It is possible to use soap for laundry, but there are drawbacks and it requires divorcing oneself from every idea of modern practice.
First and foremost one needs plenty of *HOT* and soft water. Warm will do in a pinch but hot to boiling is better. Degrees of hard water can be dealt with, but once one reaches a certain stage better leaving off using soap and going with detergent. Ditto if water is rich in iron, rust or substances.
Soap on its own will soften hard water but use of it for that purpose is wasteful. Best to determine the hardness of water and then softening by means of either chemical or whole house water softener.
Of the chemical means there are two: non-precipitating(best) and precipitating(not so much).
The first would be complex phosphates, two of the most common are sodium tripolyphosphate (STPP) and sodium hexametaphosphate (STHP). You can read about all such builders here:
http://www.chemistry.co.nz/deterg_inorganic.htm
Where phosphates are used with softening water for laundry it is also best to add same to the first rinse water. This keeps the remaining soap in laundry from binding to the minerals found in the fresh water.
When using soap laundry is best pre-soaked in warm water with a bit of soda added. This "breaks" the oils and light soils from the fabrics. Inclusion of soda raises the pH which helps break the often acid nature of certain soils do they are less likely to interfere with subsequent soap washing. The other method is to do a warm prewash *break* cycle with just water and perhaps soda. This is how commercial laundries do and five to six minutes of such cycle replaces the overnight soaking done in domestic settings.
Washing with soap requires hot water cycles of not very long duration. Better to wash laundry twice for five or six minutes using two cycles of fresh soapy water rather than one long. This helps prevent dirt and soils from becoming re-deposited back onto laundry.
The first and perhaps second rinses should be hot or at least the same temperature was the wash. Many commercial laundries would use boiling hot water for the first rinse. In the domestic setting laundry was often boiled. Reason for this is to flush out soils and soap residue from textile fibers while the weave was still open. Using cold water would cause the weaves to constrict trapping said soils and oils.
When using soap there should be at least two hot then followed by two more warm rinses, then finally one cold. In all there should be about five or whatever amount of rinses it takes to ensure all soap and dirt is removed.
In commercial laundries when using soap for cleaning the subsequent rinses could also be baths for bleaching, rust removal, or bluing. Depending what was being done those processes would perhaps add one or more rinse to the lot.
It cannot be stressed enough that when using soap for laundry one cannot skimp on rinsing. Lack of which is the main reason for residue which leads to all sorts of ills.
Will give the woman in OP this; soap is not particularly suited for modern automatic washing machines, especially domestic H-axis washers. Probably the best machines are the various semi-automatics such as twin tubs or wringer washers.
This is because first they often nearly all had solid tubs, thus removing the worry about deposits between tubs. Next because one lifted laundry out of soapy water and into another tub of hot water you left much of the muck and scum in the first tub.
