Soil-Applied Iron EDDHA Fertilizer for Correcting Iron Deficiency Symptoms In a Mature Pecan Orchard with Alkaline and Calcareous Soil

Susceptible plants in New Mexico’s Pecos Basin frequently exhibit symptoms of one or more micronutrient deficiency due to the region’s alkaline and calcareous soils.  Pecan (Carya illinoinensis) trees, which are considered relatively insensitive to low soil iron (Fe) availability, will often in the Pecos basin exhibit interveinal leaf chlorosis symptoms characteristic of Fe deficiency.   In orchards where Fe chlorosis symptoms occur, affected trees are found distributed in no apparent pattern.    Foliar application of ferrous sulfate is reportedly ineffective in correcting these symptoms and symptomatic trees decline over time.  Soil-applied Fe-EDDHA can be used to correct Fe deficiency in some crops grown in alkaline and calcareous soils, but this has not been tested in pecan.   On 16 July 2009 fifteen trees exhibiting apparent Fe deficiency symptoms and five adjacent asymptomatic trees were selected in a mature ‘Western’ pecan orchard in the Pecos Basin.  Symptom severity was quantified for each tree through SPAD (“greenness”) readings taken on 8 leaflets.  Each of the symptomatic trees was given one of three Fe-EDDHA fertilizer treatments (fertilizer was broadcast to the entire area within the dripline of the tree canopy) on 6 August 2009: 1) 200 g Fe-EDDHA per tree, 2) 100 g Fe-EDDHA per tree, or 3) 0 g Fe-EDDHA.  On 30 April and 27 May 2010 Fe-EDDHA fertilizer treatments were repeated for each tree, but the fertilizer was applied as a drench in a circle around the dripline of each treated tree.  The impact of Fe-EDDHA applications on symptom severity was evaluated with SPAD readings taken on each symptomatic and asymptomatic tree 20 September 2009 and 7 July 2010.  A single broadcast Fe-EDDHA application did not affect  severity of apparent Fe deficiency symptoms in 2009, but by July 2010, after three fertilizer applications, SPAD readings of about half of the treated trees had increased relative to that of untreated trees of similar 2009 symptom severity.  In 2010 there was no clear difference in tree response between the 100 and 200 g tree^-1 treatment rates and treated trees with improved SPAD readings still had lower SPAD readings than that of asymptomatic trees.  The results show that soil-applied Fe-EDDHA may alleviate Fe deficiency symptoms in mature pecan trees grown on alkaline and calcareous soils.