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C.A. Sanchez, S. Swanson and P.S. Porter

Five field experiments were conducted from 1986 to 1989 to compare broadcast and band P fertilization of crisphead lettuce (Lactuca sativa L.) on Histosols. Rates of P were 0, 50, 100, 200, and 300 kg P/ha applied broadcast or banded. Broadcast P was surface-applied and disked into the soil 1 day before bedding and planting. Banded P was placed in strips 8 cm wide, 5 cm below the lettuce seeds at planting. Lettuce yields were significantly(P < 0.01) increased by P rate in all experiments. However, significant rate-by -placement interactions indicated that response of lettuce to P varied by placement. Lettuce yields were generally optimized with a band P rate one-third of that required with broadcast placement. Analysis of soil samples collected in the lettuce bed after fertilization indicated that banded P increased available P in the lettuce root zone compared to broadcast fertilization. Lettuce leaf P concentration increased with P rate and generally was greater when P was banded. The critical concentration of P in lettuce leaf tissue at the six- to eight-leaf stage was 0.37%. Banding P fertilizer did not reduce the availability of other essential nutrients, as indicated by tissue analysis.

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N.M. El-Hout, C.A. Sanchez and S. Swanson

Potassium is often considered the nutrient element most limiting to crop production on organic soils. On Histosols in southern Florida, K2SO4, rather than KCl, is often used for lettuce (Lactuca sativa L.) production to minimize the risk of salt injury. However, recent soil-test calibration research suggests that current K fertilizer recommendations for lettuce may be too high. Four field studies were conducted from 1989 to 1991 to evaluate the response of five lettuce types to K rate and source. The five lettuce types evaluated were leaf, bibb, boston, romaine (cos), and crisphead. Two sources of fertilizer K (K2SO4 and KCl) were evaluated at rates ranging from 0 to 600 kg K ha-1. Lettuce showed a minimal or no response to K fertilization. Potassium chloride had detrimental effects on lettuce only when applied at rates in excess of those required for optimal production. These studies showed that K fertilizer recommendations for lettuce produced on Histosols in Florida can be reduced. Furthermore, KCl, a more economical source, is suitable when the K is applied at appropriate rates.