Soil water-stable aggregate (WSA) stability is one of the most important indicators of soil health, because it influences chemical, biological and other physical properties. At the same time, WSA formation, stabilization and degradation are also some of the most complex processes that occur in the soil, making them difficult to fully understand. In particular, there is a lack of research on WSA stability in the Baltic region. To gain a better understanding how aggregation occurs in Estonian pedo-climatic conditions, this study was conducted in 2014-2015 in a sandy loam Stagnic Luvisol (LV-st) (WRB, 2014). Potato and barley plots were analysed in a three-year crop rotation (potato → spring wheat → barley) with straw removal. The nitrogen (N) fertilization treatments were 0, 40, 80, 120 and 160 kg ha-1 yr-1 N, both without and with 40 Mg ha-1 fermented cattle farmyard manure (FYM) application prior to potato planting in the previous autumn. WSA stability was determined by Eijkelkamp's wet sieving apparatus from air-dried soil samples of less than 2 mm in diameter. The study revealed a negative correlation (r = −0.16) between increased N rates and WSA stability, regardless of FYM applications. Although soil organic carbon (SOC) content increased with additional N fertilization rates, the reduction in soil acidity (pH KCl) levels caused by N fertilization, most likely repealed the positive SOC content effect on WSA stability. In general, compared with sole N fertilization, FYM application had a positive effect on WSA stability. However, even though WSA stability did not always increase with FYM applications, it still had a positive effect on bulk density, SOC content and soil acidity levels. Further research is needed in Estonia due to the complexities involved in the soil aggregation process.

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The stability of the soil aggregates is an important soil quality indicator, as it affects the soil’s overall functionality. As the soil aggregates are highly affected by agricultural practices, it is essential to know how crops interact with the aggregation process. Therefore, for obtaining more knowledge, this research was conducted in Estonia in an organic crop rotation field experiment from 2012/2013 through 2015/2016 to study the effects of crops (potato → spring barley undersown with red clover → red clover → winter wheat → pea) under different treatments (TC—control; TW—winter cover crops; TW+M—TW with farmyard manure 40 Mg ha−1 per crop rotation). The results showed that in the topsoil (5–10 cm), the soil water-stable aggregate (WSA) content (determined by the wet sieving method) from highest to lowest was following: pea (61.7%), winter wheat (61.6%), spring barley (61.5%), red clover (59.3%), potato (57.1%); whereas in the subsoil (30–35 cm): potato (50.6%), pea (48.5%), red clover (47.9%), spring barley (47.7%), winter wheat (46.4%). Therefore, potato was a noticeable crop, as among the crops, it had the lowest WSA content in the topsoil, while highest in the subsoil. The results shown gave an assumption that the after-effects of some crops (foremost with pea) were noticeable in the soil properties during the following crop. In the topsoil, the differences between crops were significant among crops just for TW and TW+M treatments. In TW, potato was lower than spring barley and winter wheat, but not significantly lower than pea or red clover. In the subsoil, significant differences between crops were observed for TC and TW treatments: in TC, potato was just significantly greater than red clover (but similar to other crops), and in TW, significantly greater than winter wheat. Furthermore, in the topsoil the soil organic carbon (SOC) content was not significantly affected by crops, and the use of winter cover crops generally increased the SOC content while concurrently decreased the WSA content and the soil maximum water holding capacity. This was probably caused by the additional tillage operations which cancelled out the possible benefits for the soil aggregates. As a consequence of the constantly declining SOC content, caused by the weakened soil aggregates, the plant-available P and K contents, especially in the absence of manure applications, decreased as well, probably due to the combination of fixation and removal of plant biomass. Therefore, it is expected that by continuing this trend, the plant growing conditions decline, which in turn will have a negative effect for the aggregate formation and carbon sequestration, which are essential for plant growth.

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