Reducing Tillage
Reducing
Tillage
Although tillage is an
ancient practice, the question of which tillage system is most appropriate for
any particular field or farm is still difficult to answer. Before we discuss
different tillage systems, let’s consider why people started tilling ground.
Tillage was first practiced by farmers who grew small-grain crops, such as
wheat, rye, and barley, primarily in western Asia (the Fertile Crescent),
Europe, and northern Africa. Tillage was primarily practiced because it created
a fine seedbed, thereby greatly improving germination. It also gave the crop a
head-start before a new flush of weeds and stimulated mineralization of
organic nitrogen to forms that plants could use. The soil was presumably
loosened by a simple ard (scratch plow) in several directions to create fine
aggregates and a smooth seedbed. The loosened soil also tended to provide a
more favorable root-ing environment, facilitating seedling survival and plant
growth. Animal traction was employed to accomplish this arduous task. At the
end of the growing season,
the
the
entire crop was harvested because the straw
also had considerable economic value for animal bedding, roofing thatch, brick
making, and fuel. Sometimes, fields were burned after crop harvest to remove
remaining crop residues and to control pests. Although this cropping system
lasted for centuries, it resulted in excessive erosion, especially in the
Mediterranean region, where it caused extensive soil degradation. Eventually, deserts spread as the climate became drier.
Ancient
agricultural systems in the Americas did not use intensive full-field tillage
for grain production, as they did not have oxen or horses to perform the
arduous tillage work. Instead, the early Americans used mostly direct seeding
with planting sticks or manual hoes that created small mounds (hilling). These
practices were well adapted to the staple crops of corn and beans, which have
large seeds and require lower plant densities than the cereal crops of the Old
World. Several seeds were placed in each small hill, which was spaced several
feet apart
from the next one. In temperate or wet regions
the hills were elevated to provide a temperature and moisture advantage to the
crop. In contrast with the cereal-based systems (wheat, rye, barley, rice) of
growing only one crop in a monoculture, these fields often included the
intercropping of two or three plant species growing at the same time, like the
corn, bean, and squash of the Three Sisters system in North
America. This
hilling system was
generally less prone to
erosion than whole-field tillage, but climate and soil conditions on steep
slopes still frequently caused considerable soil degradation.
A third ancient tillage
system was practiced as part of the rice-growing cultures in southern and
eastern Asia. There, paddies are tilled to control weeds and puddle the soil to
create a dense layer that limits down-ward losses of water through the soil.
The puddling process occurs when the soil is worked while wet—in the plastic or
liquid consistency state; see chapter 6—and is specifically aimed at destroying
soil aggregates. This system was designed to benefit rice plants, which thrive
under flooded conditions, especially relative to competing weeds. There is
little soil erosion because paddy rice must be grown either on flat or
terraced lands, and runoff is controlled as part of the process of growing the
crop. Recent research efforts have focused on less puddling and ponding to
conserve soil health and water.
Reducing Tillage
Full-field
tillage systems became more widespread because they are better adapted to
mechanized agriculture, and in time some of the traditional hill crops like
corn became row crops. The moldboard plow was invented by the Chinese 2,500
years ago but was redesigned into a more effective tool in England in the
1700s. It provided weed control by fully turning under crop residues, growing
weeds, and weed seeds. Its benefits were compelling at first; it allowed for a
more stable food supply and also facilitated the breaking of new lands in the
Americas. The development of increasingly powerful tractors made tillage an
easier task (some say a recreational activity) and resulted in more intensive
soil disturbance, ultimately contributing to the degradation of soils.
New technologies
have lessened the need for tillage. The development of herbicides reduced the
need for soil plowing as a weed control method. New planters achieve better
seed placement, even without preparing a seed-bed beforehand. Amendments, such
as fertilizers and liquid manures, can be directly injected or band-applied.
Now there are even vegetable transplanters that provide good soil-root contact
in no-till systems. Although herbicides often are used to kill cover crops
before planting the main crop, farmers and researchers have found
that they can obtain good cover crop control
through well-timed mowing or rolling (figures 16.1, 16.7)—greatly reducing the
amount of herbicide needed. If there is sufficient cover crop biomass, the mat
acts as an effective barrier to weeds and provides nearly complete control. Reducing Tillage
Increased
mechanization, intensive tillage, and erosion have degraded many agricultural
soils to such an extent that people think tillage is required to provide
temporary relief from compaction. As aggregates are destroyed, crusting and
compaction create a soil “addicted” to tillage. Except perhaps for organic
production systems, in which tillage is often needed because herbicides aren’t
used, a crop produced with limited or no-tillage can generate better economic
returns than one produced with conventional tillage systems. Managing soil in
the right way to make reduced tillage systems successful, however, remains a
challenge
