Contents & Foreword, Characterizing And ... - IRRI books

gave a positive yield response with application of 75 kg K ha –1 . In the third, fourth,and fifth trials, there was no increase in grain yields due to either increases in soil Kstatus (third and fifth trials) or low in-crop rainfall (fourth trial).The prevalence of high soil K status in Java needs to be considered along withthe K balance of the soil when formulating K fertilizer recommendations. A positivesoil K balance was shown to occur when K was applied to some treatments in Jakenanrainfed rice trials. In the treatments at Jakenan where N and P were added without K,the K balance was negative, whereas adding NPK (120:18:75) gave an increase insoil K (Wihardjaka et al 1998). Trials with irrigated rice in Java (Dobermann et al1996a), however, showed that K uptake was greater than the amount applied, thuscreating a negative K balance in the soil at the end of the season. Cropping systemswith a negative K balance are unsustainable in the long term and K fertilizers need tobe applied to reduce the risk of K deficiencies. Thus, there is only one recommendedrate for the whole of Java: 26 kg K ha –1 (Karama et al 1998).Lowland rice has been shown to be more responsive to fertilizer K in Sumatrathan in Java. This reflects the fact that low-K areas are more prevalent in Lampung(Fig. 5). Unlike in Java, however, soil P maps are not related to soil K maps. Resultsfrom soil K mapping in Lampung are supported by a soil survey conducted in 1998for the Acid Soils Project, which found that 25 of 32 lowland sites had low soil K(166 mgK kg –1 ) (Karama et al 1998). Despite the need for K fertilizer to be applied in thelowlands of Sumatra, no K fertilizer recommendations specific to that region havebeen developed.Developing K recommendations is hampered by the difficulty in relating soil Kextracted by 1M NH 4 Oac to grain yields. Dobermann et al (1996b) concluded thatpredicting crop K uptake in irrigated rice using a static soil test was not practicalbecause too many soil properties (extractable K, Ca, and Mg combined, CEC, organicmatter, clay content) were required to give accurate results. K uptake in irrigatedlowland rice has been shown to be related to soil K (r 2 = 0.82) as measuredusing mixed-bed ion exchange resin capsules (Dobermann et al 1996b). Ion exchangeresins have the advantage of being able to measure the amount of K that can be extractedfrom a soil over time rather than measuring K in solution at a particular moment.Dynamic measurements of K are required for intensive cropping systems andwhere the soils have K-fixing properties.Data used to test the validity of predicting total K uptake at harvest in lowlandrice were derived from NPK trials conducted at 11 sites in five countries includingIndonesia with K as the only limiting factor (Dobermann et al 1996b). K fertilizerrates varied between sites from 25 to 100 kg K ha –1 , soil pH ranged from 5.7 to 8.5,and clay contents ranged from 25% to 57%. This same method could possibly be usedto predict K uptake by rainfed lowland rice in locations within Java and Lampung.182 Clough et al