Soil aggregate stability can be influenced by applying different forms of lime, e.g. calcium carbonate (CaCO3), oxide (CaO) or hydroxide (Ca(OH)2). The mode of action is probably a result of both chemical form and the calcium ion per se. When CaO or Ca(OH)2 encounters clay, different reactions take place at the micro-aggregate level. These include cation exchange, flocculation and agglomeration, carbonation and cementation through pozzolanic reactions. CaCO3 can also influence aggregate stability, probably through cation exchange and flocculation, but not through carbonation and pozzolanic reactions, giving a more resilient soil. From an environmental perspective, this can help to mitigate phosphorus (P) losses, as aggregates that do not slake when exposed to stresses such as water are prone to not lose the particulate P bound to the clay surfaces. This use of specific combinations of liming materials, termed ‘structure liming’ can also result in finer aggregate size distribution in the seedbed. From a farm management point of view this can facilitate tillage and secure establishment, assisting rapid crop emergence.
Yield response to structure lime amendments varies with different crops. This is not surprising as lime affects soil physics, chemistry and biology. Changes in any of these properties influence crop growth through complex mechanisms. Crop yield response to structure liming is thus the net result of processes that can both enhance and impair crop growth.
Trials at the Swedish University of Agricultural Sciences Uppsala during 2011–2014, comparing calcium hydroxide and a commercial mix of calcium hydroxide (20%) and calcium carbonate (80%), showed inconsistent yield results in cereal crops with both increases and decreases of ± 10%. Yield enhancements can be attributed to a finer tilth, giving improved protection against evaporation in the very dry spring and early summer of 2013. A possible explanation for the yield depressions is a combination of reduced availability of P in combination with low diffusion under dry conditions.
Aggregate stability, measured as decreased turbidity of water leached from aggregates subjected to irrigation in a rain simulator, was increased with structure lime in many of the different projects presented. The aggregate stability increased with increasing application rates.
A comparison between ground limestone and structure lime was carried out and the effect on plant pathology/health, crop nutrition, soil structure/aggregate stability and economics including crop yield were examined in a crop rotation trial. Preliminary results show similar effects for ground limestone and structure lime on aggregate stability and aggregate size distribution. Results for crop yield will be compiled and published in 2019, and the study will continue until 2020, to scrutinise the long-term effect.