Phosphorus and Potassium Nutrient Management of Mungbean (Vigna radiata L.) in Sandy Soils

Abstract

<p>Mungbean (<i>Vigna radiata</i> L.) is an excellent alternative protein source for healthy, vegetarian diets due to its nutritional superiority. As a legume, it is also beneficial in intensive cropping systems due to nitrogen fixing characteristics and potential adaptability to broad agro-climatic conditions. However potential yield is not achieved in most cropping areas due to biotic and abiotic stresses, including nutrient imbalance. Appropriate nutrient management based on understanding different soil phosphorus (P) and potassium (K) requirements can maximize yield in sustainable ways. Mungbeans have a high requirement for P and K and deep placement of K with sufficient topsoil P may increase yields in clay soils. However, there is a need for further understanding to ascertain the true characteristics of mungbean interaction with placement of P and K in sandy soils. This thesis was constructed to provide understanding of the P and K nutrition of mungbean, with the particular aim of determining an optimum P and K fertilisation strategy for mungbean sown under P and K limited sandy soil conditions. An additional aim was to understand P uptake dynamics through root-nutrient interaction as P supply changes. A review of literature revealed gaps in understanding mungbean P and K nutrition.</p> <p>Sandy soils with low P and K are prevalent through major mungbean production areas in tropical countries including Myanmar. An initially examined P and K requirements of two Australia commercial varieties, Jade–Au and Cellera II–AU was undertaken. The application of P at the rate of 120 mg/ kg resulted in the highest vegetative yield and P and K recovery despite tissue P and K concentrations remaining lower than critical levels after 44 days of growth. Phosphorus utilisation efficiency (mg DM/mg P in shoot) declined with increasing P application rate in both varieties and a larger maximum shoot yield produced in Jade–Au at 60 mg P/ kg, whilst 120 mg kg P/kg was required in Cellera II–AU. Based on low shoot tissue P and K concentrations and rooting volume effects, a second experiment studied root responses such as root mass density, root distribution down the profile and available rooting volume on plant P and K recovery and growth in pots ranging from 25 to 150 cm deep. A 50 cm depth was selected for future studies due to the root mass density (RMD) responses in the surface 25 cm best reflecting normal rooting behavior. In 50 cm deep pots, RMD was ~ 3, 2 and 1.2 times that of 100 and 50 cm and 25 cm pots respectively, and plant growth and P uptake per unit root mass was ~1.6 times that of 150 cm pots.</p> <p>Phosphorus is deficient in most tropical legume growing regions and is often stratified due to natural topsoil P replenishment by crop residues and broadcast fertiliser application, and subsoil P depletion by deep rooted crops. Deep placement of P has been suggested to increase the effectiveness of fertiliser application in soils with a long cropping history in Australia. This cropping history has also often exhausted K reserves. Precise fertiliser placement may be important for P and K management and root responses where P and K mobility increases in sandy soil. Different placement approaches, such as uniform, shallow and deep banding of P and/or K on root behaviour showed that P improved overall root mass density (RMD) which consequently increased K uptake by up to 5-fold. Potassium alone did not increase RMD and decreased K recovery to less than 50% of applied K. Phosphorus was immobile in this soil and limited leaching of P was observed despite the low P buffer capacity. In contrast, K leached >10cm down the soil profile over the course of the experiment despite watering strategies designed to minimize leaching. Placement method did not affect P and K acquisition efficiency although it increased ~ 5 - 6 times in the absence of P. Placement depth was not significant for mungbeans grown for 5 weeks although P uptake was doubled when K was co-located in the band</p> <p>Research on co-application of P and K has produced variable results. Factors contributing to these variable results include placement depth, rhizosphere pH, and the counter-ion of the K source were investigated using isotopic ³²P labelled fertiliser techniques. Phosphorus was applied alone or mixed with four K sources (nitrate, chloride, sulfate, or potassium phosphate). There was no significant difference between treatments in fertiliser P recovery with either surface or deep placement. The only significant effect was higher shoot and root yield and higher fertiliser P and K recovery in most treatments compared with the treatments were applied alone, with the highest values observed where P was co-located in a shallow K sulfate band. The increased P uptake was not attributable to increased root growth in the fertiliser band or to a difference in band or rhizosphere pH as these were not significantly different in this treatment from the other K source treatments.</p> <p>Legume crops vary widely in their capability to acquire and use P. The adaptability of mungbean to changes in P supply in the immediate root zone and the ability to up-regulate P transporters as soil solution P concentrations vary is still less understood. Therefore a final experiment was undertaken to identify the root plasticity of P uptake per unit root length/mass when a low P status mungbean plant experiences a short term adequate P supply. Plants were grown in either low (1.6 mg/L), medium (3.3 mg/L) or high (6.6 mg/L) nutrient solution for 33 days and then transferred to an adequate P solution containing (8.2 mg/L)with isotopic ³²P labeled P for an additional 4 and 9 days. Phosphorus uptake was monitored by in situ counting of the ³²P in the youngest fully expanded leaves and plants were harvested 4 and 9 days (d) after transfer. Short term adequate P supply increased plant growth, P uptake and P allocation to shoots. After 4d in the transfer solution 20.5% of the P in the low P plants was derived from the transfer solution compared to 10.7% in the high P plants, perhaps indicating rapid upregulation of P uptake. By day 9 these values had increased to 23.2 and 17.1%, respectively. Between 4 and 9 days fine root length increased 4 fold in the low P plants compared to a 2 fold and one-tenth increase in medium and coarse roots. The rate of P uptake from the transfer solution (Pdfs, mg/cm/d) was 2.8 in the low P plants at 4d compared to 0.2 in the high P plants. These results suggest that an increase in fine root length under increased P supply was the primary mechanism responsible for the increased P uptake in mungbean plants initial low P into high P solution situation. It is worth noting that this increase in root length was not observed in soil grown roots of low P status mungbean plants encountering a highly enriched P zone. Hence, it remains unclear which is the most important strategy mungbean plants use (up-regulation of P transport per unit root length, or rapid increase in fine root production) to respond to localized increases in P supply.</p> <p>This thesis contributes new knowledge on P and K application strategies for mungbean production in sandy soil. Key findings shows that root P access can increase with finer root development by P application in low P conditions and co-location of P and K in a band improved applied nutrient recoveries and efficiencies. Band placement of P together with K₂SO₄ irrespective of application depth may improve mungbean crop production. However, in field conditions, the relative differences in mobility of P and K in sandy soils growing mungbean under monsoonal rainfall suggest that broadcast application of K, and banded application of P, may stimulate root growth enough to exploit a larger soil volume and recover K that may otherwise leach below the root zone. Stratified soil test analysis is important before recommending this P and K management strategy to achieve a significant increase in mungbean productivity in each soil type and agro-climatic condition.</p>