Soil potassium dynamics under intensive rice cropping. A case study in the Mekong Delta, Vietnam

Keywords:potassium, nutrient budgets, nutrient depletion, fertilizer, kinetics, adsorption,desorption, fixation, release, modeling, rice cropping system,NaTPB-extractable K, NH 4 OAc-extractable K, sedimentation.&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Rice cropping has been greatly intensified in many Asian countries during the last decades to meet the increasing demand for food by the increasing population. There is some concern now that the increased crop yields and nutrient withdrawal, in combination with unbalanced fertilization, lead to potassium (K) depletion of the soil and to K deficiency in rice. However, reports about crop response to K fertilizer application in wetland rice cropping systems show conflicting results, and there are no proper guidelines for K management.This study was set-up to increase the understanding of K budgets of rice cropping systems, and of K dynamics in soil, to be able to develop K management schemes for the various rice cropping systems. The study was carried out in the Mekong Delta of Vietnam, a major rice growing area. Field studies have been carried out to quantify the inputs and outputs of K in various rice cropping systems. Kinetics of K adsorption-desorption and of K fixation-release in soil has been studied in the laboratory to understand the behavior of K in soils. Pot experiments have been carried out to study effect of water management, K fertilizer application on soil K pools and K uptake, and examine the rate of changes of various K pools in soil by rice, grown under controlled conditions. Models were used to analyze and predict changes of K pools in soils over time, using rate constants and initial pool sizes as derived from the laboratory and pot experiments.K budgets were assessed for areas, representing double and triple rice cropping systems on flooded alluvial soils. Partial K budgets proved inadequate and a differentiation between pools according to the availability of K in sediments was useful. K balances were always positive for total K, and negative forK(NH 4 OAc) unless about 80 kg ha -1 yr -1 fertiliser K was applied, while balances for K(NaTPB) were in between. Removal of rice straw was the largest K output, sedimentation the largest input for total K.The order of the amounts of extracted K was 0.01 M CaCl 2 < 1M NH 4 OAc pH 7 < 1 M HNO 3 < 0.2 MNaTPB. All methods were well correlated with plant uptake of K. Simple first order equations adequately described adsorption, fastening and desorption of K. The rate coefficient of K removal withNaTPBdecreased over time and could be accounted for by the Yang and Janssen equation.&nbsp;In a greenhouse experiment, water management between two successive crops had no influence on soilK(NH 4 OAc) and K(NaTPB), and on K uptake. BecauseK(NaTPB) did not decrease over time, it was concluded that the intermediate pool in a three-pools model could not be assessed by K(NaTPB). Hence the test of athree-poolsmodel failed.&nbsp;Two models were tested using the results of greenhouse experiments. A so-calledSeries model, in which K flows from minerals to an intermediate pool, next to the labile pool of exchangeable K according to first-order kineticswas able to predict the dynamics of soil K pools over time and K uptake in the future.Also areduced Series model with only two pools was well applicable.The so-called Parallel model in which K moves from each of the pools to the solution pool turned out to be useful for a first interpretation of experimental data to identify the sources of K uptake, but it was unable to predict future soil K changes and hence future K uptake.Based on application of the Series model, a decision support system scheme on soil K management was outlined.

Bibliographic Details

Main Author:	Nguyen, M.H.
Other Authors:	Oenema, Oene
Format:	Doctoral thesis biblioteca
Language:	English
Subjects:	cropping systems, fertilizers, intensive cropping, nutrient deficiencies, oryza, potassium, rice, vietnam, intensieve teelt, kalium, kunstmeststoffen, rijst, teeltsystemen, voedingsstoffentekorten,
Online Access:	https://research.wur.nl/en/publications/soil-potassium-dynamics-under-intensive-rice-cropping-a-case-stud
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