"Het gewas is klein."
Translation:The crop is small.
Check the meanings available on the OED (I don't know how to take a screenshot with my mobile, sorry, and that's the only device where I have it -I cannot copy/paste from the app) and then you'll see what I meant.
And precisely, if you analyse the meanings you propose (#2, first link and #4, second link), you'll notice that they are not exactly the same. What is more: Meaning number 4 on the second link says that "harvest" means "gathered crops", while normally "crop" means plants that were gown as to be harvested.
Granted: right at the bottom they mention that they can be used as synonyms, but nevertheless there are subtle differences.
On the other hand, if you check on http://www.macmillandictionary.com/thesaurus-category/british/crop-farming under harvest you'll find the following meanings:
harvest (noun): the activity of collecting a crop
harvest (noun): the time when a crop is collected
harvest (noun): the amount of a crop that is collected
harvest (verb): to collect a crop from the fields
Crop is defined as:
"a (1) : a plant or animal or plant or animal product that can be grown and harvested extensively for profit or subsistence an apple crop a crop of wool
(2) : the total yearly production from a specified area
b : the product or yield of something formed together the ice crop
c : a batch or lot of something produced during a particular cycle the current crop of films
d : collection a crop of lies"
While harvest is defined as:
" 1 : the season for gathering in agricultural crops the beginning of the harvest<pre>
2 : the act or process of gathering in a crop assisting neighbors in their harvest 3 a : a mature crop (as of grain or fruit) : yield bountiful harvests b : the quantity of a natural product gathered in a single season the salmon harvest timber harvests 4: an accumulated store or productive result a harvest of revenue"</pre>
So, there is a slight difference: a crop is still on the land, while a harvest is not, since it's been harvested (as redundant as that may sound).