During the British Iron Age, large tracts of land in Southern and Eastern Britain were used to produce crops and the Celts who lived there were skilled arable farmers. Pytheas of Massalia visited the Southern coast of Britain around Kent and the Thames in about 322BC. Unfortunately there are no surviving copies of his journal 'Peritou Okeanou' (About the Ocean) but later classical writers have quoted his work and they provide us with the earliest written account of Britain: "he saw plenty of corn in the fields in the south-east but also noted the gradual disappearance of various kinds of grain as one advanced towards the north". Strabo also repeated what Pytheas reported from Thule which he said was bordering on the frozen zone (perhaps the Shetlands): "...and he noticed that the farmers gathered the sheaves into large granaries, in which the threashing was done. Threshing floors being useless on account of the rain and want of sun." Thule was also described as being "destitute of the cultivated fruits, and almost deprived of the domestic animals; and the food of the inhabitants consisted of millet, herbs, fruits, and roots. When they had corn and honey they made drink of them". 
Just as today farming practices were not uniform throughout Britain. In some areas small granaries were raised above the ground on four posts but underground pits were more commonly used to store the surpluses of grain. These underground pits were timber-lined and their excavation, at places like Danebury Iron Age Fort, has revealed that offerings to the gods were placed at the bottom of the pits before harvest and possibly after a good harvest. Religion and farming were closely linked in Iron Age Britain. The classical author, Diodorus Siculus, was probably quoting the earlier writer, Posidonius, when he stated: "In reaping their wheat they cut off the ears from the stalk, and house them in pits under ground; then they take and pluck out the grains of just enough of the oldest of them to last for the day, and after they have bruised the wheat make it into bread."  Farming methods remained substantially unchanged amongst the British peasantry from the Iron Age to the seventeenth century.
Oats, rye and millet are thought to have been introduced to Britain during the Iron Age. Spelt wheat was introduced around 500BC which is the Middle Iron Age and together with Emmer wheat became the one of the most important crops of the Iron Age. However, the farmers of this period also continued to grow barley and the less productive Einkhorn wheat. The productivity of ancient crops is studied at Butser Iron Age Farm. At this experimental farm it has been shown that good crop yields were probably achieved by the Celtic farmers. Some of these varieties may also have been more nutricious.
An organised system of land mangagement can be traced in Britain throughout the Bronze and Iron Ages. The high level of organisation can be seen in the way that the fields and paddocks were laid out in a structured way and on a vast scale. Celtic Britain, which spanned much of these periods, was an organised agricultural community and the 'celtic fields' of the Iron Age saw the introduction of a pattern of regular rectilinear fields. The evidence for this is clear from aerial photographs of land which has remained undisturbed. Demand for land was great and even the slopes of hill sides were cultivated.
Celtic field system - Burderop Down, Wiltshire. (c) Crown Copyright. NMR
Celtic fields can be seen in the bottom left corner of this photograph. They appear to be quite small compared with the modern fields which surround them.
In the later Iron Age (about 100BC) woodland was cleared at an unprecedented rate and some heavier soils were drained and made into farming land. By the first century BC the advanced Belgic tribes of Southern Britain certainly had an improved ard (simple plough) which had an iron ploughshare to move the soil to one side. The Romans are usually credited with introducing the coulter to Britain but the earliest example of an iron coulter, recorded in northern Europe, was found at the Iron Age fort of Bigbury in Kent. In fact, Pliny noted that the Celtic plough was superior to the Roman type which replaced it. (Think this was Rhaetia not Britain)
The island of Britain is recorded by the ancient writers as being very populous and therefore it is not surprising that there is evidence of soil depletion dating to the Iron Age. However, according to the Roman reporter, Pliny the Elder, the British farmers invented the practice of manuring the soil with various kinds of mast, loam, and chalk. He described how chalk was dug out from 'pits several hundred feet in depth, narrow at the mouth, but widening towards the bottom. In 70 A.D. he wrote:
'.the chalk is sought from a deep place, wells being frequently sunk to 100ft, narrowed at the mouth, the vein spreading out within as in mines. This is the kind most used in Britain. It lasts for 80 years and there is no instance of anyone who has put it on twice in his lifetime.' 
These chalk pits can be found in Kent and are called Deneholes.
Few vegetables were known in Britain prior to the Roman Invasion of the country. However, Celtic beans and fat hen were grown and a kind of primative parsnip was found in Britain at that time. Herbs would probably have been the main way to get your 'greens'.
Julius Caesar tells us: "The most civilised people are those in Kent which is entirely a coastal area; they have much the same customs as the Gauls. Most of those living further inland do not sow corn but live on milk and flesh and wear clothes of animal skins". 
Strabo declared that some of the British farmers had no idea of husbandry, "nor even sense enough to make cheese, though milk they have in plenty." The comment about cheese may well be a prejuded one but certainly the Celts would have had a lot of milk from cows, goats and sheep. Cattle was king in the Celtic world and a man's wealth was measured by the number of his herd. The Celts introduced, the now extinct, Celtic Shorthorn cattle to Britain. The cows would have provided good milk and the bullocks would have been slaughtered for meat. From the archaeological remains of bones, found at Danebury Hill Fort, it has been estimated that two-thirds of the meat eaten by Iron Age Britons was beef.
Undefended settlements have been found in a few upland areas and these are thought to have been summer settlements. The movement with the pastoral herds in the summer continued with the hafody in Wales until relatively recent times. However, most farming communities were settled in lowland areas during the Iron Age. In many areas hill forts had an associated second enclosure with birch fencing for coralling their cattle and other livestock. In the case of multivalate hillforts the livestock may have been herded in between the inner and outer defences to protect them from warring parties and cattle rustlers. Julius Caesar informs us that when he attacked Cassivellaunus at Bigbury Hill that they found great quantities of cattle there.
From the number of butchered bones of differents animals found, at Danebury, we see that the consumption of meat from sheep overtook that of pig. Butchered sheep bones, dated to this period, indicate that most sheep were not butchered as lambs, for meat. Rather, they were kept alive for wool production and when finally butchered, at a mature age, were prepared as mutton. Mutton and some lamb meat accounted for about a quarter of the total consumption of meat. The Soay, Manx, Hebridean and Shetland breeds of sheep which are still found on these islands are the direct descendents of the ancient sheep farmed in Iron Age Britain. They are quite goat-like in appearance and don't provide a great deal of meat. Their main use would have been to provide milk and wool. Unlike modern breeds of sheep their wool can be pulled from their backs without shearing. Woolen garments such the British hooded cloak (birrus) were a major export in the Iron Age. Wool and animal skins were used to make clothing during the Iron Age.
The Celts kept a domesticated pig which like other domesticated animals of the Iron Age were smaller than its modern counterparts. It may have looked something like a cross between the contempory Tamworth pig and the ancient wild boar which roamed the woodlands of ancient Britain. The domesticted pig rather than the wild boar would have provided the British Celts with most of their ham, sausages and bacon which together would only have accounted for about one tenth of the total meat consumption.
The Iron Age inhabitants of Britain kept domesticated geese but, according to Julius Caesar, it was unholy to eat them: "They think it is wrong to eat hares or chickens or geese but they breed them as pets". 
* The word corn is used, in the traditional British sense, to mean cereal crops such as wheat. Americans and Canadians etc mainly use the word corn when they are talking about maize. We are not talking about maize which did not exist in this part of the world at that time.
 Diodorus Siculus - Book V chap II
 Pliny the Elder
 Julius Caesar
 Julius Caesar
Page Created 6/11/06
Courtesy: Butser Ancient Farm
Pytheas' Journey c.322BC
Celtic image of wheat - shown on a British Coin minted by Cunobelinus. The depiction of wheat on coins suggests that it was important to the life and economy of the region.
The people of Britain began farming about 5,000 years ago during the Neolithic period (New Stone Age). The Bronze and Iron Ages witnessed a number of advances in farming. Iron Age Celtic Britain consisted almost exclusively of settled farming communities who tended their crops and livestock. The earliest written information about Britain records that the Celts of Southern and Eastern Britain were skilled arable farmers. Archaeological evidence indicates that a mixture of pastoral and arable farming was practised throughout Britain. Nevertheless, the balance between these farming methods in any given area would have been dependent, to some extent, upon the geographical location and trading relationships of the different tribes.
Strabo wrote that Britain:
"produces corn*, cattle, gold, silver and iron. These things are exported, along with hides, slaves and dogs suitable for hunting. The Gauls, however, use both these and their own native dogs for warfare also."