AMELIORATION OF POTASSIUM DEFICIENCY IN RAINFED CROPPING SYSTEMS IN THE SOUTH BURNETT REGION OF QUEENSLAND

Abstract The Ferrosols of the South Burnett region were considered to be good soils in the early days of agriculture. Continuous cropping of these soils has resulted in a reduction in their chemical and physical fertility. One aspect of this reduction has been the decline in K status, especially in the subsoil. The change to conservation tillage has accentuated nutrient stratification giving high nutrient concentrations in soil surface layers (0-10cm) and lower concentrations in the subsoil (20-40cm). The stratification could be attributed to the lack of mixing of surface applied fertilisers, the annual return of crop residue to the soil surface, and the cycling of nutrients from deep soil layers to shallow layers through nutrient uptake by plant roots. Dry periods during the summer cropping season are common due to the highly variable, summer-dominant rainfall pattern of the South Burnett. As the topsoil dries out, crops will forage for moisture and nutrients from lower in the soil profile where K reserves are smaller. The combination of dry periods and stratified K reserves has resulted in an increasing incidence of K deficiency symptoms in summer crops. The objective of this research was to investigate K application strategies for conservation tillage farming to improve profile K distribution in the South Burnett region in order to overcome the negative K balance and the crop K deficiencies associated with the stratified profile and dry weather. To meet these objectives requires the application of K in bands. Published studies have shown that for soils that have stratified nutrients, if the soil K is low, especially in the subsoil, and dry conditions have existed over the growth period, then there is a significant increase in crop yield with subsoil banding of fertiliser over broadcast fertiliser placement. There are a number of issues that needed to be investigated to determine the optimum method for applying fertiliser K in a band. When banding immobile nutrients such as K there is the potential for there being insufficient root volume in contact with the fertiliser for optimum plant uptake. There are a number of options available to overcome this potential problem. The first would be to take advantage of root proliferation, a plant’s natural response to a heterogeneous supply of nutrients. A significant increase in root length density is promoted by concentrated zones of K in this soil when the ratio of the K concentrations in the fertilised zone to the K concentration in the unfertilised zone is greater than 9 (Chapter 6 and Chapter 7). However, this root proliferation is not to the extent that would be expected with N and P, and it can not be stated with certainty that it is K promoting root proliferation, and that this is not a form of secondary promotion such as where the concentrated K is resulting in a concentrated zone of N in the form of NH4+. The results also show that root proliferation does not appear to be promoted when the ratio is equal to or less than 4.5 (Chapter 3). Another option is to add N and/or P in a fertiliser band with K to promote root proliferation. Combinations of these nutrients applied together in a band in the Ferrosol soil of the South Burnett will remain in the position of application (Chapter 5), ensuring the maximum potential from root proliferation could be achieved. There were concerns regarding the toxic effects that can occur when blending high concentrations of different nutrients together. However, there were no adverse effects on root access and root absorption from concentrated fertiliser blends containing K, N and P (Chapter 4). The addition of N and/or P in a fertiliser band will promote root proliferation over having K only in the fertiliser band (Chapter 6). However, this increased root volume in contact with the K fertiliser did not result in an increased absorption of K from the fertiliser band by the plants. Although plant tissue K was below adequate concentrations, mass balance calculations of K showed that there was sufficient applied fertiliser K for adequate plant tissue K concentrations to be reached. Modelling indicated that under the initial conditions and the conditions prevalent at 45 DAS, the plants would have been able to absorb sufficient K for plant tissue K concentrations to be above adequate levels. The most likely reasoning for the insufficient plant uptake of K was soil K was rendered unavailable to the plant. What happened to the original K which was initially plant available can not be confirmed conclusively. It was hypothesised that over the growth period there was fixation of K by the soil, due to the high concentration of K in the fertiliser band (movement of K into ‘fixed’ pools in the soil), resulting in reduced levels of plant available K. Root volume in contact with fertiliser can be increased by increasing the soil volume fertilised, although this would reduce the concentration of the nutrient and reduce the potential for root proliferation. This concept was explored with a pot trial that exposed plants to different volumes of soil fertilised with the same quantity of K fertiliser (Chapter 7). The volume of soil fertilised has no effect on the cumulative plant uptake of K up to 70 days after sowing. It was concluded that this occurred as a result of the increased root length density in concentrated K zones (root proliferation, although not to the extent that would be expected with N and P), and physiological plasticity (increased plant uptake rate). When 25% or more of the soil volume contains the fertiliser K there was sufficient root volume in contact with the fertiliser to permit uptake and therefore no need for root proliferation. Where smaller soil volumes were fertilised; root length density increased in the vicinity of the fertiliser to the point where sufficient root volume was in contact with the fertiliser. When small soil volumes are fertilised with K, such as in a band, there is also an increase in root length density in the vicinity of the fertiliser, but not to the extent required for adequate plant uptake of K. In this situation it appears that there is physiological plasticity (increased plant uptake rate of K) to make up the shortfall. When only a small proportion of the soil is fertilised, such as in a band, the volume of soil in contact with the K fertiliser is minimised and, consequently the quantity of K that can be fixed by the soil is reduced. This results in a higher concentration of plant available soil K, in the soil solution and as exchangeable K. This greater plant available K is able to buffer increased uptake rates that occur to compensate for the small volumes of soil containingK fertiliser, and the resultant lack of root volume in contact with the K fertiliser. However, this increased uptake rate on its own is insufficient for adequate K uptake, requiring root proliferation to increase the root volume in contact with the K fertiliser to a point where the combined plasticity is sufficient to ensure adequate plant uptake of K