I think you should to try to adjusting the pH after autoclave, and for avoid contamination in hoagland solution you can use fungicide to avoid fungal contamination  

Dear Waseem,

We make Hoagland solution regularly and don't encounter the problem of precipitation. It ihas very low concentrations of elements, so it should not precipitate. We have a 10x stock solution in fridge which we dilute when the working solution is needed.

It is generally circulated using a water pump in hydroponic systems, and that allows inserting filters to trap microbes. Usually the stagnant solution allows build up of fungi and bacteria. Do you heat the bottle and mix using a magnetic stirrer during the preparation? It may help overcome precipitation if you don't.

I will send you the paper we used to prepare the solution for growing Arabidopsis (when I find it).

I have attached the paper I mentioned before. Here is a sentence on how the water pump works:

Commercially available fountain water pumps were used to deliver nutrient solutions to the planters from the reservoirs beneath the planters at a flow rate of 700 mL min−1. Plants in both the control and test planters were automatically watered once a day in the middle of the 14-h light cycle for 15 min using analog trip timers thus delivering approximately 10.5 L of the respective nutrient solution. The solution drained into the 57-L reservoir below each planter whereby the solution was re-circulated for the duration of the growing cycles. Ten liters of fresh deionized water was added to each nutrient tank every 2 weeks to replace evaporative loss

Thanks

I will try to follow the guidelines you all mentioned. Let see