The potential of local sediments to improve barley yield by increasing phosphorus availability and decrease aluminium toxicity in Kenya

Abstract

Ferralsols are among the most widespread soil types in East Africa and are intensively used for agriculture. However, their strong weathering, low pH, aluminium (Al) toxicity, and phosphorus (P) fixation severely limit crop productivity. These constraints are commonly addressed through the application of mineral fertilisers, which are costly and may further increase soil acidification over time. This thesis investigates the potential of locally available volcanically influenced sediments from the Kenyan Rift Valley as a more sustainable soil amendment for Ferralsols from a biogeochemical perspective. Due to their relatively high pH, silicon (Si) content, and enrichment with nutrients such as P, these sediments may reduce Al toxicity, improve P availability, and enhance crop yields. Two sediments from Baringo and Nakuru were tested in field and laboratory experiments using a Ferralsol in Eldoret, western Kenya. In the first field experiment, the sediments were applied at two rates (1 vol.% and 3 vol.%) to a Ferralsol and compared to an untreated control (Chapter 2). Barley (Hordeum vulgare L., ‚Hessekwa‘) was sown on all plots after sediment incorporation. The 3% Baringo treatment showed particularly strong effects: Soil pH increased from 4.7 to 7.0, P availability increased, and Al availability decreased significantly. This resulted in a yield increase of 1061%. Other treatments showed only moderate or no improvements. These results suggest that volcanically influenced sediments may be a promising, locally available, and natural option for improving the fertility of acidic soils, although effectiveness strongly depends on the specific sediment and application rate. Since the underlying mechanisms could not be clearly distinguished in the first experiment, a second field experiment was conducted on the same experimental site the following year (Chapter 3). The effects of the sediments were compared to two globally established soil improvement methods: liming and straw return. New plots were established with liming, and half of each plot was amended with straw. Again, the 3% Baringo treatment resulted in a significant increase in soil pH, P availability, and yield, while liming showed similar but slightly lower effects. The untreated control produced no grain. These findings indicate that pH increase is a driving factor for the observed yield improvements. However, unlike liming, the 3% Baringo treatment also significantly increased Si availability, which may explain its better performance. Neither in the other treatments (Baringo 1%, Nakuru 1%, and 3%) nor in the straw addition treatments were positive changes in nutrient availability or yields observed. To better understand the effects of sediment additions on the mobilisation of Si, Al, P, and iron (Fe) in Ferralsol, three laboratory incubation experiments were conducted, using soil slurries from the Kenyan field site (Chapter 4). The incubations were carried out as follows: (i) sediment addition at two concentrations (1 wt.% and 5 wt.%) over 28 days; (ii) an additional treatment with 1 wt.% lime over 61 days; (iii) sediment and straw addition under anoxic conditions over 80 days. The results showed that both sediments and lime resulted in only short-term increases in P concentrations in the soil solution. It is assumed that the released P was quickly re-adsorbed or precipitated with other elements. Nevertheless, a long-term reduction in available Al concentrations was observed in all Baringo sediment and lime treatments. The third incubation experiment clearly demonstrated the strong influence of Fe minerals on P fixation in Ferralsol. Through the reductive dissolution of Fe(III) phases, P concentrations in the soil solution increased tenfold, independently of sediment addition. The results of this thesis demonstrate that the use of local sediments, can have a high potential for more sustainable agriculture on acidic soils in Kenya.