Optimum pH range for formation of Schiff bases What is the optimum range of pH for the formation of Schiff Bases by the addition of hydroxylamine to an aldehyde or a ketone?

According to Peter Sykes' A Guidebook to Mechanisms in Organic Chemistry,

"The initial addition step or the dehydration step can be made rate limiting at will, depending on the pH of the solution. At neutral and alkaline pHs it is generally dehydration, that is the slow and rate limiting step while at acidic pHs it is the initial addition which is slow and rate limiting. This clearly has significance in preparatory terms, and formation of such derivatives of carbonyl compounds tends to exhibit pH optima."

My doubts are : 


How does the pH change the rate limiting step? That is, why is dehydration the rate limiting step at neutral and alkaline pHs and why is addition the rate limiting step at acidic pHs?
What is the significance in preparative terms that Sykes is taking about?
How will this significance determine the pH and what is that pH (or the range of pHs)?
Dehydration requires protonation of the hydroxy intermediate, this clearly is slower at neutral or alkaline pH. Hydroxylamine is basic so at acidic pH it protonates making it non-nucleophilic hence lower pH slows the rate. 
Time taken for the reaction to go to completion. If the rate is slowed then other competing reactions (e.g. self-condensation) can and do reduce your yield.
The pH will vary for each substrate and needs to be experimentally determined. This is where a literature search saves you a lot of grief.