Soil fertility affects weed and crop competition

B. Frick and E. Johnson - Scott Research Farm


Making nutrients available to the crop generally means making nutrients available to weeds. Crops and weeds have the same basic nutrient requirements. How can soil fertility be managed to give the greatest advantage to the crop?


All plants require the same basic nutrients but plants differ in the way they respond to nutrient availability. They differ in their ability to access nutrients because of differences in their root structures or mycorrhizal associations. They also can differ in their ability to tolerate nutrient imbalances, or in their efficiency at converting nutrients into growth. Maintaining or improving soil fertility is thus an element of weed management.

There is an advantage in placing nutrients where they are more readily accessed by the crop than by weeds. Although crop competitiveness may improve with improved nutrient status, some weeds are more effective at utilizing excess resources than are crops. Nutrient levels are generally recommended on the assumption that herbicides will be used, and that weeds are not an important consideration. Lower nutrient availability means less available for weed growth as well as for crop growth.

Higher nutrient levels stimulate the competitive ability of wild oats, green foxtail and barnyard grass. Other weeds might be limited by nutrient levels that are adequate for crop growth. Redroot pigweed, for instance, is especially sensitive to low phosphorous levels, and will not grow well if phosphorous is depleted. Wild mustard is also sensitive to low phosphorous levels, but lamb quarters may be more abundant in soils with phosphorous deficiencies. One author claimed, after reviewing numerous research reports on fertilization’s effect on weeds, that "weeds are capable of absorbing nutrients faster and in relatively bigger amounts than crop plants and thus profit more from fertilization. In the presence of a high weed population density, fertilizer application may stimulate weed growth so greatly that the crop plants will be overgrown and suppressed.

Wild oat-wheat competition experiments showed that wild oats were better able to compete with wheat at higher nitrogen levels. Wild oat seed production increased with nitrogen, while wheat yield decreased. Wheat yield increased with added nitrogen only if wild oats plants were less than 2% of the total plant counts. In another study, total weed density (several species) was highest at lowest nitrogen level in cereal crops. Therefore, results from experiments investigating nitrogen effects on weed competition have been contradictory. The effect of fertilizer nitrogen on weed-crop competition is largely dependent on fertilizer placement. Generally, in studies where weed growth has been favoured over crop growth, the nitrogen has been broadcast. Banding nitrogen close to the seedrow allows the crop roots to use applied fertilizer more efficiently than weeds. A number of studies have been conducted which show that banding nitrogen can favor the crop over weeds.

Phosphate fertilizers applied at seeding have been shown to increase crop competitiveness.

Long-term application of manure or other phosphate sources may have similar effects. Manure should be composted to kill most weed seeds. Use soil tests to determine appropriate amounts to add. In one study where herbicides were not used, researchers found that soil amendments (cattle manure and potato compost and alternating years of legume green manure) substantially reduced the weed biomass, possibly by improving crop competitiveness.


Competition between crops and weeds for nutrients, and for other factors (light, space, water) at different nutrient levels, are complex interactions that depend on many factors - crop species, weed species, moisture, nutrient release timing, nutrients’ positional availability, nutrient ratios, etc. Actions that alter nutrient availability will affect the weed community and the crop’s competitive ability.


Funding provided by the Canada-Saskatchewan Agri-Food Innovation Fund

Originally published in Research Report 2002, Canada-Saskatchewan Agri-Food Innovation Fund