Craig Scanlan

Understanding soil potassium (K) availability is critical for developing K fertilizer recommendations, especially in regions with low soil K reserves. The quantity/intensity isotherm and K release were used to evaluate the dynamics of K in loam-and clay-textured soil types developed from highly weathered parent materials in Western Australia (WA). Twenty-one soil types with a wide range of properties were collected from 0–10, 10–20 and 20–30 cm depths on farmland in WA. The equilibrium activity ratio of K+ varied from 0.1 × 10-3 to 45.6 × 10-3 (mol L-1)1/2 and was significantly higher in surface soils than subsurface soils. The labile K (KL) ranged from 0.03 to 2.18 cmolc kg−1, and the contributions of non-specifically adsorbed K towards KL were >50 % in surface soils but lesser in subsoils. The potential buffering capacity (PBCK) ranged from 5.4 to 185 cmolc kg−1 (mol L-1)-1/2 and was significantly correlated with soil effective cation exchange capacity (ECEC), pH and clay content. The ECEC alone explained 94.5 % of the variation in PBCK. The cumulative amounts of K released after 213 h ranged from 27 to 833 mg kg−1, with a rapid release of K up to 22 h followed by a gradual release until the end. In conclusion, most soils, especially surface soils, had adequate K availability for crop production but it is vulnerable to losses and depletion. Given the overall low ECEC and PBCK, split or delayed K applications are recommended, alongside practices that enhance K buffering capacity in WA soils.

Context Current decision support systems (DSS) for phosphorus (P) fertiliser were developed using data from historical cropping systems. An understanding of how soil properties and rainfall influence wheat (Triticum aestivum) response to P fertiliser in current systems is required to optimise P management. Aims The aims of this study were to: (1) assess the soil properties that have the greatest influence on relative yield; (2) examine how rainfall conditions influence relative yield; and (3) examine whether there were interactive effects between rainfall and soil properties on relative yield. Methods Forty P rate-response field experiments were completed in Western Australia. Regression tree modelling, soil test calibration curves and the sliding window approach were used to examine relationships between soil properties or rainfall and relative yield. Key results Phosphorus buffering index (PBI) was important for determining the factors that influence relative yield. For sites with PBI 0-10 cm <56 (n = 30), regression tree modelling showed rainfall before sowing and soil pHCa were important factors (R2 = 0.59). For sites where PBI >56 (n = 10), relative yield was closely related to plant-available P at 0-10 cm and the r-value for the calibration curve was 0.95. Conclusions Rainfall and soil pHCa influence wheat response to P where PBI <56 is attributed to an accumulation of soil P after decades of fertiliser applications and the availability of stored soil P to crops. Implications Pre-sowing rainfall should be included in DSS so that grain producers can make informed, tactical decisions about P fertiliser applications for wheat at sowing.

Context
Long-term negative potassium (K) balances in crop production have depleted soil K levels in Western Australia (WA). Previous research has focussed on sand-textured soils, but recently, monitoring of crops grown on loam-textured soils has shown deficient or marginal shoot K concentrations where Colwell K 0–10 cm is above current critical levels.

Aims
The aims were to examine whether grain yield responses to fertiliser K can be detected on loam-textured soils and if soil test calibration curves can be identified for these soils.

Methods
Eight field trials were conducted with wheat on loam-textured soils. The same experimental design was used at all sites; six levels of K applied at sowing, from 0 to 200 kg K ha−1 with one treatment including a split application. Soil and plant test calibration curves were modelled using measurements from the trials.

Key results
Grain yield responses of 0.69 to 1.37 t ha−1 to fertiliser K (P < 0.05) occurred in 4 of 8 trials. Relative yield was closely related to soil exchangeable K and the goodness of fit of the soil test calibration curves increased as the depth of sampling increased. The best soil test calibration curve was for sampling 0–40 cm.

Conclusions
This research confirms that on some loam-textured soils, yield loss is occurring to K deficiency if no K fertiliser is applied.

In no-till cropping systems, banding of phosphorus (P) fertiliser at seeding results in low use efficiency due to chemical reactions in soil. Foliar P has the potential to allow grain producers to respond tactically with P application after sowing when P supply from soil and fertiliser is not meeting crop demand. The objective of this study was to evaluate the effectiveness of foliar P on wheat grain yield, grain quality, biomass yield, P uptake and P use efficiency indices. Nine field experiments were conducted to investigate the response of wheat to foliar P. Three rates of P, 0, 2.5 and 5.0 kg/ha, as phosphoric acid (H3PO4 85%) were applied to wheat at three different growth stages: first tiller emergence (Z21), first node detection (Z31) and flag leaf emergence (Z39). Grain yield responses ranging from 176 kg/ha to 505 kg/ha to foliar-applied P were observed in six out of nine experiments. The percent grain yield response to foliar P was negatively related to the P buffering index (PBI, 0–10 cm soil depth), which is attributed to greater sorption by soil of the foliar P at the higher PBI levels. Mean agronomic efficiency (AE) across the experiments was 111 kg/kg P but reached up to 232 kg/kg P. It was also evident that foliar P has the potential to improve P concentration in shoots and grains and increase P uptake but with no or minimal effect on grain quality. Our results suggest that a combination of tissue testing at the seedling stage and soil P buffering can be used to guide when foliar P application is likely to increase grain yield in wheat.

Context
The NaHCO3-extractable soil K test (Colwell K) is the most frequently used method for soil K across Australia, but there is still uncertainty about the suitability of K testing guidelines for heavy-textured soils.

Aims
To determine whether Colwell K is suitable for predicting plant-available K on loam- and clay-textured soil types developed from highly weathered soil parent materials in Western Australia (WA).

Methods
Nine soil K testing methods were used to determine plant-available K on 21 soil types with a wide range of properties collected from three depths on the northern, central and southern farmlands in WA.

Key results
Quartz and K-feldspars were the dominant minerals in all soils, while limited white mica (1–2%) was identified in less than 10% of samples. The amount of K extracted by silver thiourea was only about 70% of the amounts extracted by NaHCO3 (Colwell K) and NH4OAc. Soil non-exchangeable K extracted by nitric acid and sodium tetraphenyl borate were from similar K pools, while aqua regia K was 1–6 times higher than these values.

Conclusion
There was no systemic difference or proportional bias between NH4OAc K and Colwell K, and both had good model performance (R2 > 0.86) for total K uptake by wheat in a single growing season in a wide range of soils.

Implication
In this diverse range of soils formed from weathered parent materials, the reliance on exchangeable K for plant K uptake suggests that plant-available K will be vulnerable to rundown due to negative K balances.

Factors affecting fertilizer decisions made by grain growers are changing in the context of changing climatic conditions and growing volatility in global fertilizer and grain markets. To ensure sustainable development of grain industries in light of this uncertainty, research, development, extension, and adoption activities associated with growers’ fertilizer decisions need to be focused on factors to which they are most sensitive. The aim of this paper is to understand the factors that have the greatest influence on grain producer’s fertilizer strategies, how these factors have changed over recent years, and what is the relative importance of agronomic, socioeconomic, and logistical factors affecting these strategies. A telephone survey of 425 grain-growing businesses in Western Australia was conducted, and survey results were analyzed statistically. We show for the first time that grain growers’ fertilizer decisions are most sensitive to agronomic factors (especially the amount and distribution of rainfall). Logistic factors (such as difficulties fertilizing increasing areas in short periods of time) are growing in influence as farm size, cropping areas, and the number of fertilizer applications within seasons increase. Fertilizer decisions have become less sensitive to socioeconomic factors over the last 10 to 15 years. To ensure sustainable development of grain production, research through to adoption activities should focus on agronomic issues (such as seasonal forecasting) and logistic issues (such as improving planning, organizational, and technical capacity for developing and implementing fertilizer strategies).

Abstract

Negative potassium (K) balances on farmlands globally are widespread because fertiliser K input is often less than losses (leaching) and removal of K in hay, straw and grain, which leads to a rundown of plant-available K. When soil K reserves are not large and the plant-available K pools are not well buffered, the risk of K rundown in soils is high. In the south-west of Western Australia, soil K rundown, particularly by continuous cropping or in systems where a large portion of crop biomass is removed, is increasing the prevalence of crop K deficiency even on soils where K was not previously a limiting factor for crop yields. While fertiliser K is required for adequate supply of plant-available K, maximising K use efficiency is also important for cropping profitability and sustainability in dryland agriculture. Plant K uptake and use efficiency can be affected by soil types, crop species and sequences, seasonal conditions, and K management. In water-limited environments, crop K nutrition, especially root access to subsoil K, plays a crucial role in promoting root growth, regulating plant water relations and alleviating biotic and abiotic stresses. Optimised use of both soil and fertiliser K is increasingly necessary to sustain crop yields under stressed conditions in the context of K rundown in soils.

Purpose Soil water repellence causes uneven soil wetting which can constrain dryland crop and pasture establishment and yield. The same processes are likely to affect nutrient availability from soil and fertiliser, but the effects of repellence on crop growth and nutrition per se have seldom been reported. Here, we investigated early wheat (Triticum aestivum cv. Mace) growth and nutrient uptake responses to repellence. Methods Wheat was furrow-sown in severely repellent sandy loam soil (with a wettable furrow base to allow for germination) or completely wettable soil, under uniform plant density and variable topsoil thickness (20 or 100 mm) and fertiliser band placement (below or away from the seed). Tiller number, shoot dry matter, shoot N concentration, total nutrient uptake, and root length density (RLD) were determined. Results Contrary to expectations, repellence significantly increased tiller number (by up to 2 tillers per plant), shoot dry matter (by 82%), shoot N concentration (by 0.3% N), and total nutrient uptake (by 87%) at 51 days after sowing, regardless of topsoil thickness and fertiliser placement. In the furrow, RLD of repellent treatments was also nearly double that in wettable treatments when fertiliser was banded below the seed. Results suggest that preferential soil wetting of the furrow in repellent treatments favoured plant nutrient uptake under regular but low water supply. Conclusion We conclude that for water-repellent soils with limited water supply, water harvesting techniques such as furrow sowing and banding wetting agents could boost water and nutrient uptake and early crop growth.

A controlled-environment study was conducted to explore possible synergistic interactions between the feremycorrhizal (FM) fungus Austroboletus occidentalis and soil free-living N2-fixing bacteria (diazotrophs). Wheat (Triticum aestivum) plants were grown under N deficiency conditions in a field soil without adding microbial inoculum (control: only containing soil indigenous microbes), or inoculated with a consortium containing four free-living diazotroph isolates (diazotrophs treatment), A. occidentalis inoculum (FM treatment), or both diazotrophs and A. occidentalis inoculums (dual treatment). After 7 weeks of growth, significantly greater shoot biomass was observed in plants inoculated with diazotrophs (by 25%), A. occidentalis (by 101%), and combined inoculums (by 106%), compared to the non-inoculated control treatment. All inoculated plants also had higher shoot nutrient contents (including N, P, K, Mg, Zn, Cu, and Mn) than the control treatment. Compared to the control and diazotrophs treatments, significantly greater shoot N content was observed in the FM treatment (i.e., synergism between the FM fungus and soil indigenous diazotrophs). Dually inoculated plants had the highest content of nutrients in shoots (e.g., N, P, K, S, Mg, Zn, Cu, and Mn) and soil total N (13–24% higher than the other treatments), i.e., synergism between the FM fungus and added diazotrophs. Root colonization by soil indigenous arbuscular mycorrhizal fungi declined in all inoculated plants compared to control. Non-metric multidimensional scaling (NMDS) analysis of the bacterial 16S rRNA gene amplicons revealed that the FM fungus modified the soil microbiome. Our in vitro study indicated that A. occidentalis could not grow on substrates containing lignocellulosic materials or sucrose, but grew on media supplemented with hexoses such as glucose and fructose, indicating that the FM fungus has limited saprotrophic capacity similar to ectomycorrhizal fungi. The results revealed synergistic interactions between A. occidentalis and soil free-living diazotrophs, indicating a potential to boost microbial N2 fixation for non-legume crops.

Approximately 5 and 29% of soils used for crop production globally have a sand or gravelly subsoil. The proportion of cropping soils with sand subsoil is greatest in Africa and Australia and Oceania. The countries with the greatest area of soil with gravel subsoil used for cropping are India and China. Sand and gravel subsoils have a limited capacity to supply soil and water to crops. Sand subsoils by definition have low clay content and, as a result, low water and nutrient storage capacity. The capacity of gravel subsoils to store water and nutrients decreases as gravel content increases. Although crop roots can access water and nutrients from these subsoils, the depth of these resources and physical constraints to root growth limit the efficiency of their use. Sand and gravel subsoils can constrain root growth although the mechanisms differ. Root growth is constrained in sand subsoils by constraints that can develop under crop production: compaction and aluminium toxicity. The impact of gravel subsoils on crop growth depends upon the penetrability of the gravel layer by crop roots. For impenetrable gravel layers, the properties of the topsoil will have the greatest influence on crop growth. For penetrable layers, root depth or length decreases as gravel content increases. There is potential to adapt agronomic management to maximise production on soils with sand or gravel subsoils. Split applications of nutrients can minimise leaching risk. There is evidence that the constraints that develop on sand subsoils due to crop production can be ameliorated profitably.