This method for ‘Nitric K’ has been used in the sugar industry of Queensland for several decades (e.g. Chapman (1971)). It is based on the extraction of soil K+ by boiling with 1.0 M nitric acid (HNO3) for 30 min at an initial soil/solution ratio of 1:40 (Haysom 1971). The difference between this measure of fixed K+ and exchangeable K+ equates to non-exchangeable available K+ (Chapman 1971; Martin and Sparks 1985).
Diagnostically, the test estimates easily mineralisable K+ available for crop growth. The values are influenced by soil type and texture and by the addition of sugar-mill by-products such as mill mud and boiler ash. Mean values (units of cmolcK/kg) in surface soils (0–250 mm) monitored by BSES Ltd and reported by Chapman et al. (1981) reveal region to region variability as follows: Mossman–Cairns = 2.1; Babinda–Tully = 2.1; Ingham = 1.7; Burdekin = 2.8; Mackay = 1.3; Bundaberg = 0.9; and Maryborough–Rocky Point = 0.8. Nil or little crop response by Queensland sugar cane to applied K fertiliser is expected when the root zone exceeds 0.6 cmolcK/kg by this test (Anon 1998). As soil extracts are further diluted 1+1 prior to analysis, the final soil/solution ratio is 1:80.
Extracting Solution – 1.0 M HNO3
Dilute 66.67 mL of nitric acid (HNO3, 15 mol/L; high quality) to 1.0 L with deionised water and store in a clean plastic container.
0.5 M HNO3
Dilute the 1 M HNO3 Reagent in the ratio 1+1 with deionised water for preparation of standards. Also use this solution to dilute any over-range sample extracts.
Primary Potassium Standard – Nitric K
1 L contains 391.0 mg K+/L (10.0 mmolcK+/L).
Dissolve 0.7456 g potassium chloride (KCl; previously dried at 110°C for 2 h) in about 300 mL of 0.5 M HNO3 then make to 1.0 L with 0.5 M HNO3.
Working Standards for ICPAES – Nitric K
Pipette 1.0, 2.0, 4.0, 6.0, 8.0 and 10.0 mL of Primary Potassium Standard – Nitric K into clean 200 mL volumetric flasks. Add 100 mL Extracting Solution (1.0 M HNO3) to each volumetric flask and make to 200 mL with deionised water and mix well. These solutions contain 0.05, 0.10, 0.20, 0.30, 0.40 and 0.50 mmolcK/L, which correspond to 1.96, 3.91, 7.82, 11.73, 15.64 and 19.55 mg K+/L. For a final soil/extract ratio of 1:80, these are equivalent to soil concentrations of 0.40, 0.80, 1.60, 2.40, 3.20 and 4.00 cmolcK/kg, or 156.4, 312.8, 625.6, 938.4, 1251.2 and 1564 mg K+/kg. Store in clean, sealed plastic bottles.
Working Standards for Flame Photometer – Nitric K
Pipette 1.0, 2.0, 3.0, 4.0, 5.0 and 6.0 mL of Primary Potassium Standard – Nitric K into clean 200 mL volumetric flasks. Add 100 mL Extracting Solution (1.0 M HNO3) to each volumetric flask and make to 200 mL with deionised water and mix well. These solutions contain 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30 mmolcK/L, which correspond to 1.96, 3.91, 5.87, 7.82, 9.78 and 11.7 mg K+/L. For a final soil/extract ratio of 1:80, these are equivalent to soil concentrations of 0.40, 0.80, 1.60, 2.40, 3.20 and 4.00 cmolcK/kg, or 156.4, 312.8, 469.2, 625.6, 782 and 938.4 mg K+/kg. Store in clean, sealed plastic bottles.
Weigh 2.5 g of air-dry soil (<2 mm) into 250 mL conical flask, graduated at 200 mL. Also bring a suitable laboratory hotplate to operating temperature using a previously checked setting so as to achieve gentle refluxing at 100°C. Add 100 mL of 1.0 M HNO3 Extracting Solution to each conical flask, add two clean glass beads to each flask, and place these on the pre-heated hot plate without delay. Insert a small, clean glass funnel in the neck of each flask, then allow these flasks to reflux gently for 30 min. (Temperature of the extract solution should be maintained as close as possible to 100°C during this period.)
After 30 min of gentle reflux, remove the flasks from the hotplate, then, with minimum delay, rinse and remove the funnels. Encourage rapid cooling to ≈23–25°C by running tap (or cooling) water for 1–2 min or by partly submerging the conical flasks in a cold water bath for a similar time. (Ensure no coolant water contaminantes the contents of the flasks, which should all receive the same attention.)
Dilute the contents of each of the now cooled conical flasks accurately to 200 mL with deionised water and mix well. Next decant a portion of each extract into a suitable centrifuge tube, then centrifuge to clarify the soil extracts prior to instrumental analysis. (It is important to follow boiling and cooling times carefully and to ensure soil extraction conditions are adhered to.)
Analyse for K+ in the clarified aliquots and K Working Standards by ICPAES, by AAS, or by FES, using procedures similar to those described in Method 15A1 or 18A1. Wavelengths for K by ICPAES or AAS = 766.5 nm. Use Working Standards as appropriate.
Report nitric acid-extractable K+ (mg K+/kg) on an air-dry basis. To convert to cmolcK+/kg, divide mg K+/kg by 391.