Hey mods, first answer on here, so please let me know if this fails to meet your standards. Thank you!

This is an interesting question that I think requires a little bit of reframing. As you mentioned, the Pontic-Caspian steppe is quite fertile and productive (given Ukraine's current role as a leading grain producer), and indeed, what we find is direct evidence for agricultural production on the steppe for quite some time. A few examples include that of the Bug-Dniester Culture, which resided on the Ukrainian steppe sometime during the 5,000s BCE. At Bug-Dniester sites like LBK-Ratniv 2, archaeologists have found samples of grain, including einkorn and emmer, indicating some form of agriculture.

Other cultures down the line would also showcase examples of agricultural production. The Mikhaylovka culture, dated to around the 3,000s BCE, was known to also have had some form of agriculture. As Professor David Anthony recounts, "emmer wheat, barley, millet, and 1 imprint of a bitter vetch seed… a crop grown today for animal fodder" had been found at one site. In fact, Anthony posits that the proto-Indo-Europeans like practiced agriculture on the steppes. I would be hesitant to equate the practices of the proto-Indo-Europeans directly with later steppe groups like the Mongols and Pechenegs.

As we continue moving forward, we find evidence of other cultures on the Pontic-Caspian steppe that practiced a form of agriculture. Srubnayan settlements, dated to the early-to-mid 1,000s BCE were noted as having agricultural elements. Settlements by the Srubnaya were relatively permanent and not nomadic. These areas, according to Professor Barry Cunliffe, did contain many pastures that pastoralists could rotate between, but in general Srubnayan communities were permanent structures that emerged as "the result of the development of cereal growing as an adjunct to the herding economy."

As for your latter question, there is some debate on whether climate change in the late-1000s BCE may have influenced the move away from settled agricultural to nomadic pastoralism, but I'm not really sure on that as a mechanism, so I'll leave some possible readings in my sources.

TLDR: My point is that while we have this popular imagery of the steppe as being populated by nomadic pastoralists, the reality is that agriculture did exist well into the past. In fact, one source I do want to point out here is Claudia Chang's "Rethinking Prehistoric Central Asia: Shepherds, Farmers, and Nomads." Although her work is focused on the region of the Kazakh steppe, what she reveals to us is that a number of steppe communities may have practiced limited agriculture and lived in semi-permanent settlements. Her work reminds to us about the gradient nature of nomadism in steppe societies, and how in many cases, pure steppe nomadism may not have been the reality, and I suspect that to be true here.

Sources:

• Giedre Motuzaite Matuzeviciute, “Neolithic Ukraine: A Review of Theoretical and Chronological Interpretations,” Archaeologica Baltica 20, no. 1 (2014).

• Giedre Motuzaite Matuzeviciute, “The earliest appearance of domesticated plant species and their origins on the western fringes of the Eurasian Steppe,” Documenta Praehistorica 39, no. 1 (2012).

• David W. Anthony, The Horse, the Wheel, the Language: How Bronze-Age Riders from the Eurasian Steppes Shaped the Modern World (Princeton: Princeton University Press, 2007).

• Barry Cunliffe, The Scythians: Nomad Warriors of the Steppe (Oxford University Press: Oxford, 2019).

• Claudia Chang, Rethinking Prehistoric Central Asia: Shepherds, Farmers, and Nomads (Routledge: New York, 2018).

First, I'd like to agree with u/NomadEmpiresPod in that there is a common misconception that there wasn't agriculture in Eurasian steppe regions until modern times. I have an old answer where I write a little about this, but at least on the Central Asian side of the steppe there is evidence of agriculture in river valleys going back far, like 6,000 BC, so on a similar time scale to evidence on the Ukrainian steppe.

Also as I note there, in both ancient and modern periods a lot of nomads on the steppe weren't as agriculture-free as popularly believed. As I discuss here, even the Scythians of the Pontic Steppe engaged in quite a bit of agriculture and even urbanism - the grain that Athens imported during the Peloponnesian War via the Black Sea had to be grown and harvested by someone, after all. The popular image of nomadic Scythians specifically comes from the "Royal" Scythians, who were just one group/caste of Scythians in the area.

Lastly I have an answer I wrote that quickly goes through the modern history of the Pontic Steppes (it's important to keep in mind that from the late Medieval period to the 18th century this was a borderland known as the "Wild Fields", and as such was basically a warzone and not the greatest place for large scale agricultural settlement, although people did still farm there). But I also talk a bit about the technical sides of things: especially the further you go west east on the Pontic-Caspian Steppe, the drier it gets, and even today wheat production is dependent on incredibly variable rainfall and has to deal with massive shifts in temperature thanks to the Continental climate zone it's in (you have to be ready to handle -40C to 40C over the course of the year). Also, the whole steppe isn't chernozem - a lot of the best chernozem is in river valleys, or in the forest steppe zone just north of the steppe proper, or in mixed with kastanozem "brown" soil, which is OK to farm in, and salty solonchak soils, which are absolutely horrible to farm in. So: you need to really know the local conditions, have access to water and other resources, and also have a great deal of luck on your side in order to engage in large-scale agriculture on the Pontic-Caspian steppe, and it's something that even in the Soviet and post-Soviet eras of modern industrial agriculture it can be an iffy undertaking if you miscalculate.