Yes, phosphates are surely better to use, I did not mention them as generally they're not easy to be found...
Over here especially.
Your best bet is over ebay and internet, hardwares and such..under the full name.
Sodium tripolyphosphate, or sodium triposphate.....these are the two (actually one same thing, just called differently) that best suits the detergent water softening ingredient application, even though, also trisodium phosphates finds/found application in detergents and cleanng, IIRC this last one is a stronger type.
http://en.m.wikipedia.org/wiki/Trisodium_phosphate
http://en.m.wikipedia.org/wiki/Sodium_triphosphate
Baking soda works fine if used correctly as for washing soda, but as mentioned and especially the last may get problematic if too much is used and you are not practical about, as it will give problems of precipitation build up....so the classical white streaks on darks, chalky marks and stiff laundry due to build up imprisoned in fabrics, ...classical result of crappy ( badly formulated) powder detergents, so ones which are all about soda.... And not containing, or at best, very few of the alternative *better* builders and water softeners.
If in a correct amount and in well formulated detergents these alkali do not give probs anyway.
Alternative builders and w softweners:
Powder detergents, do not contain only washing soda or baking soda for water softening function, once as Stan said they relied on phosphates that also played a cleaning function, but today have been replaced and taken out in the United states even by dishwasher products, this not because alternative ingredients are thought working better, but because phosphates have been held responsible for algae growth in rivers, lakes and such, long story, if you type stpp or phosphates in the website search box you'll surely get many info from past posts and threads, as mentioned they have been replaced partially by washing soda (sodium carbonate) and alkali in general, and partially by zeolites and carboxilates, also phosphonates (which are not related to phosphates) which are found in greater quantity and especially in the *better, top of the line detergents*...these are also the main ingredients found in laundry water softener such as Rain Drops or Calgon etc (once they were all stpp).
They will also potentially produce precipitation versus the phosphates, but not like the soda, plus they will also be more easy to be rinsed than soda.
Cheap powders tends to have more soda than the rest, or even totally exclude these other water softeners, this because policarboxilates and zeolites are more expensive than soda.
If you can get your hands on some stpp then I strongly reccomend you to use it, otherwise baking soda or alkali should be fine....as long as you do not use too much of them.
If you think it may be best, you can also try to go with some calgon.
Most baking soda brands as for the leading Arm and Hammer box it should state the various use and dosifications for cleaning, laundry, deodorization, carpet cleaning etc....and depending on your water hardness you should be able to detect which is the amount that will work for you pretty easily.
[this post was last edited: 7/30/2014-09:07]