The machair of the Outer Hebrides, Scotland, is a cultural landscape, designated as a Site of Special Scientific Interest. Crofting on the machair is a sustainable low-input, low-output system, producing crops for livestock fodder, which has been practiced for hundreds of years. Recent demographic changes have affected the agricultural practices, with unknown, but potentially negative effects on these marginal soils. The present work investigated the effects of some of these changes in agricultural practices, in particular the effect of amendment with kelp (Laminaria digitata), a brown seaweed, ploughing and land use on soil stability, water repellency and soil microbiology. Furthermore, the effect of kelp on plant seed germination and root development was assessed. The results showed that machair soil is resilient towards amendment with kelp and synthetic NPK fertilizer in a single growing season, and that this resilience was not caused by rapid degradation of kelp. There was evidence that kelp inhibits soil fungi, but increases total microbial biomass and activity. The soil was slightly to severely water repellent. The causes of this repellency could be attributed to a number of factors including plant organic matter, manure, land use and organic matter within aggregates. Kelp increased seed germination in a number of key native machair plants and crops, suggesting that kelp may affect native plant community composition and crop yield. Also, a promoting effect on root development of kelp was seen in some plant species, with a threshold concentration above which it became inhibiting. Furthermore, these results confirmed the presence of plant growth regulators in kelp. The above findings are relevant not only to the machair, but also to other marginal sandy soils, and to the use of seaweed species as organic alternatives to conventional synthetic fertilizers.