bitstream/item/210624/1/Potassium-Management-in-Humid-Tropical-Oxisols.pd

BASTOS, J. B.

CRAVO, M. D. S.

SMYTH, T. J.

English

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12íii' 10S8:E.~ 6>-iij 4Õ•....2•oBanded P(kg/ha/crop)o Oo 11II 24• 44688 176 264 352Cowpeas. •• • Oc:, c:,c:,:E.!!! 3>-2Ia~ 1ooBanded P(kg/ha/crop)o Oo 11II 22• 44oo 88Total Applied P (kg/ha)Figure 5. Cumulative corn and cowpea yields in rela-tion to total applied P during four years in Manaus.ImplicationsResulrs from this study have been used in the farmer-dernonstration trials which UEPAE/Manuas hasestablished as its participation in the State Rural In-tegration and Development Programo Soil-test criticallevels for P have aided in the selection of appropriatesites for com and cowpea trials. In P deficient areas,cowpea trials were established with and without 22kg/ha of banded P.CLAYEY OXISOLSPotassium ManagementIn Humid Tropical OxisolsManoel S. Cravo, EMBRAPAThomas Jot Smyth, N.C. State UniversityJoaquim B. Bastos, EMBRAPAAbundant rainfall in the Central Brazilian Amazon,and the naturally low cation exchange capacities ofthe regions' s clayey Oxisols, promote the rapid leachingof potassium from the soils. ative soil K reserves canbe expected to be small, since clay mineralogy ispredominantly kaolinitic. These conditions suggest thatcrop production on these soils will require continualK fertilization, and that management systems shouldbe considered to minimize K losses.The objectives of this project were 1) to establishK response curves and soil-test calibration data for themain annual crops cultivated in the region, and 2) todetermine whether split applications would improvethe efficiency of K fertilization.Yield Response to Broadcast KThe first phase of this project examined the yieldresponse of a com-cowpea rotation to annual broad-cast rates of O, 16.5, 33,66, and 132 kg K/ha, ap-plied at com planting during two consecutive years.Although maximum com yields were obtained in bothyears with 66 kg K/ha, yield differences between 33and 66 kg K/ha were not significam. Cowpea yieldresponses to residual fertilizer K were not significamin eirher crop. Measuremems at harvest of K uptakein ali crops indicated that maximum fertilizer K utiliza-tion (39%) occurred with the application of 66 kgK/ha. Periodic soil K measurernents to a 60 cm depthindicated significam increases in subsoil K only in thetreatrnent with 132 kg K/ha.Effects of Split ApplicationsA second K experiment was established in December,198 3 to evaluate whether split K applications mightreduce fertilizer K requirements or improve the effi-ciency ofK fertilization in a com crop. Three methodsof K application were combined wirh four K rates ina factorial design with four replications, Porassium rateswere O, 16.5, 33, and 49.5 kg K/ha. Methods of Kapplication were 1) 100% broadcast ar planring, 2)50% broadcast at planting and 50% sidedressed ar 55days, and 3) divided equally among three broadcastapplications-at planting, 25 days and 55 days. Timesfor side-dressed K applications were selected to coin-97CLAYEY OXISOLScide with applications, in order to avoid additionallabor inputs. Potassium was applied to corn in an an-nual rotation with cowpeas.Yield data for four consecutive crops of the corn-cowpea rotation are shown in Tables 1 and 2. FoliarK levels are not shown but ranged from 1.13 to 2.05%in the 1984 corn crop, from .90 to 1.83% in the 1984cowpea crop, and from .79 to 1. 71 in the 1985 corncrop. Yields and leaf K levels in ali crops increasedsignificantly with rates of applied K, whereas meaneffects of split applications were not significant. Cornyields in 1984 tended to increase with the number ofsplit applications to the maximum K rate. LeafK levelsfor this crop suggested luxury K consumption when49.5 kg K/ha was parcelled into three applications.Foliar K levels for corn in 1985 were inferior to cornin 1984 for identical fertilizer K treatments. The narivesoil K supply may have declined by the second yearof cultivation.The 1984 cowpea crop showed a lower yieldresponse to residual K fertilizer treatments, cornparedto the response in corn. In 1985, cowpea yields ingeneral were decreased in the residual K fertilizertreatrnents. Plant and soil analyses for the latter crophave not been completed.Soil and leaf analyses for corn in 1985 suggestedthat Mg was a potential constraint for this crop. FoliarMg concentrations declined below the recommendedsufficiency range with increasing rates of applied K(Figure 1). Soil Mg levels for ali treatrnents were .1mel 100 ml and represented less than 5% of the ef-fective CEC. Ratios of soi! exchangeable Mg:K decreas-Table 1. Yield response in 1984 of a corn-cowpea rotation to rates and methods of K application to thecorn crop.AppliedK 2 3 MeanSplit Applications2 3 MeanSplit Applicationskg/haO16.53349.5MeanLSD .05K ratePlacementRate X placement---------------------co rn y ie 1d (ti h a) ---------------------1.2 1.2 1.1 1.21.8 1.8 1.8 1.81.7 2.0 2.0 1.92.1 2.4 2.8 2.41.7 1.9 1.9 ~.4nsns-- cow pea y ie 1d .0.7 0.5 0.6 0.60.8 0.6 0.8 0.70.7 0.8 0.9 0.80.7 0.8 0.8 0.80.7 0.7 0.80.1nsnsTable 2. Yield response in 1985 of a corn-cowpea rotation to rates and methods of K application to thecorn crop.AppliedK 2 3 MeanSplit Applications1 2 3 MeanSplit Applicationskg/haO16.53349.5MeanLSD .05K ratePlacementRate X placement..................... corn yield (t/ha) .1.6 1.5 1.5 1.52.3 2.0 1.8 2.02.3 2.3 2.4 2.32.7 2.9 2.1 2.62.2 2.2 1.90.2ns0.4. cow pea y iel d .0.8 0.8 0.8 0.80.7 0.6 0.4 ~60.6 0.7 0.7 0.60.7 0.9 0.6 0.70.7 0.7 0.60.1nsns98CLAYEY OXISOLSTable 3. Total K uptake and fertilizer K utilization efficiency (UE) as a function of rates and placementof K fertilizer for corn.1984 Corn 1985Applied Split K Uptake K Utilization Eff. K Utiliz.K Applic. Corn Cowpea Corn Cowpea 80th Uptake Eff.kg/ha ············kg/ha············ ••••••••••••••••••••••• % ••••••••••••••••••••••• kg/ha •••••• % ••••••O 1 19.7 10.1 10.72 20.5 9.8 11.53 19.5 8.8 11.516.5 1 26.6 16.8 41 44 84 20.9 592 24.4 16.0 27 38 66 18.0 413 26.0 11.8 37 13 50 19.3 4933 1 24.9 16.2 15 20 35 21.3 302 32.9 18.9 39 28 68 23.8 383 30.3 17.6 32 24 58 25.2 4249.5 1 32.2 16.7 25 14 39 25.2 282 38.7 17.7 38 16 54 24.8 273 39.7 19.4 40 20 60 23.2 24LSD .05K rate 4.0 2.8 2.5Placement ns ns nsRate X placement ns ns nsed frorn 1.8 to 1.3 as K rates were increased from Oto 49.5 kg/ha. Lower values of this ratio were notobtained due to the low levels of excl1angeable K(21- 3O ppm) maintained in the topsoil.Total K uptake at harvest was determined for thethree initial crops (Table 3). The efficiency of fertilizerK use was determined by comparing the amount ofK applied each year to the amount detected in the cropsat harvest. Native soil K uptake for each crop wasassumed to be constant for all K treatments, andequivalent to the average of the rreatrnents withoutK. For crops in 1984, rhe effects of split applicationson fertilize r K utilization varied with the rates of Kapplied. Total K uptake and utilization of fertilizer Kwere increased by split applications as K rates increas-ed. For com in 1985, however, K uptake was lowerwith split applications at the rate of 49.5 kg K/ha.ObservationsIt is too early to draw firm conclusions frorn thisstudy. Although current data have not indicated a clearyield advantage from splir K applications, this prac-tice may help reduce fertilize r K losses by maintain-ing a greater proportion of this element in cropresidues.3 0.3YieldíãJ:: 2 0.2 r-- (li~ !li-"C ~Q)>=c.c~I: c... Mg 0.1 -ooIo o0.5 2.01.0 1.5Leaf K (%)Figure 1. Relationship between K content in corn earleaf, corn yields and Mg content in the leaf.99

Potassium management in Humid Tropical Oxisols.

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Potassium management in Humid Tropical Oxisols.

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