Habitats and soils of Belarus
It's true that the land of Belarus is "spread widely with forests and marshes" and a lot of the soil is sandy.
The book Belarus and Moldova: country studies (1995) by Helen Fedor (link goes to a 300-page PDF) includes a chapter "Physical Environment" in the Belarus part (by Jan Zaprudnik and Helen Fedor) which says (emphasis mine):
Northern Belarus has a picturesque, hilly landscape with many lakes and gently sloping ridges created by glacial debris. In the south, about one-third of the republic's territory around the Prypyats' (Pripyat' in Russian) River is taken up by the low-lying swampy plain of the Belarusian Woodland, or Palyessye (Poles'ye in Russian).
Belarus's 3,000 streams and 4,000 lakes are major features of the landscape and are used for floating timber, shipping, and power generation. [...]
Nearly one-third of the country is covered with pushchy (sing., pushcha), large unpopulated tracts of forests. In the north, conifers predominate in forests that also include birch and alder; farther south, other deciduous trees grow.
I found a paper "Soil classification in Belarus: history and current problems" (2018) by Viktar Tsyrybka and Hanna Ustsinava which goes into a lot of detail about the soil composition throughout Belarus, including a map showing all the regions of the country and their predominant soil types. The situation is summarised by the German Agricultural Society (DLG) in an article on Belarus as follows:
Too much poor land
Describing the climate in Belarus needs just a single sentence:
The autumn is wet, spring cold and summer dry.
The soils differentiate a little more: head west towards the Lithuania or the borders to Ukraine and you’ll find areas of good farmland. Turn eastwards and the fields are sandy.
Between 60 and 70 % of soils can be cropped, the rest is pasture, often peat or fenland.
The Encyclopedia Britannica also has this to say in its article on Belarus, section entitled "Soils":
About three-fifths of Belarus is covered by podzolic soils. On the uplands these soils are mainly clay loams developed on loess subsoils, which can be productive with the use of fertilizers. The plains and lowlands have mostly sandy podzols of low fertility interspersed with swampy clays, which have a high humus content and can be very fertile when drained.
Agriculture in Belarus
Agriculture is a major, although declining, sector of the Belarusian economy, and it was one of the major agricultural regions of the USSR. But all of the information I've been able to find about Belarusian agriculture is from the Soviet and post-Soviet eras, which wouldn't be relevant to a poem written by someone who died in 1897 (during whose lifetime Belarus would have been part of the Russian empire).
However, I'd like to highlight two phrases in the last quoted paragraph above: "can be productive with the use of fertilizers" and "can be very fertile when drained". This suggests that, although agriculture is now a thriving part of the Belarusian economy, it took a lot of effort to make the land viable for it, effort which might well have been made mostly after the time this poem was written. This guess is borne out by the Encyclopedia Britannica in the section on "Agriculture, forestry, and fishing" (emphasis mine):
The agricultural sector in Belarus, which employs about one-tenth of the labour force but constitutes a diminishing proportion of GDP, is dominated by large collective and state farms. Private holdings were permitted for household use during the Soviet era, but, while their number increased dramatically following independence, they remained small in size. In the early 21st century a significant number of collective farms were sold to private or state-controlled companies.
Most of the country has mixed crop and livestock farming, with a historic emphasis on flax growing. (During the late Soviet era the Belorussian S.S.R. produced about one-fourth of the U.S.S.R. total.) Potatoes, sugar beets, barley, wheat, rye, and corn (maize) are other important field crops; a large percentage of the grains are used for animal feed. Cattle, poultry, and pigs are the main livestock. Considerable areas of the swampy lowlands have been drained since the late 19th century, with much of the reclaimed land being used for fodder crops. Dairying and truck farming are locally important in the vicinity of Minsk. Nearly two-fifths of Belarus is covered by forests, which are exploited for the production of wood and paper products. Most of the country’s small fish yield results from aquaculture.
In short, the tense of your question should be changed: Belarus was hard to farm, in the 19th century, due to its natural habitats and soils, but thanks to considerable human effort, it is now a more fertile country where agriculture has boomed for decades.