultimately making for maybe a 1-round distraction of some troops. Obscuring Mist is centered on the caster, so no help there. Wind Wall would only reach 25 feet high and 50 feet across, so it won't be much impediment to enemy archers (especially since the archers will be firing upward, lobbing arrows over to your troops indirectly from above). Silence just won't help often enough. Calm Emotions ends when the subjects are damaged. Prayer is horribly short on duration and doesn't extend very far, so it's inferior to Bless most of the time. Single-target spells are wasteful in mass combat. Enthrall is broken if the subjects are attacked. Etc.
During transit, of course, the clerics can prepare lots of Create Water, Purify Food & Drink, and Create Food & Water spells. But once the day of battle is nearly upon them, they should prepare to fight. Unless they're evil-aligned or something, they can probably convert spells to Cures when needed.