EFFECTS OF CULTIVATED BRACHIARIA GRASSES ON SOIL AGGREGATION AND STABILITY IN THE SEMI-ARID TROPICS OF KENYA

Abstract

Soil aggregation is a key short term indicators of soil quality attributed to changes in land management. A study was conducted to investigate changes in the size distribution and stability of soil aggregates in a structurally unstable sandy loam soil following cultivation of Brachiaria grass in semi-arid region of Kenya. Brachiaria grass cultivars included Brachiaria decumbens cv. Basilisk, B. brizantha cvs Marandu, MG4, Piata and Xaraes, B. humidicola cv. Llanero and B. hybrid cv. Mulato II which were compared with two locally cultivated forage grasses (Chloris gayana cv. KAT R3 and Pennisetum purpureum cv. Kakamega 1) and a bare plot (negative check). The grass treatments were evaluated with fertilisers application (40 kg P applied at sowing and 50 kg N ha−1 in each wet season) and with no fertiliser applications. Aggregate size fractions were isolated using the wet sieving method. Aggregation based on the proportion of small macro-aggregates (250–2000 μm) increased in soils cultivated with all grass types compared to the control and was greatest in soils under B. hybrid cv. Mulato II. Aggregate stability in terms of mean weight diameter (MWD) differed among the grasses and was highest in soils under cv. Mulato II and cv. Marandu with MWD of 4.49 and 4.31 mm, respectively. Changes in small macro-aggregates fraction was positively correlated with particulate organic matter (POM) (r=0.9104, P= 0.001), microbial biomass carbon (MBC) (r=0.5474, P= 0.01), soil organic carbon (SOC) (r=0.3654, P= 0.05) and root biomass (r=0.4977, P= 0.01). This indicated that the binding agents were important in the aggregation of soils cultivated with Brachiaria grasses.