I figured I'd share what I've been working on. This is just a draft, but should help with the Astragalus IDs. This includes only the Texas material and will likely need a couple of New Mexico species added before this is complete. Also, I have not included the key to varieties of A. mollissimus yet. Distributions are based on Turner et al. 2003 (Atlas of the Vascular Plants of Texas).

Robust white-flowered species
,1. Plants glabrous or nearly so............A. praelongus
,1. Plants hairy..........................................2
,2. Hairs on calyces appressed, not bluish; pedicels attached at the middle of flat, proximal part of calyx; leaves green; adaxial side of leaves glabrous or nearly so......A. racemosus
,2. Hairs on calyces not appressed, bluish; pedicels attached at the base of flat, proximal part of calyx; leaves silvery; adaxial side of leaves hairy......................S. nuttalliana

Prostrate perennial species
,1. Plants glabrous or nearly so.......A. lentiginosus var. higginsii
,1. Plants hairy.......................2
,2. Leaflets of apical leaves more than 2.5x longer than wide....3
,2. Leaflets of apical leaves less than 2.5x longer than wide....4
,3. Plants producing many long creeping stems; apical leaves with strongly conduplicate leaflets.......A. crassicarpus var. crassicarpus
,3. Plants appearing nearly caespitose with few very short stems; leaflets conduplicate or not....A. lotiflorus
,4. Leaflets glabrous or nearly so adaxially; stems shaggy..............A. plattensis
,4. Leaflets densely hairy, silvery; stem hairs various.................5
,5. Plants strictly and regularly strigose; hairs appressed......................A. missouriensis
,5. Hairs shaggy, if appressed, some hairs not so producing a silvery effect.....A. mollissimus

A. plattensis and A. crassicarpus are very close and should be keyed in the same couplet once I figure out the finer distinctions.

Key by fruits
,1. Fruits glabrous or nearly so..........2
,1. Fruits hairy..........................10
,2. Fruits at least 3x longer than wide..........3
,2. Fruits less than 3x longer than wide.........6
,3. Fruits trigonous, not falcate; plants erect perennials with white flowers.......................................................A. racemosa
,3. Fruits not trigonous, falcate at least proximally; plants prostrate annuals with purple to occasionally white flowers (usually due to fading)........................................4
,4. Fruits not prominently grooved abaxially, appearing somewhat flattened; banner purple, wing petals white for apical 1/2 to 2/3rds, not purple tiped; plants large, prostrate.............A. lindheimeri
,4. Fruits prominently grooved abaxially, rounded in cross-section; banner purple to lavender or nearly white, wing petals usually white with purple or lavender stripe on ventral margin extending to tip, sometimes entire tip purple or purplish; plants small, usually ascending early on and becoming prostrate later.............5
,5. Leaves truncate or emarginate.....A. nuttallianus
,5. Leaves not as above...............A. austrinus var. austrinus
,6. Fruits mottled; leaves slenderer than A. gracilis; likely absent in the area.......A. ceramicus var. filifolius (likely absent)
,6. Not as above..............................................................................7
,7. Fruit narrowing proximally nearly to a stipe; robust, white-flowered plants...........A. praelongus
,7. Not as above..........................................................................8
,8. Fruits strongly falcate, not inflated.............A. mollissimus
,8. Fruits either not falcate, or inflated............9
,9. Fruits papery, not pigmented at maturity; plants essentially glabrous.....A. lentiginosus var. higginsii
,9. Fruits not papery, often pigmented; plants strigose.......................A. crassicarpus var. crassicarpus
,10. Fruits tiny (less than 5 mm long), roughly oblong-oval, and numerous along the inflorescence, often more than 10.....A. gracilis
,10. Fruits larger (more than 5 mm long), various in shape, and fewer, sometimes numerous usually less than 10............11
,11. Plants spring ephemerals; fruits linear..........A. austrinus (var. trichocarpa or var. austrinus depending on hair density)
,11. Plants larger perennial; fruits not linear...........12
,12. Fruits narrowing proximally to a stipe............Sophora nuttaliana
,12. Fruits not narrowing proximally...................13.
,13. Immature fruits obscurred by the corolla; mature fruits usually halfway obscured by the sepals....................Oxytropis lambertii
,13. Not as above; calyces obscuring less than a quarter at maturity or absent altogether.........................14
,14. Fruit hairs not appressed............A. lotiflorus (in part; cleistogamous plants)
,14. Fruit hairs appressed................15
,15. Mature fruits less than 2x longer than wide (not including style)..........A. plattensis
,15. Mature fruits more than 2x longer than wide (not including style)..........16
,16. Fruits gradually curved abaxially; leaflets produced during flowering usually oval or eleptical, less than 3x longer than wide...............................................A. missouriensis
,16. Fruits straight for first 2/3rds then curving sharply abaxially; leaflets produced during flowering usually narrowly elliptical, more than 3x longer than wide.............A. lotiflorus