Ask a gardener why Jersey grows things so well and you'll usually hear about the climate: the mild winters, the long light. That's half the story. The other half is underfoot, and it arrived on the wind.
An inheritance from the Ice Age
During the last glaciations, sea level fell far enough that the floor of the English Channel lay exposed, a cold, dry plain of glacial silt. Wind picked that silt up and carried it south, and a film of it settled over Jersey, in places metres deep. Geologists call it loess; it is the westerly edge of the same wind-blown blanket that covers much of northern Europe's best farmland.
Most of Jersey's soils today are derived from these drift deposits: loess above all, with blown sand near the bays and alluvium in the valley bottoms. The result, across most of the Island's plateau, is a deep, fine, silty loam: fertile, generally well drained, yet moisture-retentive. It is the soil that made Jersey an agricultural island, and it is almost certainly the soil under your lawn.
But not everywhere, and not uniformly. That's where irrigation gets interesting.
The same island, three different thirsts
The plateau loams hold water well in the body of the soil, but their fine, silty texture has a habit: the surface dries and tightens first. A lawn on loess can look parched on top while the root zone below is still adequately wet. Or, after a long dry spell, the surface can shed light watering before it soaks in at all. Judging these soils by eye, from above, is genuinely difficult. Judging them by a sensor at root depth is not.
The thin coastal and valley-edge soils are the opposite character. Along the north coast, the headlands, and the steeper valley sides, the drift cover thins out and the soil sits shallow over rock. These soils drain fast, hold little, and warm early (it's why they grew the Island's earliest potatoes), but a lawn on them runs out of water days before its plateau neighbours. Two properties a mile apart can need watering schedules that barely resemble each other.
The low, silty flats have the loess problem in reverse. Where redeposited silt has collected on flat ground, water can sit. There's a spot in the east of the Island whose old Norman-French name translates, roughly, as "bad mud", named centuries ago for soil that waterlogs. Lawns on ground like this are damaged more by overwatering than by drought.
What this means for watering
The point is not that Jersey soil is difficult. It is that Jersey soil is specific. Depth of drift, distance to rock, position on the slope: these vary field by field, and they decide how fast a given lawn dries, how deeply water penetrates, and how much a root zone can hold in reserve.
A conventional irrigation timer knows none of this. It waters the shallow zone and the deep zone, the free-draining corner and the damp one, with the same minutes, which means somewhere on the property is always getting it wrong.
This is why Greensward starts every installation with the soil rather than the system: sensors placed at root depth in each zone, so the watering decision is made from what that particular patch of Ice Age inheritance is actually doing, not from what a timer assumes the average lawn might need.