Nutrient absorption of Brachiaria brizantha and the weed Urena lobata as influenced by pH.
The differential competitive ability between weeds and crops is influenced by several factors, such as the pH of the substrate where plants grow. The pH variation effects on plant development and on the nutrients phosphorus, potassium, calcium and magnesium in the shoots and roots of the marandu grass (Brachiaria brizantha) and the weed Urena lobata were studied. Nutritive solutions with pH's of 3.5, 4.5, 5.5 and 6.5 were prepared. The experiment was carried out under greenhouse conditions for 42 days. The forage grass and the weed showed different responses to pH variation in the nutritive solution. The marandu grass showed a greater sensitivity to increasing pH by increasing the dry matter accumulation of shoots and roots. However, the dry matter production of shoots and roots of the weed was not affected by pH variation. The marandu grass showed a greater nutrient absorption capacity at pH 6.5 while the weed had a higher nutrient absorption ability at extreme pH's (3.5 and 6.5). Regardless of the nutritive solution, pH and part of the plant analysed, the phosphorus, potassium, calcium and magnesium contents were normally higher in the weed than in the grass.