During the last two million years, volcanic vents on the eastern side of the Rift Valley produced large amounts of lava that flowed eastward and covered most of what is now the Laikipia District, covering the underlying granite basement rock.  Most of the lava flows on the Laikipia plateau are of phonolite, a potassium and calcium-rich greenish lava.   The lava-covered area forms a slightly undulating plain occasionally cut by drainage systems.  The phonolite lava is 300 to 650 feet deep and can best be seen on the sides of rivers where it forms steep escarpments.

When phonolite weathers on the poorly draining flat plains, it forms ‘black cotton’ soil, which expands when wet and contracts and cracks when dry.  When the soil is wet, it’s sticky and slippery, making it difficult to drive on.  Black cotton soil covers 43% of the Laikipia District.

Water is scarce on the plateau and is confined to a few rivers and springs.

Being near the equator means that Laikipia has no distinct seasons but rather periods of wet and dry.  Typical of semi-arid areas, rainfall varies significantly from year to year.  Rainfall in Laikipia is influenced by three distinct rain periods in Kenya:  the ‘long’ rains usually fall from April through May, the ‘short’ rains in November and the ‘continental’ rains fall sporadically from July through September.  The months of December, January and February are generally the driest in Laikipia. Most rain falls from heavy thunderstorms rather than light, steady rain and varies from south to north, with the northern locations receiving the least amount of rain. 

Average annual rainfall ranges from less than 500 mm (19.5 inches) in the driest areas to over 800 mm (31 inches) in the wettest.  The average annual rainfall on Sosian is between 500 and 600 mm (19.5 to 23 inches).  Drought episodes are cyclical, occurring approximately every 10 years.

Being near the equator also means that the sun’s intensity is nearly equal throughout the year so that daily temperatures do not fluctuate greatly.  Temperatures vary with altitude and season but generally range from 22 to 26 degrees C (71 to 79 F).   Minimum temperature is 6 to 14 degrees C (43 to 57 F) and maximum is 35 degrees C (95 F).

Acacia bushland and fire-adapted grassland are common in the semi-arid central parts of Laikipia, the result of human activities such as frequent burning, charcoal production and over-grazing of livestock.

Sosian Ranch is comprised of three main vegetation types: mixed Acacia bushland, open grassland and some Acacia woodland.

The most common vegetation community of Laikipia is the Acacia grassland that dominates the open lava plains and occurs mostly on black cotton soil.  On Sosian, these grasslands with some scattered Acacia woodland cover the plains on the northern half of the ranch.  Acacia species found in this area include A. drepanolobium (‘whistling thorn’), A. seyal, and A. Senegal.

The southern half of Sosian is comprised of mixed bushland dominated by Acacia species such as A. mellifera, one of the most drought-resistant species, A. brevispica (‘wait-a-bit thorn’), A. Senegal and A. nilotica (‘Egyptian thorn’).  Other species that form thickets include Crotons, Eucleas (ebony) and Grewias.  Scattered Boscia and Euphorbia trees are also found in the south.


Riverine vegetation, including several fig species and Acacia xanthophloea (yellow-barked acacia or ‘fever’ trees), lines the Ewaso Narok River.  Several luggas or seasonally dry riverbeds dissect the ranch, including the Pinguone Lugga on the northern boundary, the Samburumburu Lugga, which runs from the northern plains down to the Ewaso Narok River and Acacia Dam Lugga in the centre of the ranch.



Savannah-associated ecosystems like those on Sosian are very dynamic and in a constant state of change.  Old photos from 70 years ago reveal that the vegetation in Laikipia was much different than today.  Savannah ecosystems are not simply open grasslands that most people think of; rather they are a matrix of trees, bushes and grass, where climate, soil, wildlife, livestock, fire, and human activities all contribute to their composition.  If all browsing animals (animals that feed on woody plants) were eliminated, the land would quickly become overgrown with trees and thick bush; conversely, if too many browsers are concentrated in a given area, it will become more open and grassy.

The woody plants are always trying to expand their range, but browsing animals such as the diminutive dikdik keep them in check by feeding on young trees.  Older trees are damaged or destroyed by feeding elephants.  This creates more open ground for the grasses.  The effect of elephants on trees can easily be seen in the southern part of Sosian.  These ‘disturbances’ and many others contribute to the rich ecosystem that is Sosian, where habitat exists for a wide variety of wildlife.  However, this balance can easily be destroyed by overstocking of livestock as is visible on some of the neighboring communal and smallholder land.  On Sosian, domestic stocking is carefully limited so as to contribute to the diversity of habitat.