In this article we will discuss about the competition of weeds and crops for mineral nutrition, soil moisture and light.
Analysis of a large number of weeds reported by different authors during the last five decades have shown that the weeds absorb and accumulate mineral nutrients in higher concentrations than the common crop plants. For instance, Amaranthus viridis accumulated up to 3% N, Digitaria sanguinalis up to 3.36% P2O5, and Portulaca sp. up to 4.57% K2O, on dry weight basis (Table 2.1).
Among tine micronutrients, depletion of zinc by weeds has been studied most extensively. A weed like Setaria lutesens contained as high as 585 ppm Zn. At this level it amounted to removal of about three times more zinc by the weed than that removed by an average cereal crop.
The author found that weeds associated with sugarcane at Delhi removed 162 Kg N, 120 Kg P2O5, and 203 Kg K2O per ha. Patel (1983) reported from Gujarat per ha loss of about 31 Kg N, 19 Kg P2O5, and 41 Kg K2O through weeds from rice fields. In Tamil Nadu such losses were estimated at 26 Kg N, 4 Kg P2O5, and 21 Kg K2O per ha.
In Bihar, maize fields were estimated to lose about 118 g Zn, 45 g Cu, 190 g Mn, and 4765 g Fe per ha in weeds. In soybean fields, weeds removed 86 Kg N/ ha. Porwal and Gupta (1988-89) estimated average nutrient removal by weeds from wheat fields as 32.45 Kg N/ha and 5.07 Kg P2O5 per ha during the crop season.
The potato fields lost on an average 63 Kg N, 11 Kg P2O5, and 88 Kg P2O5 per ha through this pest. Numerous other similar studies have been reported from different states of India with considerable variations, depending upon the weed flora composition and the weed intensity.
It is important at this stage to point out that the above examples of nutrient losses through weeds should not give an impression that in a weedfree situation the quantity of nutrients removed by the crop shall be similar to that removed by the weeds and the crop combined in a weedy field.
Contrary to it, the quantity of mineral nutrients absorbed by a weedfree crop is always much more than those otherwise removed by the weeds and the crop together, from a weedy field. For example, Dashora et al. (1990) reported that a weedfree mustard crop removed 99.1 Kg N/ha against only 59.1 Kg N/ha removed by a weedy crop and its associated weeds, combined.
Weed-crop competition for soil moisture is particularly apparent in dry farming areas where it is already limited. During a dry spell, the crop plants exhibit the moisture stress symptoms much earlier in a weedy field than in a nearby weedfree field. Weeds often have higher water requirements than the crop plants.
In a study the consumptive use of water of Chenopodium album was found to be 550 mm against 479 mm of wheat crop. Besides their high water requirements, many a weed possesses root system which is capable of depleting even the deep seated soil moisture, spoiling thus the farmlands permanently. Alhagi camelorum and Pluchea lanceolata are two such weeds with us. Further, the weedy fallows conserve much less moisture for the crop than a weedfree fallow.
Weeds compete with crops for light needed for photosynthesis by shading the young crop seedlings. A visit to the weedy fields of crops like groundnut, cotton, potato, and sugarcane at their seedling stages will easily convince anyone of this phenomenon.
Most weeds put forth rapid initial growth, particularly during the rainy season, to shadow the slow growing young crop plants below. The crop seedlings thus become weak and, sometimes, wither away under the mat of weeds whence resowing of the field becomes necessary.