The more you grow, the more you need.
Back in 2011, CSBP set up an experiment on a deep sand near Bolgart to investigate the effects of liming on potassium (K) fertiliser requirements in a paddock with seven crops in nine years.
The surface soil pH was 5.2, but the sub soil was very acidic (4.2 – 4.3 between 10 and 40cm). Soil reserves of K were very low.
Lime sand was applied in 2011 (3.0 t/ha) and 2014 (2.6 t/ha), and four rates of K (0, 15, 30 and 60K) was applied to each of the seven crops grown up to 2018 – with and without lime.
There were small but significant responses to lime in 2012 wheat (+0.14 t/ha) and 2014 wheat (+0.26 t/ha), but much bigger responses in 2016 barley (+0.95 t/ha) and 2018 wheat (+0.42 t/ha).
And as expected from low soil reserves, there have been strong responses to K in every crop.
The first signs of lime increasing K fertiliser requirements were in canola grown in 2017, but in the following wheat crop the responses to lime were clearly increasing the need for K (Figure 1).
Figure 1. Lime sand applied in 2011 (3.0 t/ha) and 2014 (2.6 t/ha) increased the 2018 wheat yield response to potassium fertiliser.
Soil tests in December 2016 showed that lime increased soil pH by about 0.5 of a unit down to 25cm, but there was no effect on where previously applied K was found down the soil profile.
This suggests that the effects of lime on crop K requirements had little to do with direct effects on K availability. Rather, it is more likely that K requirements increased because of the increase in yield potential and higher rates of removal.
Increasing nutrient inputs is often required to take advantage of investment into the removal of soil constraints, and to replenish the stocks depleted by increased productivity.
As lime has increased yield potential, so has the need for K.
Figure 2. The response to muriate of potash applied in previous years can be seen this year in wheat sown by the farmer.