It is not uncommon for me, while walking across a field of our farrrm, to take a swing at a large turd with my boot to spread its nutritive value out across the grass. Observation shows that the evidence of sheep dissipates much quicker in then this way and makes for a more even sward. It must be a common reflex among rural folk who are known as shit-kickers by city dwellers. Those same city-dwellers think that milk comes from a bottle and ham is delivered from heaven in uniform slices encased in plastic and weighing precisely 150g. Any biologist would suspect such precision in anything derived from the living world.
Since they came across the English Channel, farmers have been mixed farmers – raising both animals and arable crops like wheat, beans and potatoes. It’s different in the tropics with paddy fields and two cash crops a year but in these our islands of the WEA where we have weather as well as climate, it was impossible to obtain cash crops sustainably every year. It was recognised that the soil became depleted if wheat Triticum aestivum or barley Hordeum vulgare was grown in successive years, so that yields dwindled to almost nothing and the farmer went bust. A rotation of several different crops seemed to restore fertility to the soil so that tons of grain and straw could be carted off the fields in alternate years and this system could be driven on in perpetuity. 1) Spring corn 2) rye-grass Lolium perenne and clover Trifolium repens hay followed by grazing 3) wheat if the land was strong, barley or oats Avena sativa else 4) Turnips Brassica napus 1) Spring corn . . . and so on till the end of time. In particular places you might observe more exotic variants: clover - potatoes – wheat – turnips – potatoes – wheat which gave 4 cash-crops in 6 years or even a seven year cropping with 3 years of wheat and 1 of oats interleaved with turnips, clover and beans. The addition of clover and turnips turned out to be a lot more productive than previous systems which left fields 'fallow' every few years. Fallow seemed to restore fertility although folks were unable to deduce why an obviously unproductive year was an investment. Now we reckons it's from the wild clover and its relatives. But a year without food production was also a benefit in making fungal, bacterial and insect pests work harder for their living
Nobody knew what clover and beans did for the soil but somehow those leguminous plants put the land in better heart. Likewise the dung from sheep or cattle, produced as they grazed the pasture or chomped through a desolate field of turnips, seems to give something back to the soil so that wheat grew far better after that treatment. Research carried out in Rothamstead in the second half of the 19thC showed a) that nitrate in the soil were vital for fertility b) that bacteria in the root-nodules of legumes was capable of ‘fixing’ atmospheric nitrogen and leaving it behind in the soil as nitrate. Martinus Beijerinck, a researcher from Wageningen in the Netherlands was the first to culture a root-nodule microbe on a petri-dish in his lab. He called it Bacillus radicicola but it has been renamed as Rhizobium leguminosarum. I spent the Summer of 1976 in Wageningen doing desultory research in corn and potatoes but I didn't make the sort of discoveries for which Beiierinck became renowned.
Thus Rhizobium spp. bring something to the fertility table. The other bucket of crop rotation inputs are more in the nature of stopping food falling off the table. Animal dung is rich in minerals, particularly phosphates, which are essential in the equations for plant growth. Sure plants protosynthesise; capturing energy from the sun to make glucose the energy from which drives everything else. But DNA has a backbone of alternating phosphates and sugars and proteins are made up of nitrogen-rich amino acids: neither of these can be made from scratch by plants and animals. So cow dung is a matter of recycling essential nutrients back onto the field either directly or by gathering a steaming hape of the stuff in the farm-yard and carting it back out to the fields. Forking tons of shit out over a field by hand was one of the tasks that made farming so labour intensive. Everyone heaved a sigh of relief when 19thC ingenuity developed muck-spreaders that would let horse-power do the work