The soil may have been brackish, but this wasn't their main setback.
The Jamestown colonists didn't even attempt to plant crops for several years after their arrival. Their first ship brought jewelers and smiths to work the gold they assumed they'd find, but didn't have a real plan for agriculture. The majority died of starvation and disease, but the survivors were sustained by meager leftover travel supplies from newly arriving ships, and by raiding neighboring natives for their corn.
Less than a decade later, separatist Pilgrims landed in New England, and by contrast, grew crops immediately, and cultivated diplomatic relations with their neighbors. The Pilgrims settled in a higher latitude with a shorter growing season, but during their first drought they had already stored enough supplies to share with local natives.
Jamestown could have been on a similar footing if they'd prioritized survival and diplomacy over finding treasure for the crown, the chartering company, and themselves.
>The Jamestown colonists didn't even attempt to plant crops for several years after their arrival
Source? I'm pretty sure they planted corn and wheat as soon as they could, in the first month of arrival. "The 15th June we had finished our fort... we had also sown most of our corn on two mountains. It sprang a man's height from the ground." https://en.wikipedia.org/wiki/Edward_Maria_Wingfield
By the third year (1609) they had cleared and planted at least 40 acres https://www.nps.gov/parkhistory/online_books/hh/2/hh2b2.htm
> Less than a decade later, separatist Pilgrims landed in New England, and by contrast, grew crops immediately, and cultivated diplomatic relations with their neighbors.
And, as I understand it, settled into areas which had previously been cleared and cultivated by the natives but had been relatively recently abandoned.
https://discover.hubpages.com/education/The-Pilgrims-and-the... "The Pilgrims decided to establish their colony in an area that had been cleared and abandoned by the Patuxet Indians. One colonist remarked, “Thousands of men have lived here, which died in a great plague not long since; and pity it was and is to see so many goodly fields, and so well seated, without men to dress and manure the same."
That's one amazing head start. And, had they not had it, the Pilgrims probably would have died, too.