During WWI the Germans were continually starved for food supplies, largely due to blockades by the British of the North Sea. Similarly, in WWII a oft-cited reason for the campaign into Russia is that the Germans wanted to take possession of the fertile lands of Ukraine and beyond.
However, is there any particular reason why Germany was short on food? Their lands were quite extensive (esp with Austria-Hungary as their ally in WWI) and fertile. Shouldn't this have been sufficient to feed their population? Where did they get their agricultural imports from anyways? Their colonies were not exactly breadbaskets...
And during WWII was the arable land motif mostly an "excuse" to justify German geopolitical designs? Having taken posession of France and Poland endowed the Germans with significant additional arable territory, after all...
Germany relied on imports for a third of its food supply prior to the war. It also relied on imports for critical farming supplies such as fodder for stock animals and nitrates for fertiliser. The British blockade strangled Germany of these things. Germany had no pre-War plans in place for fighting a prolonged war. It had no stockpiled foodstuffs ready to feed its rapidly expanding military and no stockpiles of fodder or fertiliser for the farms.
Now, war always increases the demand for food. Armies March and fight on their stomachs and any modern army requires a logistics train that has about as many men assigned to it as the army itself. Everyone needs to be fed and they all need to be fed everyday. Because of the transportation requirements, more food needs to be available for transportation then would normally be called for. Added to this, the distances involved means more food is tied up in the logistical system on its way to its destination.
This doesn’t even take into account the requisition of draft animals for war work. Horses were requisitioned from all over Germany for hauling guns, wagons, cavalry mounts and every other job that required more strength then a man could provide. This took the draft animals from the farms. no draft animals means little to no ploughing of fields, leading to lower crop yields. The shortage of nitrates for fertiliser also played a major role in lower crop yields. Farm animals such as cattle, sheep and pigs were also starved of fodder due to the blockade. This means thinner animals and subsequently less meat after slaughter.
The blockade also led to shortages of essential machine components and lubricants. The demand on manufacturing meant more wear on machines and without these imports, German manufacturing suffered from irreparably broken machinery. This included rolling stock, railway carriages and the like. Germany was not only struggling to produce the food it needed but it also struggled to maintain the industries that processed the foods and the transportation required to get it to where it was needed.