ABSTRACT
Soil systems contain an enormous stock of unicellular eukaryotic organisms that are not animals, plants, or fungi and that constitute a biological kingdom of their own. Protists and nematodes are more similar in function than in form. Traditionally, soil protists were considered to be primarily consumers of bacteria. Plant-pathogenic nematodes affect the primary productivity of plants by altering plants' uptake of water and nutrients. Both nematodes and protists have the ability to make physiological adaptations to survive under unfavorable conditions such as freezing or drying. Conservation biological-control approaches can modify the soil environment, or management practices can protect and enhance antagonistic organisms that reduce crop damage from pests. Protists increase plant biomass not only by releasing nutrients from bacteria and other soil organisms that they “graze” on but also by producing analogs of plant hormones such as auxin.
