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Weed learning module
How does soil fertility influence the competitive balance
between crops and weeds?
Hypothesis:
Increasing soil fertility only improves the competitive ability of crops up to a point; beyond that point, further increases in fertility do not improve crop growth but continue to increase the density and growth of weeds.
Reasoning:
Weeds are adapted to rapidly take up the nutrients that are released by decomposition of organic matter following a soil disturbance like tillage. Many species, like common lambsquarters and several pigweed species germinate in response to the presence of nitrate. Normally, plants take up nitrate as fast as it forms by decomposition. Plants are killed by soil disturbance, and these weeds use the presence of nitrate as a cue to indicate the absence of competition. Thus, we might expect to see more of these weeds when there is more soluble N in the soil, especially early in the season when the crop is not taking it up.
Weeds also commonly have N, P, K and Ca tissue concentrations that are 1.5 to 3 times higher than concentrations in the crops with which they are competing. Since most weed species apparently need higher levels of mineral nutrition than crops for optimal growth, we would expect weed growth to continue to increase beyond the point at which crop yield is maximized.
Crop seeds are generally bigger than
weed seeds. Larger image. |
Also, in the early stages of growth, weeds are more dependent on soil sources of nutrients due to their small seed size (photo, right). In contrast, large seeded and transplanted crops rely partially on stored nutrients for early growth and can therefore thrive at more moderate soil nutrient concentrations. Finally, most crops have mycorrhizae (symbiotic fungi) associated with their roots that help them absorb dilute solutions of nutrients, especially phosphorus. Some common weeds have micorrhizae also (common ragweed, black nightshade), but many do not (notably, most species in the mustard, goosefoot, pigweed and smartweed families). Thus, for several reasons we expect excessive nutrient levels to aid weeds more than crops.
The experiment: As part of the project, we have set up an experiment at the Martens' farm in which two types of compost are applied annually at 5 different rates (0, 0.6, 1.5, 3 and 6 ton/a) before corn in 2004 and 0, 0.15, 0.3, 0,6, and 1.2 ton/a before soybean in 2005). One compost was made from chicken manure and had a test of 4.0-5.2-2.4 N-P2O5-K2O. The other was made from chicken litter and had a test of 1.9-3.6-2.4. Thus, the two types of compost were similar in K, but the first had a higher concentration of N and P and a lower C:N ratio.
In addition to measuring corn yield, we identify and count weeds before inter-row cultivation. In mid-August, we centered four 0.5 m2 (5 ft2) quadrats on crop rows in each plot and counted the weeds and measured the height of each individual.
2004 Results:
Compost increased corn yield up to a rate of 1.5 ton/a (see figure 1). The two types of compost had similar effects on corn yield.
Figure 1. Response of corn yield to compost. applied the previous fall. Soluble refers to the high N compost made from chicken manure, whereas organic refers to the low N compost made from chicken litter.
Number of weeds did not differ among rates of compost at either of the counting dates for any species. Thus, higher N fertility did not appear to be promoting weed seed germination in 2004.
Weed size did not respond to compost rate for any species in the high N "soluble" compost. With the low N "organic" compost, however, lambsquarters, foxtail and common ragweed all increased in size with compost rate (Figure 2). Unlike corn which reached peak production at about 1.5 ton/a of compost, these weeds continued to increase in size up to the highest compost rate. Thus, as hypothesized, the high compost rates favored weed growth without corresponding increases in yield. The leveling off of corn yield could not have been explained by greater weed growth, however, since even at the largest size, none of the species were abundant enough to have affected yield. Also, the height and number of quackgrass shoots, did not respond to compost rate, and this was the most abundant weed species in the field.
Figure 2. Height of three weed species in relation to application rate of the low N compost.
2005 Results:
Results from the 2005 soybean crop differed in detail from results for the 2004 corn crop, but the bottom line message was similar. Soybean yield was not significantly affected by compost rate, though there may have been a slight increase in yield from 0 to 0.15 ton/a.
The only species to show a response of plant height to compost rate in 2004 was Powell amaranth (a pigweed species). In the high N compost, Powell amaranth height continued to increase up to the highest application rate. Note that this species did not respond to application rate of either compost type in 2004.
Unlike 2004, weed density responded to application rate for both types of compost. In the low N compost, the response of individual species was too variable to be statistically significant, but when species were combined into groups, the density of both annuals and perennials increased with application rate. For the high N compost, density of common ragweed and Powell amaranth (a pigweed) but not giant foxtail responded to application rate.
Thought question 1:
The greater size and density of some weeds at higher nutrient rates did not affect corn and soybean yields in this experiment. Does that mean that these results do not have practical importance? Explain. When you have thought about this question, read these comments.
Thought question 2:
Why did weed size respond to the low N compost, but not the high N compost in 2004? When you have thought about this question, read these comments.
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Design by Craig Cramer.
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