It might help if I put these questions into some kind of context.
There is actually a long tradition of Marxist research into agrarian questions going back to Marx and Engels. Lenin and Kautsky also wrote important articles on the question. Michael Perelman, the moderator of PEN-L and LBO-Talk regular, has also written on the topic: "Farming For Profit In A Hungry World: Capital And The Crisis In Agriculture."
The context for Marx's examination of the agrarian question was the general crisis of soil fertility in the period from 1830 to 1870. The depletion of soil nutrients was being felt everywhere, as capitalist agriculture broke down the old organic interaction that took place on small, family farms. When a peasant plowed a field with ox or horse-drawn plows, used an outhouse, accumulated compost piles, etc., the soil's nutrients were replenished naturally. As capitalist agriculture turned the peasant into an urban proletariat, segregated livestock production from grain and food production, the organic cycle was broken and the soil gradually lost its fertility.
The need to artificially replenish the soil's nutrients led to scientific research into the problem. Justin Von Liebeg was one of the most important thinkers of the day and he was the first to posit the problem in terms of the separation between the city and the countryside.
While the research proceeded, the various capitalist powers sought to gain control over new sources of fertilizers. This explains "guano imperialism," which I referred to in my post on Peru the other day. England brought Peru into its neocolonial orbit because it was the most naturally endowed supplier of bird dung in the world. In 1847, 227 thousand tons of guano were imported from Peru into England. This commodity was as important to England's economy as silver and gold were in previous centuries.
There was also a desperate search for bones. Over a ten year period, the value of English imports rose from 14,000 pound sterling to 254,000. Raiding parties were dispatched to battlefields to scavenge bodies of dead soldiers. Their bones were desperately needed to replenish sterile soil.
The United States followed suit. There had been a big crisis in upstate NY and the mid-Atlantic states in the mid 1800s. This prompted Congress to pass the "Guano Act" of 1856, which eventually led to the seizure of 94 islands in the Pacific Ocean, rich sources of guano.
Von Liebeg theorized that such measures would eventually fall short. Even with such substitutes, the soil tended to lose its nutrient properties so long as the artificial divide between town and countryside was maintained. Not only was the countryside losing its productivity, the town was being swamped with human waste which was no longer being recycled. London had such a terrible problem with open sewers that Parliament was forced to relocate to a location outside the city during the summer months. The stench was unbearable.
The neo-Classical economists tended to view soil fertility as a given, like some kind of natural law. Ricardo and Malthus both regarded it as an exhaustible resource. Thus, the problem of overpopulation was tightly coupled to the existing practices of capitalist agriculture, which was to exploit the soil and then abandon it when it lost its fertility. This has been the main character of Malthusianism until the modern era. It accepts the limits imposed by the capitalist mode of production as eternal.
Scientists like Von Liebeg, on the other hand, supported the notion of soil improvement. This meant looking at the relationship between society and nature in ecological terms. The solution to the problem was the reintegration of the town and country. This overlapped with Marx's own exploration of the problems in Capital. In volume three of Capital, the discussion of farming is framed within this general dialectic. Soil fertility could only be ensured over the long run through the abolition of the capitalist system, which would allow food production to take place along sound, ecological guidelines.
The concluding paragraphs of the chapter on "The Transformation of Surplus Profit into Ground-Rent" in V. 3 of Capital are a succinct description of the problematic:
"All criticism of small-scale landownership is ultimately reducible to criticism of private property as a barrier and obstacle to agriculture. So too is all counter-criticism of large landed property. Secondary political considerations are of course left aside here in both cases. It is simply that this barrier and obstacle which all private property in land places to agricultural production and the rational treatment, maintenance and improvement of the land itself, develops in various forms, and in quarreling over these specific forms of the evil its ultimate root is forgotten.
"Small-scale landownership presupposes that the overwhelming majority of the population is agricultural and that isolated labour predominates over social; wealth and the development of reproduction, therefore, both in its material and intellectual aspects, is ruled out under these circumstances, and with this also the conditions for a rational agriculture. On the other hand, large landed property reduces the agricultural population to an ever decreasing minimum and confronts it with an every growing industrial population crammed together in large towns; in this way it produces conditions that provoke an irreparable rift in the interdependent process of social metabolism, a metabolism prescribed by the natural laws of life itself. The result of this is a squandering of the vitality of the soil, which is carried by trade far beyond the bounds of a single country.
"If small-scale landownership creates a class of barbarians standing half outside society, combining all the crudity of primitive social forms with all the torments and misery of civilized countries, large landed property undermines labor-power in the final sphere to which its indigenous energy flees, and where it is stored up as a reserve fund for renewing the vital power of the nation, on the land itself. Large-scale industry and industrially pursued large-scale agriculture have the same effect. If they are originally distinguished by the fact that the former lays waste and ruins labour-power and thus the natural power of man, whereas the latter does the same to the natural power of the soil, they link up in the later course of development, since the industrial system applied to agriculture also enervates the workers there, while industry and trade for their part provide agriculture with the means of exhausting the soil."
Louis Proyect
(http://www.panix.com/~lnp3/marxism.html)