ASSESSING SOIL POTASSIUM RESERVES IN SUGARCANE SOILS

DURING the period 1996-2000, the potassium (K) budget for the Australian sugar industry indicates that more K is being removed in harvested product in a green cane trash blanket system than is being applied in fertiliser. The deficit is therefore being supplied from soil reserves of exchangeable K (Exch K) and non-exchangeable available K (NEAK). Currently, NEAK is assessed in sugarcane soils using nitric acid (Nitric-K) and K fertiliser recommendations at a particular exchangeable K value are adjusted according to Nitric-K levels. A newly developed method using tetraphenyl borate to extract soil K over a 1 h period (TB-K) has been shown in glasshouse experiments to be better correlated with K uptake by forage crops than Nitric-K. In these studies Nitric-K often overestimated soil K reserves. Surface samples (0–25 cm) from representative soils of the Johnstone, Proserpine, Plane Creek, Herbert, Mackay, Bundaberg, and Isis districts were analysed for Exch K, Nitric-K and TB-K. Median values indicated that on a district basis, Bundaberg soils were lowest in both Exch K and TB-K. The ratio of TB-K/Exch K was used to assess the relative amounts of NEAK in the soils. On a soil order basis, Ferrosols had the smallest range in the ratio and the median value was less than 2 indicating these soils had very limited soil K reserves. Dermosols, Vertosols and Sodosols showed the greatest ranges in the ratio, and high ratio values, probably associated with alluvial influence. Therefore, apart from Ferrosols and Chromosols, it is not possible to generalise on the magnitude of the ratio, nor infer the relative amounts of soil K reserves at the soil order level of classification. The magnitude of these K reserves needs to be characterised at the soil series level of classification for soil survey purposes, and at individual block scale to inform appropriate application rates of K fertiliser. Very limited trial data suggest that TB-K responds to crop removal of K, or addition of K fertiliser, in the same way as Nitric-K. It was also shown that the TB-K pool was plant available in one soil, but not in another. This raises the question of how best to assess the availability of reserve K in sugarcane soils and warrants further investigation.