While good plant nutrition is an important contributor to a vigorous, high-yielding crop, weed growth is also increased by nutrients. Some studies show that weeds can take up nutrients more quickly than crops in early growth stages and can actually accumulate higher concentrations of many nutrients than crops do, depleting soil nutrients and reducing crop yield (Blackshaw et al., 2003). Getting nutrients to the crop and not to the weeds is therefore an important tool for producing a vigorous and competitive crop.
Fertilizer placement and timing can be manipulated to increase the availability of nutrients to the crop and not the weeds.
Fertilizer placement can enhance crop competitiveness and reduce interference from weeds. In general, banding fertilizer below the soil surface, rather than broadcasting, helps seedlings get to the nutrients more quickly, increasing crop competitiveness. In a Saskatchewan study, weed density, biomass and N uptake was 20 to 40% lower and spring wheat yield was12% higher where N fertilizer was side-banded rather than broadcast (Kirkland and Beckie, 1998). Similar results have been found in Alberta studies on N placement in both spring wheat and winter wheat (Blackshaw, 2004; Blackshaw et al., 2004).
Different nutrients and fertilizer formulations require different treatment. For example, nitrogen is highly soluble in water and is rapidly moved away from its original placement. Therefore, banding nitrogen is a short-term measure and is most effective when done as close to seeding as possible. That said, it is important to note that placing large amounts of nitrogen fertilizer close to the seed may damage seed and thus reduce competitiveness.
Adequate phosphorus levels are important to rapid early development. Unlike nitrogen, phosphate is not very water soluble, and thus not very mobile. Therefore, phosphate fertilizer should be placed close to the seed.
Timing of fertilizer application
Applying fertilizer to coincide with nutrient uptake by the crop is another way of giving the crop a competitive edge over weeds. In an Alberta study on spring wheat, applying N fertilizer in spring rather than in fall resulted in higher wheat yields and lower total weed densities and biomass in many cases (Blackshaw et al., 2004). Different weeds responded differently to the time of fertilizer application in this study; biomass of wild oat, wild mustard and common lambsquarters was reduced but biomass of green foxtail was not. In another study, application of nitrogen as a sideband at the time of planting had the fewest weeds but the impact was relatively small except for the sweep seeding which had higher weed numbers (Table 1; Derksen, unpublished). In this study, canola yield was also lowest in the sweep seeding treatment (Table 2).
Table 1. Impact of nitrogen fertilization on weed numbers over time.
|Weed density (plants/m2)|
Table 2. Impact of seeding system and nitrogen fertilization on canola yield.
|Canola yield (kg/ha)|
Banding nitrogen fertilizer near the seed row, rather than broadcasting, gives the crop a competitive advantage over weeds.
Applying N fertilizer in spring rather than in fall can reduce weed density and growth and increase crop yield.
Blackshaw, R.E. 2004. Application method of nitrogen fertilizer affects weed growth and competition with winter wheat. Weed Biol. and Management. 4:103-113.
Blackshaw, R.E., R.N. Brandt, H.H. Janzen, T. Entz, C.A. Grant and D.A. Derksen. 2003. Differential response of weed species to added nitrogen. Weed Sci. 51:532-539.
Blackshaw, R.E., L.J. Molnar and H.H. Janzen. 2004. Nitrogen fertilizer timing and application method affect weed growth and competition with spring wheat. Weed Sci. 52:614-622.
Kirkland, K.J. and H.J. Beckie. 1998. Contribution of nitrogen fertilizer placement to weed management in spring wheat (Triticum aestivum). Weed Technol. 12:507-514.