I found this that may help
Onychoschizia or splitting of the fingernails is a common problem seen by dermatologists. The term onychoschizia includes splitting, brittle, soft or thin nails. Onychoschizia is more common in women.
Only very rarely are internal disease or vitamin deficiencies the reason (iron deficiency is the most common). One tip is that if the fingernails split, but the toenails are strong, then an external factor is the cause. Basically brittle nails can be divided into dry and brittle (too little moisture) and soft and brittle (often too much moisture).
The usual cause is repeated wetting and drying of the fingernails. This makes them dry and brittle. This is often worse in low humidity and in the winter (dry heat). The best treatment is to apply lotions containing alpha-hydroxy acids or lanolin containing lotions such as "Elon" (by the "Dartmouth" company) to the nails after first soaking nails in water for 5 minutes.
Wearing gloves when performing household chores that involve getting the hands wet is very helpful in preventing brittle nails. Cotton lined rubber gloves can be purchased in stores.
If soft, consider that the nails may be getting too much moisture or being damaged by chemicals such as detergents, cleaning fluids and nail polish removers (the acetone containing removers are somewhat worse than acetone free). Some feel that once a week application of clear nail prep once a week may help. Nail polishes with nylon fibers in them may add strength.