The sentence is correct but it's unusual to see this phrasing in a newspaper. You can interpret "This when..." as meaning "This thing that I have just mentioned is occurring when..." It emphasizes that the thing is occurring even though the next thing is true.
For example,
Soundings taken by the British gunboats in their ascent in 1900 gave 63 1/2 fathoms of water in two places, and this when the water at Ichang was rather less than 6 feet above zero mark!
means "...and the soundings taken by the British gunboats did this when..."
Or
...when taking a journey, he arms himself, and seeks to go well accompanied; when going to sleep, he locks his doors; when even in his house he locks his chests; and this when he knows there be laws and public officers...
means "...and he does all these things when he knows (etc.)"
So in the newspaper article, the two sentences
We have not seen national will on a similar scale since. This when we urgently need an agricultural initiative comparable in its transformative capacity today.
can be interpreted as
We have not seen national will on a similar scale since, and this is a fact even though we urgently need an agricultural initiative comparable in its transformative capacity today