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Decades of heavy phosphorus (P) fertilization of vegetable crops in the Salinas Valley of California has increased soil test P (STP) levels, with bicarbonate-extractable P (P bc ) values >50 mg.kg -1 now common. To evaluate the response of lettuce (Lactuca sativa L.) to P fertilization in fields with elevated STP levels, 12 trials were conducted in commercial fields during 2002-03. Initial P bc at the trial sites varied from 53 to 171 mg.kg -1 . In each trial, four replicate plots receiving the growers' P application were compared with paired plots in which no Pwas applied. Leaf P was monitored at midseason and at harvest. At harvest, mean whole and marketable plant mass and percent of marketable plants were recorded. A significant increase in lettuce yield with P fertilization was achieved at only one trial site, a spring planting with 54 mg.kg -1 P bc ; at all other sites, including three with P bc <60 mg.kg -1 , P application resulted in no significant yield increase. Phosphorus application resulted in only a marginal increase in plant P uptake; in the nonresponsive fields leaf P concentration of nonfertilized plots was in excess of established sufficiency levels. In a laboratory study, the correlation of P bc to bioavailable P (P ba ) was evaluated using 30 representative Salinas Valley soils; P bc varied among these soils from 15 to 177 mg.kg -1 . P ba was estimated by P adsorption on an anion resin membrane during a 16 hour incubation. The effect of temperature on P bioavailability in six of these soils was estimated by conducting the P ba incubation at 5, 15, and 25 °C. P ba was highly correlated with P bc (r = 0.89), and increased about 40% across soils with each 10 °C increase in soil temperature. Therefore, P bc was determined to be an accurate reflection of bioavailable P in these soils, although the addition of a temperature correction factor in setting threshold values is desirable.

Lettuce Response to Phosphorus Fertilization in High Phosphorus soils