Negative interest rate and land
By neutral, the author probably refers to something that can hardly be influenced by external forces. Money can be affected by market forces and by central banks, and the consequences are variations on inflation and exchange rates. However, if you have gold or land, these will hardly lose their relative value, at least that was the idea in the 50s-60s. 

Coming back to the negative interest rates, for a rational investor that would need to decide between investing his money in return for a negative interest rate, or use the money to buy gold or land, which yield a neutral or positive interest rate, he/she would take the latter.