Northern soils: are low potassium reserves the tip of an iceberg? by Mike Bell, Agency for Food and Fibre Sciences/Farming Systems Institute, and Phil Moody, Department of Natural Resources and Mines

Farmers working in the field

'Take home' messages

  • Soil potassium reserves have declined markedly under longterm cropping, with even heavy-textured vertosols increasingly showing signs of potassium infertility.
  • The shift to direct-drill tillage systems is also concentrating the remaining potassium in surface soil that is often dry, reducing availability to plants.
  • Current soil tests are not detecting emerging problems because they do not measure all the plant-available potassium pools in the soil, and soil samples are generally taken only from the top 10 cm of the profile.

The recent National Land and Water Audit showed negative balances (implying depletion of soil nutrient reserves) for most major nutrients throughout most of the northern grains region - especially for potassium (K) and magnesium.

The grains industry in this region has generally accepted the need to address developing nitrogen deficiencies by applying nitrogen fertiliser, albeit at conservative rates, and increasingly by using biologically fixed nitrogen residual from grain legume crops.

Similarly, use of phosphorus fertilisers is increasing as crop responses are recorded more frequently. However, an initial lack of response to fertiliser applications of other nutrients like K and magnesium has generally resulted in these nutrients being omitted from normal fertiliser programs, and contributed to the often strongly negative balances for these nutrients.

Unfortunately soil reserves can last only so long, and there is increasingly widespread evidence of developing deficiencies in some of these 'neglected' nutrients - especially K.

Approximate nutrient requirement and removal in harvested product for a range of crops
Crop Yield Crop requirement (kg/ha) Removal rate (kg/ha)
species target N P K N P K
Sorghum 6 t/ha 140 25 140 90 12 18
Maize 6 t/ha 150 25 140 100 15 24
Wheat 3.5 t/ha 140 15 90 75 12.3 14
Chickpea 2.5 t/ha 120 10 110 75 7.5 22.5
Sybean 3.5 t/ha 250 17.5 150 210 21 70
Mungbean 2.5 t/ha 120 10 110 100 17.5 15
Cotton 7 bales/ha 140 30 160 51 18 40

How much K is needed to grow a crop? How much is removed?

Examples of the total crop nutrient requirement and the rate of nutrient removal are shown for relevant crops in the farming system in the table below. Note that most crops accumulate as much K as N in their biomass, and while removal rates in produce are lower, they are still quite high.

How do I know I have low soil K?

This is one area where we currently have major problems with our soil test methodology. Current soil tests measure only some of the plant-available pools of Kin the soil, and don 't measure the slowrelease ('fixed') K pools that constitute the slow-release reserves buffering against K removal in soils such as the vertosols.

These are the pools that have been depleted severely under long-term cropping, especially where yields are regularly high (with irrigation) or rates of removal are high (e.g. silage maize, hay production, high grain legume or cotton frequency), and our routine soil tests are unable to detect these changes.

Examples of the poor correlation between current soil test K and plantavailable K in a range of soils can be seen here.Relationship between soil test K and plant uptake by grass pastures

Compounding this problem has been the shift to direct-drill, in which crop residues are returned to the soil surface and not incorporated. This is resulting in increased stratification of nutrients like P and Kin the surface soil, which is most prone to drying out during periods without rain. These shallow nutrients are therefore regularly detected in the soil analyses of the standard (0-10 cm) soil sample, but are often not available for plant uptake for long periods during the growing season.

This is critical- especially for nutrients like K, which are taken up rapidly in a short time period. lf the topsoil is dry during that time, and the subsoil reserves are depleted, K deficiency can occur. In some cases, even having available K in moist topsoil is not enough, as shown in the maize K uptake data from the field experiment.

K uptake by maize grown with K applied to varying proportions of soil volume

How do I remedy low soil K?

Another conundrum. Traditionally, fertilisers have been injected (e.g. nitrogen), banded or broadcast and incorporated. In the case of K, injecting is not an option. Broadcasting in direct-drill enriches only the surface 5 cm or so, increasing the stratification problem, and K does not leach to deeper layers in clay soils. Incorporation with normal conventional tillage equipment, now that deep ploughing has virtually ceased, really treats only the top 10 cm.

Banding is an option, but plant roots do not respond well to banded K, even in the presence of nitrogen and phosphorus - there are simply not enough roots in a band to supply the peak demand periods. Growers on the red soils of the Burnett are considering an occasional broadcast application, followed by deep ploughing, to address the stratitication issue and then routinely deep-banding K during subsequent crops.

Further research in this area, and on the reaction of plant roots to different forms of banded K, is planned.

Why the tip of the iceberg?

The problems with K have crept up on the grains industry which has been lulled into a false sense of security by originally high levels of nutrient reserves and an inadequate set of soil testing methods.

The negative nutrient balances highlighted by the National Land and Water Audit suggest care needs to be taken so that similar problems do not occur with other nutrients (e.g. magnesium). There is an urgent need for a wider nutrient budgeting approach in the grains industry to identify emerging problems.

Program 4 Contact: Dr Mike Bell 07 4160 0730 email

Region North, South, West