Ground Cover Supplement : GC Supplement - Profit from crop nutrition 2013
11 northern region by dr Mike bell and chris Guppy TABleS 1 AND 2 below outline some rough ‘rules of thumb’ used to interpret phosphorus and potassium soil tests in terms of likely fertiliser responsiveness on northern region vertosols. These tables are still ‘works in progress’ and will be refined as more nutrient information comes to light via the second phase of the More Profit from Crop Nutrition (MPCN) program. At this stage, the tables might help answer the question: “Should I apply phosphorus and potassium as a surface or deep-soil application to generate a crop yield response on vertosols?” phOsphOrus Table 1 lists the critical phosphorus values below which a response to subsoil or surface fertiliser phosphorus is likely. It is important to remember that Colwell and BSeS phosphorus pools are distinct. Colwell-P measures the labile, readily available phosphorus pool while BSeS-P measures this labile pool and also a slowly released phosphorus pool. In northern soils, where phosphorus fixation is generally low, the availability of Colwell-P is excellent and when present in reasonable concentrations, the Colwell-P pool represents the crop’s major source of phosphorus. The importance of the slow-release phosphorus pool is still being explored, but at this stage it is not understood to release phosphorus quickly enough to sustain crop yields in the absence of more readily available Colwell-P. However, when reserves of slow-release phosphorus (BSeS-P) are present in a large soil volume (the subsoil root zone), the small amount of phosphorus released per unit of soil volume makes a major contribution to crop phosphorus uptake later in the season. Critical soil phosphorus concentrations differ according to crop type and different soil layers are important at different growth stages. For example, the critical Colwell-P in the 0-to-10-centimetre layer for maize and wheat is between 25 and 30 milligrams of phosphorus per kilogram, while peanuts require only 12 to 15mg P/kg with limited responses above this range. Similarly, wheat, barley and sorghum have an obligate requirement for a high phosphorus concentration in the seedling root zone in the early stages of crop growth, when grain number is being established. Typically, less than 10 per cent of total crop phosphorus uptake occurs during this early growth period, but this shallow soil phosphorus requirement forms the basis of the starter phosphorus strategies. However, as root systems develop this starter phosphorus and the shallow soil layers become relatively less important and, in some cases, such as dry seasons, insignificant. pOtassIum It is more difficult to establish rules of thumb for potassium availability, but Table 2 summarises current thinking. A particular limitation at present is that there is no commercial equivalent of the BSeS-P test to identify the presence of the slow-release potassium reserves that are present in some soils. Work to develop a potentially useful potassium test is continuing (see page 12 of this supplement). While the critical exchangeable potassium values in Table 2 will be applicable to most crops, cotton is known to have a high potassium requirement and critical values for this crop can be almost twice those reported in Table 2. More research is required to improve the precision of these critical potassium values and to understand why the values increase when soil sodium or magnesium status is high (when CeC increases). Research via the MPCN program is investigating the factors influencing potassium availability in northern region soils and will help us determine the best way to apply potassium to generate the greatest crop response. For example, whether potassium is broadcast widely to enrich a large soil volume a little, or whether it is concentrated in multiple bands at various row spacings. □ Grdc research codes uQ00066, uQ00063 More information: Dr Mike Bell, QAAFI, 0429 600 730, firstname.lastname@example.org Soil testing for crop nutrition Fact sheet: www.grdc.com.au/GRDC-FS-Soiltesting taBle 1 Critical p values used to determine likely response or drivers of phosphorus availability in northern vertosols. Surface (0 to 10cm) Subsoil (10 to 30cm) Colwell-P <20 to 25mg/kg >60mg/kg Likely to get a response to starter P Ensure good ground cover to limit erosion risk and to avoid nutrient loss in runoff <10mg/kg >100mg/kg Likely to get a response to deep P placements, unless BSES P high Unlikely to see P deficiency in your lifetime BSES P <25mg/kg >100mg/kg Limited evidence of residual fertiliser accumulation High residual fertiliser load; slowly available to surface roots <30mg/kg >100mg/kg Limited reserves of slowly available P. Consider replacement of removed P once every five years Potential to slowly replace Colwell-P reserves and support crop growth in large soil volumes ta Ble 2 Critical K values used to determine likely response or drivers of K availability in northern vertosols. Surface (0 to 10cm) Subsoil (10 to 30cm) CEC (Cation exchange capacity) ExK (cmol/kg) High Mg (>30% CEC) or Na (>6% CEC) ExK (cmol/kg) High Mg (>30% CEC) or Na (>6% CEC) <30cmol/kg 0.2 0.3 0.1 0.2 >60cmol/kg 0.4 0.5 0.2 0.3 interpreting soil test results for the northern region translating soil test results into kilograms of fertiliser per hectare can be a challenge, especially in the northern region, where there is the added requirement to sometimes place nutrients at depth Concentrations of cations are expressed in centimoles of positive charge per kilogram of soil (cmol(+)/kg). This measurement is equivalent to the previously used unit me/100g.
GC Supplement - Farm business management 2013
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