I started solids at almost 5 months, started with avocado and baby milk, the amount of nitrates in water is safe, safe for babies too! (think formula feeding babies?) But, some vegetables and fruits contain higher amounts of nitrates, some of them even heavy metals, like sweet potato, rice, carrots, fish.. so you should consider giving these foods in consideration, like avoid baby rice (better oats, or other grains), later when you move to finger foods such as puffs and teething biscuits, see for high amount of rice (due to high in arsenic) fish - mercury etc.. there is a recent study on baby food that talks about it, google it, i am sure you will find it useful. But to answer your question, use water from tap (freshly boiled preferably till 6month), use breast milk, cows milk or other milks for cooking only (ask your pediatrician). Use root vegetables in moderation (i mean don't feed a child sweet potato all week) buy organic where possible. When you boil, stream root vegetables, nitrates goes to water, so don't use that water for puree making, beter add fresh water or milk, but use cooking water used for steamed veg and fruit that has less nitrate amounts as that water gets lots of nutrients (think of broccoli, pears.. etc..) some leafy greens has big amounts of nitrates too.. kale, spinach etc, but some nitrates your body needs, and if you have a good balanced variety of food, your LO will be fine!. happy growing.