Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 224

fertility at the soil surface resulted in greater nutrient deficits for weeds.
Special attention to weed suppression near fertilizer bands may also be an
important component of success with this approach.
Temporal variation in nutrient availability
Because weed and crop species can differ substantially in their abil-
ities to absorb nutrients at different growth stages, the timing of fertilizer
application can have strong effects on weed and crop performance. In cases
where the peak period of nutrient absorption by a crop occurs after the period
of maximum nutrient absorption by an associated weed, delayed application
of fertilizer may starve the weed of nutrients during critical initial growth
stages and better match the timing of crop nutrient demand. This hypothesis
was tested by Alkämper, Pessios & Long (1979) in pot experiments in which
maize was grown with Sinapis arvensis ( Brassica kaber ) or Chenopodium album ,and
different rates of NPK fertilizer were applied either in a single dose at plant-
ing, or with one-half the total application at planting plus one-half at maize
ear emergence. Delayed fertilizer application increased crop biomass as much
as 70% and reduced weed biomass as much as 50%,compared with early appli-
cation of the same total quantity of fertilizer.
Delayed fertilizer application also improved rice yields in field experi-
ments in which the crop was heavily infested with Echinochloa crus-galli (Smith
& Shaw, 1966).Application of N in two split doses at 8 and 12 weeks after crop
emergence increased yield of rice 30% to 80% compared with an earlier set of
split applications at 3 and 8 weeks after emergence. Although weed biomass
values in the different treatments of the experiment were not reported, the
observed yield increases with delayed fertilization were attributed to a reduc-
tion in weed competition.
Both crop and weed responses to different times of N application were
measured in experiments conducted by Angonin, Caussanel & Meynard
(1996) with winter wheat and the annual dicot Veronica hederifolia. Effects of
applying 60 kg N ha 1 at the tillering (earlier) or stem elongation (later) stages
of wheat development were compared. Over a wide range of weed densities,
weed biomass production was more than twice as high with earlier N applica-
tion than with later N application.The weed species had no effect on N uptake
and yield of wheat when fertilizer was applied at the stem elongation stage,
but it significantly reduced N uptake and yield of wheat when fertilizer was
applied at the tillering stage.The investigators noted that because of its short
stature and relatively early growth and development, the major competitive
effect of V. hederifolia occurs early in the growing season. Later N application