Use of urea-sulfuric acid and iron chelate to mitigate iron chlorosis in pear

Iron chlorosis represents the most serious nutrient deficiency problem in pears grown on high clay content soils in the major growing districts of California. Iron is less available to the trees due to the high soil pH and the use of irrigation water with high pH and bicarbonates. A replicated trial was established in 1989 with the objective of evaluating the effectiveness of urea-sulfuric acid and iron chelate. Urea-sulfuric acid was shanked into the soil and compared to non-shanked areas followed by 5 monthly drip irrigation applied treatments of urea-sulfuric acid, an iron chelate (Fe-138) and a control. The shanked urea-sulfuric acid treatment was applied in the spring and fall of each year from 1989 through 1992. Trees were rated visually and the number of replants and dead trees counted prior to treatment annually from 1989 to 1992. Visual rating data were collected in May and yield data in November 1993, the year after treatment. Soil samples were taken in June 1993 from each treatment, at five positions and three depths. The second through fourth years of treatments showed that iron chelate gave the greatest improvement in visual rating response. Urea-sulfuric acid, a more economical treatment, resulted in a lesser improvement based on the visual ratings but was much better than the control. Iron chelate and urea-sulfuric acid through the drip gave the same but better visual ratings than the control during the fifth year when no treatments were applied. Shanking of urea-sulfuric acid was the least effective treatment but there was still a trend for it to improve tree health particularly during the 3rd, 4th and 5th year. Yield and tree survival was significantly improved by both drip treatments but only a trend for improvement was noted with shanked urea-sulfuric acid. Yield increased significantly with the decrease in soil pH measured directly under the emitter with the lowest yield at 7.4 pH and the highest yield at 6.5 pH. Soil pH directly under the drip emitters was reduced below that of the controls of 7.4 by drip applied urea-sulfuric acid (6.5) and chelated Fe (7.0) but not by the shanked US treatments. This in turn increased soluble phosphorus, zinc and iron while decreasing bicarbonates. The acidifying effect was most prominent in the surface 0-30 cms and decreased with depth. Along with soil pH, sodium and bicarbonates increased in the 30-60 and 60-90 cms of soil depth. High pH and bicarbonates coupled with a low calcium:magnesium ratio of the irrigation water helps explain the corresponding high soil levels and resultant difficulty correcting the iron chlorosis problem.