Content area
Full Text
In a 1937 address to the American Society of Agronomy, F. J. Veihmeyer outlined the consensus thinking of that time concerning the effect of soil organic matter (OM) on available water capacity(AWC). Based on a summary by Bouyoucos (6), the main points were as follows:
1. OM can markedly increase the total water holding capacity or field capacity (FC) of mineral soils.
2. However, it also increases their permanent wilting point (PWP). The net result is that OM does little to increase the water available to plants (AWC), which is the water contained by a soil between FC and PWP.
According to Bouyoucos (6), the opinions outlined above prevailed despite the fact that experimental data were few and contradictory. Studies relating soil OM to AWC conducted since 1937 have continued to yield contradictory conclusions. However, the consensus view remains the same. Textbooks and review articles continue to state that soil OM has little effect on AWC. For example, in a 1985 review article MacRae and Mehuys (14) stated that OM increased AWC in soils only under specific circumstances, and that such increases are the exception rather than the rule.
With the advent of chemical fertilizers, the relative role of OM in providing nitrogen, phosphorus, and other nutrients to crops diminished greatly. Because it is generally believed that OM has no significant influence on the amount of water available to plants, the overall importance of OM in modern agriculture is thought to be minimal.
This study provides evidence that the consensus view of the relationship between OM and AWC is incorrect. The last five decades of research in this area are critically reviewed to show that the research results do not support the consensus view Published data are then analyzed to demonstrate the highly significant correlation between OM and estimated AWC when the variation of soil properties other than OM content is controlled.
Literature review
Researchers have studied the relationship between OM and AWC in two main ways. Some added different amounts of OM to mineral soil and measured the effect on AWC. Others sampled horizons of naturally-occurring soils that varied in OM content and other properties believed to affect AWC. Using regression analyses, they then attempted to relate variability in OM content among the samples...