Matric potential

Problem set 2 question 4 asked whether hydraulic conductivity increases as matric potential increases from -0.5 to -0.01. I answered it does as the less negative matric potential is a sign there is more water in the pores. But in the comments wrote that isn't correct because massive clay already saturated at a matric potential of -0.5. So can clay soils have a matric potential below 0 even at saturation, and how does that occur?

In massive clay there are no aggregates. Hence, all pores are present among individual (small) clay particles and consequently those pores are all very small. In such soil, change in matric potential from -0.5 to -0.01 will not result in any change of hydraulic conductivity (ie soil's capacity to conduct or transmit water through its pores). Remember, water retention curve for massive clay is very flat, especially the portion that is close to matric potential near zero.