Saturday, 15 July 2006

Movement of Water, N-forms and Potassium in a Sandy Soil Cropped with Vegetables under Drip Irrigation.

Kamal A. Mahmoud, Peter Nkedi-Kizza, and Kelly Morgan. Univ of Florida, Soil and Water Science Dept, Gainesville, FL 32611

Over irrigation and fertilization of crops grown in sandy soils can lead to leaching of mobile nutrients such as nitrate-N below the root zone. A fertigation study was carried out at North Florida Research and Education Center, University of Florida, (UF) to investigate the effect of irrigation regimes and N- fertilizer rates on plant growth, yield, and movement of water, N-forms, potassium, and bromide in the soil profile. Four vegetable crops (bell peppers, cabbage, watermelons, and pumpkins) were grown on sandy soil plastic mulched beds during two seasons. The treatments included factorial combination of three irrigation rates (66%, 100%, and 133 % of a reference rate) and two N rates (100% and 125%) of UF recommendations. Treatments were arranged in a complete randomized block design with four replicates. Br, a tracer for water and nitrate-N was simultaneously applied with the fertilizers in all plots. The objectives of the study were to determine the best irrigation and fertilizer management practice that would give optimum crop yield and yet reduce leaching of nutrients below the root-zone (0-30 cm). The spatial distribution of soil moisture, N-forms and K with depth were determined using soil samples taken at different stages of crop growth. Data showed that bromide moved below the crop root zone even with the low irrigation level. Nitrate-N concentrations increased with the higher N application rate and it moved below the root zone even at the low irrigation level. Ammonium-N concentrations increased with the higher N application rate but most of it was retained in the 0-15 cm depth. Lower K concentrations were found with the higher irrigation rate indicating K leaching below the crop root zone. There was no significant effect of irrigation levels on the yield but there was a significant effect of N rate. It can be concluded that maximization of crop yield and minimization of leaching of nutrients below the rooting zone can be achieved through fertigation if applied according to crop requirements. In this study the best management practices would be a combination of N rate at 125% of UF recommendation and irrigation rate of 66% based on class A pan (Ep) and a crop factor ranging between 0.20 and 1.00 depending on crop age.

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