Response of wheat to the geometry and proximity of lime slotting in an acidic soil profile

Lime slotting is a strategy for the amelioration of acidic soil profiles, whereby vertical seams of lime-amended soil traverse the acidic horizon. Two experiments characterized the above-ground and below-ground response of wheat to the lime amendment of 4 cm or 8 cm wide slots traversing the 7–40 cm deep acidic (pH = 3.9), Al-toxic subsoil horizon in an 80 cm deep constructed acidic soil profile. The growth of wheat rows responded to the proximity, but not the width of the limed slots. Wheat rows positioned above or adjacent (0–4 cm) to limed slots had greater shoot growth than those in the non-amended soil profile, whereas rows positioned 4 cm or more away from a limed slot were not significantly different (at α = 0.05) to those in the non-amended profile, regardless of the width of the 4 cm or 8 cm wide limed slots. Growth responses to the limed slots were realized at the early stages of plant growth and sustained until anthesis: rows positioned above the limed slots had 83% more tillers at 28 days after sowing, 58% greater green canopy area 35 days after sowing and 13% more spikelets per ear at anthesis compared to those in the non-amended profile. A rubidium tracer within the limed slots revealed that the proximity of wheat rows to the limed slots strongly influenced the timing at which the wheat roots exploited them. Wheat rows positioned above or adjacent (0–4 cm) to a limed slot accumulated the rubidium tracer within 21 days after sowing, whereas wheat rows positioned 4, 9 or 11 cm away from the limed slot did not access it until 35, 42 or 49 days after sowing, respectively. The uptake of a rubidium tracer from acidic or lime amended soil sections was strongly correlated with the length of roots within the respective soil sections. The limed slots traversing the acidic subsoil horizon functioned in two ways. Firstly, as discrete sanctuaries for root proliferation within the acidic layer, root length density was 3–7-fold greater than in non-amended soil sections, primarily due to the proliferation of fine (≤ 2 mm diameter) roots that were inhibited in the acidic soil sections. Greater root proliferation within the limed slots enabled greater uptake of nutrients and, to a lesser extent, water from the 7–40 cm deep acidic subsoil horizon. Secondly, the limed slots acted as conduits for roots to traverse the acidic, Al-toxic horizon. Uptake of water and a strontium tracer from the 40–80 cm deep subsoil horizon (beneath the 7–40 cm deep acidic layer) demonstrated that the limed slots enabled plant rows positioned above them to acquire water and nutrients from the 40–80 cm deep horizon 1–2 weeks earlier and in greater quantities than in the non-amended profile. However, for plant rows positioned 4 cm or more away from the limed slots, root activity in the 40–80 cm deep horizon was not significantly different (at α = 0.05) to those in the non-amended profile. Therefore, responses to lime slotting amendments are likely to be greatest where limed slots are positioned beneath wheat rows, and least where they are positioned in the interrow.