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Technical annual report 2000-01



Technical annual report 2000-01


Dept. of Regional Development, Primary Industry, Fisheries and Resources technical annual report; Department of Regional Development, Primary Industry, Fisheries and Resources technical and annual report; Reports; PublicationNT; Technical bulletin (Northern Territory. Dept. of Primary Industry and Fisheries) ; no. 295




Made available via the Publications (Legal Deposit) Act 2004 (NT).






Agriculture -- Northern Territory -- Periodicals; Fisheries -- Northern Territory -- Periodicals

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Dept. of Primary Industry and Fisheries

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Technical bulletin (Northern Territory. Dept. of Primary Industry and Fisheries) ; no. 295



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Technical Annual Report 2000/01 19 Equation 1, base saturation levels in the trial plot Formula for deriving base saturation exchangeable bases (mEq/100g) multiply by 100% Total CEC (mEq/100g) Total CEC = 1.8 mEq/100g The base saturation levels based on soil samples from the trial area equal 99%. This essentially means that although the quantity of ions held by the soil is small they are readily exchanged. Table 8. Breakdown of soil CEC measured within the trial area Calcium 72.8% Potassium 8.3% Magnesium 16.5% Sodium 2.4% Conclusion and recommendations: Cation exchange capacity (CEC) affects the ability of soils to retain adsorbed ions. The low clay content and the type of clay present in Ruby Blain soils results in a limited ability to retain fertiliser in the upper layers of the soil. The calculation of our base saturation indicates that the cations that are adsorbed are readily available to the plant. Ca (6075%) and Mg (10-15%) dominate cation exchange ratios. K comprises approximately 7% of ions on the exchange complex. Uptake by roots occurs through two main processes, passive and active. Ca is likely to be taken up by passive processes whereas K and Mg are taken up by active mechanisms. Mass flow (passive adsorption) ensures adequate supplies of Ca. Active adsorption relies on high concentrations within the soil solution and constant replenishment from the exchange complex. If there is little exchange available from the soil complex then it must be supplied by frequent fertiliser application. Due to the low CEC, more mobile cations are not retained at the surface where most of the active uptake by plants is occurring. The dissolved mobile cations will shift to the lower levels of the irrigation zone. There is a large amount of uptake via the soil solution rather than the soil exchange complex. Monovalent cations are less attracted to the weakly charged clays. Therefore the cations that dominate the soil solution are likely to be the less mobile, more chemically attractive ions, such as Ca++ and Mg++. Dissolved cations that have shifted to deeper profiles will be available to plants with extensive root systems and mychorrizal associations. As the mobile cations are more uniformly distributed throughout the soil profile they are unlikely to affect uptake of other nutrients at the surface. The higher levels of fertiliser present in this trial were most likely shifted through excessive rain and irrigation, therefore unable to dominate the soil solution. The lower rates were able to adequately supply the plant needs, providing some base levels from which we can derive fertiliser recommendations. Despite concern that high levels of K could induce Ca deficiency in peanuts, the results of this trial indicate it is relatively difficult to achieve, given the properties of Blain soils. The kernel disorder that was seen in 1999 was not observed in any treatment. Whilst an explanation for this symptom has not been identified in this trial there have been positive outcomes from this work that further our understanding of nutritional management on the sandy surfaced Blain soils of the Top End. Short term strategies Use more frequent, smaller applications of mobile fertiliser, particularly K, S and N. Use fertigation where possible. Avoid excessively heavy irrigations when plants are small. Adjust timing of planting to avoid large rainfall events where possible. Long term strategy Use low input green manure crops to build up soil structure and assist with fertiliser retention.