This chapter tells why it is important to study soil–plant–water relations . Water is the most important substance necessary for food production . People depend upon plants for food, so the challenge of feeding a growing population is discussed . The human population growth curve, a logarithmic one, is presented . Rules of logarithms are then given . A calculation is done to show that the human population is limited by the productivity of the land . The calculation shows that it requires two square yards (16,700 cm2) to feed one person . The sigmoid plant growth curve is presented followed by a mathematical analysis of Blackman's compound interest law for plant growth . Finally, data from corn and soybean are analyzed to show that their growth rate is exponential . A biography of Napier, the inventor of logarithms, is given in the appendix.