Nutrient distribution around roots of Brachiaria,maize, sorghum,and upland rice in an Andisol

Plants grown on Andisols often have an insufficient phosphorus (P) supply, since active aluminium (AI) and iron bind P in low available forms to the plants. The objectives of the present studies were to examine the differences in growth associated with the P-uptake ability among four Gramineae, to determine which P-forms are utilised, and to relate plant growth to the distribution of nutrients in soil close to the roots. Rhizosphere soil was separated from bulk soil by using a rhizobox system. Shoot and root yields and nutrient contents of maize (Zea mays L.), Sorghum bicolor (L.), Brachiaria dictyoneura (Stapf), and upland rice (Oryza sativa L.) were determined after cultivation in rhizoboxes for 105 d. Soil was sampled at increasing distances from the roots and analysed for P compounds, other nutrients, and pH. Maize gave the highest yield by using P reserves in its large seeds, resulting in the greatest depletion of K in the root soil of maize. Brachiaria showed the highest efficiency while upland rice the lowest in using soil P, respectively. The amounts of Bray-2 P and acetic acid-extractable P were significantly lower in root soil compared to bulk soil. Soil pH increased in the root soil of all crops, mainly around the Brachiaria roots.