Can I use amount with plural nouns

Question

According to grammar, amount should only be used with uncountable nouns. But in the following TOEFL TPO question, it is used with plants.

Are they used incorrectly?

Second, the production of significant amounts of ethanol would dramatically reduce the amount of plants available for uses other than fuel. For example, much of the corn now grown in the United States is used to feed farm animals such as cows and chickens. It is estimated that if ethanol were used to satisfy just 10 percent of the fuels needs in the United States, more than 60 percent of the corn currently grown in the United States would have to be used to produce ethanol. If most of the corn were used to produce ethanol, a substantial source of food for animals would disappear.

Answer 1

'amount' can be used with countable nouns

From Oxford Learner's Dictionary:

Amount is most often used with uncountable nouns: an amount of cash/space/material/food It is also sometimes used with countable nouns

It may be a rule in your curriculum or classroom to use amount with countable nouns as a matter of developing your habit, but that does not mean it is an accurate rule for all grammar. Often times for example, it is my experience rules in an ESL classroom are intended to cultivate habits, but not represent all nuances of grammar.

Answer 2

In this case, 'plants' is an uncountable noun because in the referenced industrial usage, 'plants' are not counted by specific numbers.

I need 10 tons of corn stalks (plants) for the herd this year.

You would not say

I need 75,000 corn stalks for the herd this year.

I mean, you could, but it just doesn't work like that.

Agriculture, like other industries has its nuances. For example, it would not be unusual to hear

My nursery produces 25,000 roses (another plant) every February.

because they are sold as individual roses.