My understanding has always been that when one noun is used to describe another, the second noun takes an -(s)i suffix UNLESS the descriptive noun is used to indicate what the second noun is made from.

For example:
köpek çanakı - dog bowl
plastik tabak - plastic plate

But I see recipes that do not follow this rule.

For example:
işkembe çorbası
In this example, the çorba is made out of the işkembe so why does çorba take the -sı suffix?

http://www.turkishclass.com/forumTitle_6_5894_3

In this thread I have discussed this issue today.

I have read your question again. Actually it is different than what I explained before.

I. " noun + noun " is a noun modification (isim tamlaması
They are devided into two groups as defined and undefined. We will see this below.

II. "adjective + noun " is "sıfat tamlaması" and I don't know the English term. These don't take any suffixes.


I. noun modifications:

okul : school
bahçe : garden

1. okul bahçesi : school garden
2. okulun bahçesi : the school's garden

#1 is undefined noun modification. #2 is defined noun modification. We use defined when it is certain modifier and when it is defined both nouns take suffixes.

A noun modification is used to name an object with two nouns. It exists in English as well.

bus stop : otobüs durağı
web site : web sitesi
eye drop : göz damlası

II. "adjective + noun" phrase.
red light = kırmızı ışık
beautiful girl = güzel kız
silly boy = aptal çocuk

As you see the reason for "plastik tabak" having no suffix is the first word being an adjective.

summary:
undefined noun + noun > A + B+(s)i
defined noun + noun > A(n)in + B+(s)i
adjective + noun > A + B

Ah yes......of course "metal" in "metal box" or "plastic" in "plastic plate" are adjectives......I took the content of this lesson without questioning it - silly me

Obviously when it says:
Two nouns form a clause in three different ways in Turkish:

Case1:
The first noun tells what the second noun is made of (i.e. metal box, plastic plate...). In this case, you just write these nouns in the same order as you do in English without adding any suffixes.

metal box ==> metal kutu

plastic plate ==> plastik tabak
It is not talking about noun clauses at all :-S