Organ Postlude on "How Firm a Foundation"

I'm not familiar with the tune , so perhaps I'm a bit confused, esp. with the added countermelody. I might guess it was mostly on the right hand, and in that case I find it rather irregular (despite the regular meter and the emphatic chords that make up what I suppose to be the 'main' tune).

Since I'm no organist, I think you'd benefit a lot from the oppinion of some other members who actually are, esp. in terms of playability.

Actually, the tune starts in the pedals, and stays predominantly in the left hand. The right hand takes the melody at 24 for 4 measures but then doesn't have the melody again until 49 (at which pedals, left, and right hand all have the melody, just in different forms of augmentation and diminution).

Thanks for the input though!

You demonstrate some ideas of how to develop a melody into a longer piece. I would like to have seen a little more contrast in texture, given that the piece is essentially broken up into several different sections each based on a different deployment of the tune.

I am not convinced the pedal writing between 48 and 57 need be so high. The effect could be achieved more comfortably by playing the pedals notated an octave down and coupling them to a manual or using a soft 8' stop.

Avoid using the V symbol to indicate breaks in phrasing (which is what I assume it to mean here) as to an organist it is a pedal marking and will cause confusion. Phrasing should be indicated in the normal way using slurs, or for a breath mark use a comma.

Dynamic markings are needed at the opening. The organist will have no idea if the opening is a loud statement of the tune or a quiet introduction. Some idea of what stop combinations to use is also helpful. This does not have to be an exact list as no two organs are the same but a rough description of the sort of sound you want will do. You must also have all staves present in the score, even if they are resting.

You need to introduce some accidentals or a sense of moving to another key in music with this texture. It sounds after a time as if the music is circling round and round the same combinations of pitches and removes much of the sense of the piece progressing through its course.

on above remarks there is only few to add.

For a first quartal harmony exercise I think its promising. Especially the opening is very strong, and from the very start contains some textural variation (ms 17, swell), the arpergio-figure, which unfortunately does not return.
One minor thing in regard to harmony, I see you still are very grounded in the more common practice triads. Take for instance ms 32 which you let resolve to just old C major. The thing with quartal is that you should consider chord like CGD your new kind of consonants. When you still want the tertiary harmony you could stack multiple quarts. For example I am very fond of the following chord: CG EAD, which is C major with an added 2nd and 6th. This way the quartal and the tertian sound is combined

the melody itself sounds pentatonic, add to that the quartal harmonies, and it sounds somewhat oriental. The ways you used to prevent that are successful, although they sort of undermine the idea of quartal harmony. For this reason I would have picked another melody. But ok, good exercise!