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 suf-
ficient cover crop biomass, the mat acts as an effective
barrier to weeds and provides nearly complete control.
Increased mechanization, intensive tillage, and
erosion have degraded many agricultural soils to such
an extent that people think tillage is required to pro-
vide temporary relief from compaction. As aggregates
are destroyed, crusting and compaction create a soil
“addicted” to tillage. Except perhaps for organic produc-
tion systems, in which tillage is often needed because
Figure 16.1. Rolled-rye cover crop being prepared for row-crop planting.
herbicides aren’t used, a crop produced with limited or
Photo by Anu Rangarajan.
no tillage can generate better economic returns than one
Full-field tillage systems became more widespread
produced with conventional tillage systems. Managing
because they are better adapted to mechanized agri-
soil in the right way to make reduced tillage systems suc-
culture, and in time some of the traditional hill crops
cessful, however, remains a challenge.
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
45
1700s. It provided weed control by fully turning under
40 FP
crop residues, growing weeds, and weed seeds. Its ben-
efits were compelling at first; it allowed for a more stable
35
food supply and also facilitated the breaking of new
FC
30
e)
lands in the Americas. The development of increasingly
acr 25
powerful tractors made tillage an easier task (some say a
ons/
FP
20
recreational activity) and resulted in more intensive soil
oss (t
NT
disturbance, ultimately contributing to the degradation
15
soil l
FC
of soils.
10
New technologies have lessened the need for tillage.
5
The development of herbicides reduced the need for soil
NT
0
plowing as a weed control method. New planters achieve
0 10 20 30 40 50 60 70
better seed placement, even without preparing a seed-
residue cover of soil surface (%)
bed beforehand. Amendments, such as fertilizers and
This figure shows that:
• surface residue reduces erosion,
liquid manures, can be directly injected or band-applied.
• reduced tillage (chisel and no-till) leaves more
Now there are even vegetable transplanters that provide
residue and results in less erosion than plowing, and
• corn (circled) returns more residue than soybeans.
good soil-root contact in no-till systems. Although her-
Figure 16.2. Soil erosion dramatical y decreases with increasing surface bicides often are used to kill cover crops before plant-cover. Note: FP = fall plow, FC = fall chisel, NT = no-till; circles = corn, no ing the main crop, farmers and researchers have found
circles = soybeans. Modified from Manuring (1979).
175
Building SoilS for Better CropS: SuStainaBle Soil ManageMent