The differential competitive ability between weeds and crops is influenced by several factors, such as the pH of the substrate where the plants grow. The pH variation effects on the plant development and on the content of the nutrients phosphorus, potassium, calcium and magnesium in the shoots and roots of the marandu grass (Brachiaria brizantha) and the weed Urena lobata were analyzed. Nutritive solutions with pH values of: 3.5; 4.5; 5.5; and 6.5 were prepared. The experiment was carried out under greenhouse conditions during 42 days. The forage grass and the weed showed different responses to pH variation in the nutritive solution . The marandu grass showed greater sensibility to the pH variations, increasing dry matter accumulation of the shoots and roots when the pH values were raised in the nutritive solution . On the other hand, the dry matter production of the weed's shoots and roots was not affected by pH variation. The marandu grass showed greater nutrient absorption capacity at pH 6.5, while the weed presented higher nutrient absorption ability at extreme pH values (3.5 and 6.5) . Regardless of the nutritive solution, pH and part of the plant analyzed, the contents of phosphorus, potassium, calcium and magnesium were normally higher in the weed than in the grass, indicating that the weed has higher capacity to extract those nutrients from soil.
pasture; nutrient solution; dry matter; root; aerial part
