The competition between barley and certain weeds under controlled conditions

The nature of the competition between barley and certain annual weeds which are dominant at Woburn, under conditions where the available root space as well as the water, light, and nitrogen required by the plants are limited, has been studied. The weeds involved are Spergula arvensis Linn. (spurrey) and Matricaria inodora Linn, (scentless mayweed). Where there is a limited and fixed root space, and where the amount of available nitrogen is also limited, and where barley or each of the two weeds are grown by themselves, there is a tendency for the yield of the crop or of the weeds to reach a maximum with a definite density of plants per volume of soil. When an excess of nitrogenous fertilizer is added, a limit is reached with each of the weeds where extra nitrogen, added as manure, does not increase the growth but only raises the percentage of nitrogen in the plants as grown. Where there is a constant amount of weediness, with either of the two weeds studied, an increase in the density of barley plants diminishes the injurious effect of the weeds. The combined weight of barley and weeds is hardly ever as great as that of the barley grown in weed-free soil. In a mixed crop, the weight of both weeds steadily decreases as the number of barley plants increases. Where there is a constant number of barley plants per unit area, the increase of either of the weeds does not much effect the yield of barley till the number of weed plants exceeds the number of barley plants. The presence of a strong, well-grown crop of barley largely reduces the production of spurrey and, to a less extent, that of mayweed. With a constant density of barley and a variable density of weeds, the total weight of barley and weeds, taking the above-ground portion only, is almost constant whatever be the number of weed plants. Where the mixed plants are grown under slight nitrogen stress, the barley secures the greater part of the nitrogen absorbed, obtaining over 75 % of the nitrogen in the combined crop, even when the number of weed plants is greater than the number of barley plants. This applies to both weeds. If excess of nitrogenous fertilizer (ammonium sulphate) is added to the mixed crop of barley and weeds, above that which will increase the crop, any reduction in the yield of barley must be due to competition for root space. At the highest level of nitrogen used (which, however, did not apparently reach the maximum that the barley could utilize for increased growth), the reduction in the yield of barley with excess of spurrey present was only 12%: with mayweed, a much stronger competitor, it amounted to 45%. There is no evidence that with either of the weeds there is any special relationship between barley and the weeds. The effect seems to be only one of competition for root space or for nitrogen when there is not an excess of the latter.