Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 220

applied to moisten the soil profile before planting a crop.Surface soil can then
be allowed to dry and weeds can be killed by shallow tillage (Kempen, 1987,
pp. 37, 53). Seeds of large-seeded crop species capable of emergence from a
depth of several centimeters are subsequently planted into moist soil below
the dry surface, promoting crop emergence before the next weed cohort
germinates.
Fertility management
At least 14 mineral elements are essential for the growth and develop-
ment of higher plants (Marschner, 1995).Applications of mineral elements to
soil, especially nitrogen (N), phosphorus (P), potassium (K), and sulfur (S),
often improve crop yield. Fertilization consequently plays a key role in crop
production. Nitrogen is the nutrient whose supply most often limits the
growth and yield of agricultural plants, and it is applied in the greatest quan-
tities as synthetic fertilizer. Consequently, a large body of scientific literature
has focused on the effects of N on crops and weeds and its fate in the environ-
ment. In recent years, increasing concerns over the energy costs and environ-
mental impacts of synthetic fertilizers have led to greater interest in
alternative nutrient sources, including crop residues, animal manures, com-
posts, food-processing wastes, and sewage sludge (Parr, Miller & Colacicco,
1984).
Differential responses between species to soil fertility
conditions
For both weeds and crops, increased uptake of P, K, and especially N
can promote greater stem extension, branching, and leaf area production
(Marschner, 1995). While these responses are important for increasing light
interception, photosynthesis, and dry-matter accumulation when weeds or
crops grow in single-species stands, such responses are particularly important
when they differ among species growing in mixtures. Many weed species are
considerably more effective than crops in capturing nutrients applied in ferti-
lizers (Alkämper, 1976; DiTomaso, 1995), and increases in soil fertility may
alter canopy relations in weed-crop mixtures in favor of the weed component.
Such a situation is illustrated in Figure 5.3,which shows the effect of applying
ammonium sulfate to mixtures of barley and Brassica hirta . Application of N
fertilizer increased barley's green surface area (leaves and stems) by 10%, but
increased the weed's surface area by 706%. The large increase in the weed's
surface area was particularly marked in upper canopy levels, where the great-
est proportion of photosynthetically active radiation was absorbed. In this