Deep insertion of lime into acid organic soils

Abstract

Organic (peat) soils are inherently acid in their natural state with soil pH levels that decline from 4.5 – 5.0 in the surface layer of 0 – 75 mm to 4.0 - 4.5 at a depth of 400 mm. Traditionally lime has been surfaceapplied and incorporated into the lower soil layers by
cultivation. An alternative method of inserting 3 - 4 t/ha of lime down to 400 mm soil depth was tested on a developing Waikato deep Moanatuatua Organic soil. When two-thirds of the lime was placed within the 300 - 400 mm layer and one-third within the 200 - 300 mm layer in summer 2021, soil pH in the 200 - 300 mm layer decreased from 5.6 in 2021 to 5.3 in 2023 and significantly increased from 4.2 to 4.9 in the 300 – 400 mm layer over the same time period. Reversing the ratio of the lime rate into the two layers resulted in a
significant increase in soil pH from 4.5 to 5.3 in the 200 – 300 mm layer and a non-significant increase from 4.2 to 4.6 in the 300 – 400 mm layer. Generally, there were corresponding decreases in soil exchangeable aluminium (Al) concentration in those layers. The ratio
of lime insertion rate had no significant effect on rooting depth between years. There was a moderate correlation between soil pH and exchangeable Al when measured at depths of 0 – 400 mm. These interim results indicate that the deep insertion of lime into an acid Organic soil was effective in increasing soil pH and reducing soil Al concentration in the lower soil layers in the two years after the lime was inserted.