Saturday, 15 July 2006

Chemical extraction methods for various K pools in a dynamic soil K model. A case study in intensive rice cropping in the Mekong Delta, Vietnam.

My Hoa Nguyen, Cantho University, Vietnam, Campus 2, 3/2 street, Cantho city, Vietnam, Bert H. Janssen, Wageningen University, P.O Box 430.6700AK, Wageningen, Netherlands, Oene Oenema, Wageningen University and Research Centre, Alterra, Wageningen, Netherlands, and Achim Dobermann, Department of Agronomy and Horticulture, University of Nebraska-Lincoln, 253 Keim Hall, Lincoln, NE 68583-0915.


            In the Mekong Delta, intensive triple rice cropping is depleting soil K. Analytical methods for characterization of various soil K pools need to be evaluated for prediction of soil K deficiency risk using a simulation model. Possible methods include solution K extracted by 0.01M CaCl2, exchangeable K by 1M NH4OAc pH7, non-exchangeable K by 1M boiling HNO3, 0.2M Na TetraPhenylBorate (5 min), and total K by concentrated HClO4-HF. Two greenhouse experiments were set up, each for 3 rice crops, to study changes in soil K pools: an exhaustion experiment with 19 soils and 5 weeks per crop, and an experiment with 4 soils without or with fertilizer K, and 3 months per crop. Soil K fractions including K(CaCl2), K(NH4OAc), K(HNO3), and K(NaTPB) were measured at the beginning and at the end of each crop in the first experiment and after 3 crops in the second experiment. K(HClO4-HF) was measured only at the beginning of the experiment.

            In the exhaustion experiment, the order of the amounts of extracted K was 0.01M CaCl2 < 1M NH4OAc pH7< 1M boiling HNO3 < 0.2M Na-TPB (5 min). R2 values for the relationships between K fractions and K uptake by one and three crops  were 0.73 and 0.61, respectively for K(CaCl2), 0.88 and 0.89 for K(NH4OAc), 0.51 and 0.57 for K(HNO3), 0.58 and 0.71 for K(NaTPB) if all soils, and 0.79 and 0.90 if only clay soils were taken into account. K(NH4OAc), which includes both soluble K and exchangeable K, was used as the first pool in a three-pool soil K model. The increased correlation between crop K uptake and K(NaTPB) with successive crops, and the higher R2 values for clay soils suggested that K(NaTPB) could be used to describe soil K with intermediate availability to the crop. We developed a 3-pool soil K model (Fig. 1), including (1) labile K (LK) extracted by NH4OAc, (2) intermediate K (IK) defined as the difference between K(NaTPB) and K(NH4OAc), and (3) stable K (SK) defined as the difference between K(HClO4-HF) and K(NaTPB). The flows between these pools in the two experiments were calculated based on the net effects of forward and backward reactions.

Figure 1: The three-pool soil K model. The flows between soil pools are denoted by three letters, the first is always F (flux), and the next two letters stand for the pool from where the flux departs and where the flux arrives, respectively. L stands for labile, I for intermediate,  S for stable K, and U for crop K uptake.


In the second experiment IK was larger, whereas in the first experiment IK was smaller at the end than in the beginning. So, in the second experiment the calculated flows of K leaving the intermediate pool were smaller than the flows entering the intermediate pool, which is unlikely. The reason for the decrease in IK in the first experiment is that under successive exhaustion conditions, the soil had insufficient time to recover. In contrast, in the second experiment the extraction by the crop was less intensive and the soil had more time to recover. Buffering of interlayer K extracted with the short-term (5-min) 0.2M NaTPB method resulted in no measurable increases or decreases of the IK pool in clay soils. In conclusion, K(NH4OAc) can be used as the labile K pool in the soil K model, but net K(NaTPB) failed to act as the intermediate K pool. A longer extraction period with NaTPB may allow quantifying an additional pool of interlayer K .

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