When you translate the original sentence as

El arroz escaso de sal.

it lacks the verb. The verb is what tells you the action, and is one of the key components of a sentence. Imagine that I said

El niño hacia el columpio

Without a verb you wouldn't know if I mean "mira" or "corre" or "camina", etc. I think your confusion comes with the use of "is" and "lack".

To enhance the latest translation, it could be

The rice lacks salt.

or

The rice is needing some salt. / needs salt

or even

The rice is bland.

I understand that "is lack of salt" makes no sense in English, but "escaso" is no verb, that is why you need a verb in "El arroz escaso de sal". The tricky thing is that you "forced" an "is" in your translation into English, making a redundant "is lack of".

When you translate the original sentence as

El arroz escaso de sal.

it lacks the verb. The verb is what tells you the action, and is one of the key components of a sentence. Imagine that I said

El niño hacia el columpio

Without a verb you wouldn't know if I mean "mira" or "corre" or "camina", etc. I think your confusion comes with the use of "is" and "lack".

To improve the translation, it could be

The rice lacks salt.

or

The rice is needing some salt. / needs salt

or even

The rice is bland.

I understand that "is lack of salt" makes no sense in English, but "escaso" is no verb, that is why you need a verb in "El arroz escaso de sal". The tricky thing is that you "forced" an "is" in your translation into English, making a redundant "is lack of".