The effect of cattle manure and its humic fractions on the aggregate stability of a sandyloam soil has been studied. Cattle manure was extracted with 0.1 M NaOH and the extract was dialysed to obtain a combined fulvic acid+ humic acid fraction (FHA). Humic acid (HA) was precipitated from this extract, redissolved in 0.1 M NaOH solution and dialysed. The manure and the organic fractions were added at two rates to a sandy-loam soil and the mixtures incubated for 2 and 17 weeks.Changes in the water-stable aggregation of the soil were followed. In these tests, the effects on water stability of pre-wetting with alcohol or benzene were also examined. Addition of manure, by itself, was rather ineffectual, but stability was significantly improved after two weeks of incubation by addition of the organic fractions. FHA was much more effective than HA, particularly in increasing stability after the benzene pretreatment. There were only slight changes in aggregation after 17 weeks as compared with 2 weeks.

SUMMARY The influence of soil amendment with fulvic + humic acid (FH) fractions, obtained from manure or peat, upon the shape, size and numerical density of aggregates in a sandy loam and a clay soil was examined using micromorphological techniques and image analysis. The effect of the FH fraction from manure (FHM) was to produce larger aggregates from the smaller ones, while the FH from peat (FHP) gave rise to a higher numerical density of smaller aggregates. Although the type of structural change was similar in the two soil types, the extent of change per g of FH was greater in the clay soil. Aggregate shape varied with treatment and soil type, but both kinds of organic matter produced vughs and a narrowing of the planes in the two soils which can be considered as signs of structural improvement.

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