I've just recently been introduced to 'calcium/phosphorus' content in dog food. Momo is 6 yrs old & had a urinalysis, it showed struvite crystals in his urine. The vet said to lower the ash levels in his food which consists of minerals & calcium/ phosphorus.
I don't know much about this to fully understand, but I've read that calcium to phosphorus ratio should be 1:1. The AAFCO recommends .8 Minimum - 1.6 Max Phosphorus in dog food.
Selecting Nutritious Pet Foods
It would be ideal to get a urinalysis, as it doesn't cost much & it's also a good test for a prognosis.