There has been a lot of speculation among growers over the years concerning the compost N. northiana requires to grow well. There are those who have insisted that limestone, antimony, or other elements or compounds are required for its successful cultivation.
Well, this much I can tell you: I have tried none of those things, and my plant is 1.3 m in diameter. It has produced pitchers between 35 and 40 cm in height, and it flowered for me last month. In short, it is a fully mature plant.
I grew it from a small TC specimen, no more than 1 cm in diameter, over a period of about five years. To do so, I grew it in a well-drained mix of peat, perlite and other typical compost materials. (I don't remember what all I used off-hand, but the details are in an article I wrote which was published in CPN last summer. If you want more information, please read it.)
In addition to compost, successful cultivation requires that this species be given very warm, humid conditions, with little variation. Furthermore, lighting should be somewhat subdued, for it is easily burned.
If you are really serious about growing this species, please be sure that you can provide these conditions, for failure to do so will almost certainly result in a plant which will grow poorly. Moreover, be sure that you have room for N. northiana, for it will grow to be very large indeed, and you will quickly run out of room! If you cannot do these things, I highly recommend that you elect to grow another of the many interesting species available, for I doubt that neither you, nor your N. northiana, will be happy....