Izak De Villiers Malherbe, brother of D.F. du T. Malherbe*, attended the Boys' High School at Paarl and passed the matriculation examination of the University of the Cape of Good Hope in 1905. He continued his studies at Victoria College, Stellenbosch, and was awarded the university's Bachelor of Arts (BA) degree with honours in Zoology in 1908. While still a student he read a paper before the Victoria College Scientific Society on 'What is an animal?' (1907). Subsequently he proceeded to Germany where he studied at the Universities of Berlin and Muenchen and obtained the degree Doctor of Philosophy (PhD) at Muenchen in 1915 with a thesis titled Zur frage der unproduktivitaet des unterbodens in humiden gebieten (The question of the unproductivity of the subsoil in humid regions). On completion of his studies he was unable to return to South Africa as a result of World War I (1914-1918) and therefore continued his research at the University of Muenchen until the end of the war.
In 1919 Malherbe was appointed as the first professor of agricultural chemistry in the newly formed faculty of agriculture at the University of Stellenbosch (formerly Victoria College). He assumed duty in October 1920 and held his post until his retirement at the end of 1958. He was a South African pioneer in the field of soil fertility and fertilisation and conducted extensive fertiliser trials. His most important publication during this period was his handbook Grondvrugbaarheid (Soil fertility; Cape Town, 1933). An English translation of the sixth edition appeared in 1948, with later editions in either language following until 1962. Less comprehensive publications on the same subject were aimed at farmers, including five articles in Farming in South Africa during 1928-1931. He also published some scientific papers in the South African Journal of Science, including 'Some outstanding facts relating to the fertility of South African soils' (1929, Vol. 26, pp. 125-130), 'The potash requirements of South African soils' (with M.H. Slabber; 1930, Vol. 27, pp. 236-252), and 'Grondsuurheid en sy praktiese betekenis' (1930, Vol. 27, pp. 253-269). At the annual congress of the South African Association for the Advancement of Science in 1931 he was president of Section B and delivered his presidential address on 'The aims and objects of soil science' (Ibid, 1931, Vol. 28, pp. 23-31). Years later he contributed an article on 'Soil fertility' to the journal Soil Science (1949).
Malherbe received the Havenga Prize of the Suid-Afrikaanse Akademie vir Wetenskap en Kuns in 1960. He had an endearing personality and was popular among his students and among the farmers of the Western Cape. In 1938 he married Miss Anna Kuehn, with whom he had a son and a daughter.