Editing British Agricultural Revolution (section)

===Crop rotation===

{| class="wikitable" cellpadding="1" cellspacing="1" style="text-align:right;"
|+Crop Yield net of seed<br />(bushels/acre)<ref>{{Cite web |last1=Apostolides |first1=Alexander |last2=Broadberry |first2=Stephen |last3=Campbell |first3=Bruce |last4=Overton |first4=Mark |last5=van Leeuwen |first5=Bas  |date=26 November 2008 |title=English Agricultural Output and Labour Productivity, 1250–1850: Some Preliminary Estimates |url=http://www.basvanleeuwen.net/bestanden/agriclongrun1250to1850.pdf |access-date=1 May 2019}}</ref>
|-
!Year||Wheat||Rye||Barley||Oats||Peas &<br />beans||Growth rate<br /> (%/year) ‡
|-
!1250–1299
|8.71||10.71||10.25||7.24||6.03||−0.27
|-
!1300–1349
|8.24||10.36||9.46||6.60||6.14||−0.032
|-
!1350–1399
|7.46||9.21||9.74||7.49||5.86||0.61
|-
!1400–1449
|5.89||10.46||8.44||6.55||5.42||0.08
|-
!1450–1499
|6.48||13.96||8.56||5.95||4.49||0.48
|-
!1550–1599
|7.88||9.21||8.40||7.87||7.62||−0.16
|-
!1600–1649
|10.45||16.28||11.16||10.97||8.62||−0.11
|-
!1650–1699
|11.36||14.19||12.48||10.82||8.39||0.64
|-
!1700–1749
|13.79||14.82||15.08||12.27||10.23||0.70
|-
!1750–1799
|17.26||17.87||21.88||20.90||14.19||0.37
|-
!1800–1849
|23.16||19.52||25.90||28.37||17.85||0.63
|-
!1850–1899
|26.69||26.18||23.82||31.36||16.30||—
|-style="border-top:3px solid;"
!Est. ave.<br>seed sown<br>{{when|date=February 2025}}||2.5||2.5||3.5–<br>4.3||2.5–<br>4.0||2.5–<br>3.0||
|}
;Notes to table
Yields have had the seed used to plant the crop subtracted to give net yields.

‡ Average annual growth rate of agricultural output is per agricultural worker.
<br />Other authors offer different estimates. (1 bushel/acre = 0.06725 [[tonnes]]/[[hectare]])

One of the most important innovations of the British Agricultural Revolution was the development of the Norfolk four-course rotation, which greatly increased crop and livestock yields by improving soil fertility and reducing fallow.<ref name="Overton 1996 1"/>

[[Crop rotation]] is the practice of growing a series of dissimilar types of crops in the same area in sequential seasons to help restore plant nutrients and mitigate the build-up of pathogens and pests that often occur when one plant species is continuously cropped. Rotation can also improve soil structure and fertility by alternating deep-rooted and shallow-rooted plants. Turnip roots, for example, can recover nutrients from deep under the soil. As it is now known, the Norfolk four-course system rotates crops so that different crops are planted, resulting in various kinds and quantities of nutrients being taken from the soil as the plants grow. An essential feature of the Norfolk four-field system was that it used labour at times when demand was not at peak levels.<ref>{{Harvnb|Overton|1996|p=117}}</ref>

Planting [[cover crop]]s such as turnips and clover was not permitted under the [[Common land|common field system]] because they interfered with access to the fields. Besides, other people's livestock could graze the turnips.<ref>{{Harvnb|Overton|1996|p=167}}</ref> During the [[Middle Ages]], the [[open-field system]] had initially used a two-field crop rotation system where one field was left fallow or turned into pasture for a time to try to recover some of its plant nutrients. Later they employed a three-year, [[Three-field system|three field crop rotation]] routine, with a different crop in each of two fields, e.g. oats, rye, wheat, and barley with the second field growing a legume like peas or beans, and the third field fallow. Normally from 10% to 30% of the [[arable land]] in a three crop rotation system is fallow. Each field was rotated into a different crop nearly every year. Over the following two centuries, the regular planting of legumes in the fields that were previously fallow slowly restored the [[Soil fertility|fertility]] of some croplands. The planting of legumes helped to increase plant growth in the empty field because of the ability of the bacteria on legume roots to [[nitrogen fixation|fix nitrogen]] from the air into the soil in a form that plants could use. Other crops that were occasionally grown were [[flax]] and members of the [[Brassicaceae|mustard family]].

[[Convertible husbandry]] was the alternation of a field between pasture and grain. Because nitrogen builds up slowly over time in pasture, ploughing up pasture and planting grains resulted in high yields for a few years. A big disadvantage of convertible husbandry was the hard work in breaking up pastures and difficulty in establishing them. The significance of convertible husbandry is that it introduced pasture into the rotation.<ref>{{Harvnb|Overton|1996|pp=116, 117}}</ref>

The farmers in [[Flanders]] (in parts of [[France]] and current day [[Belgium]]) discovered a still more effective four-field crop rotation system, using turnips and clover (a legume) as forage crops to replace the three-year crop rotation fallow year. The four-field rotation system allowed farmers to restore soil fertility and restore some of the [[Plant nutrition|plant nutrients]] removed with the crops. Turnips first show up in the probate records in England as early as 1638 but were not widely used till about 1750. Fallow land was about 20% of the arable area in England in 1700 before turnips and clover were extensively grown in the 1830s. Guano and nitrates from South America were introduced in the mid-19th century, and fallow steadily declined to reach only about 4% in 1900.<ref>{{cite web |last=Overton |first=Mark |date=17 February 2011 |title=Agricultural Revolution in England 1500–1850 |url=https://www.bbc.co.uk/history/british/empire_seapower/agricultural_revolution_01.shtml |publisher=BBC History |department=British History |access-date=1 May 2019}}</ref> Ideally, wheat, barley, turnips and clover would be planted in that order in each field in successive years. The turnips helped keep the weeds down and were an excellent forage crop—ruminant animals could eat the tops and roots through a large part of the summer and winters. There was no need to let the soil lie fallow as clover would add [[nitrate]]s (nitrogen-containing salts) back to the soil. The clover made excellent pasture and hay fields as well as [[green manure]] when it was ploughed under after one or two years. The addition of clover and turnips allowed more animals to be kept through the winter, which in turn produced more milk, cheese, meat and manure, which maintained soil fertility.

The mix of crops also changed: the area under wheat rose by 1870 to 3.5&nbsp;million acres (1.4m ha), barley to 2.25m acres (0.9m ha) and oats less dramatically to 2.75m acres (1.1m ha), while rye dwindled to {{convert|60,000|acre|ha|abbr=off}}, less than a tenth of its late medieval peak. Grain yields benefited from new and better seed alongside improved rotation and fertility: wheat yields increased by a quarter in the 18th century{{sfn|Overton|1996|p=77}} and nearly half in the 19th, averaging 30 bushels per acre (2,080&nbsp;kg/ha) by the 1890s.