Thank you for the information,
Theer are some common causes of such penile itching.The common ones are irritation to soap, dermatitis, balanitis, psychogenic, psoriasis, etc.
Since you were tested negative for STD so i advise you to have the relevant investigations done like repeat urine routine examination, urine c/s, blood counts,etc.after consulting your urologist to know any associated abnormality as the infection are common causes of such itching.
You should also see that you are not allergic to any soap which you apply as the allergy can also cause such penile itching.The other cause of such itching is dermatitis.The treatment of allergy is avoidance of irritants and use of antihistamine drugs like benadryl, pheniramine, etc.
The other causes are stress, psychogenic, etc. which needs the treatment accordingly. You should see that you drink more water and avoid dehydration.
The treatment depends on the underlying cause.
Please visit here for more information:
causes of penile itch
Let me know if you have any doubt.