To understand the relationship between natural physicochemical properties of soil and abundance of citrus root nematode (Tylenchulus semipenetrans), a survey was conducted during 2009-2010 in some 37 citrus orchards, in Kohgilouyeh va Boyerahmad Province, Iran. Distribution of the citrus nematode was determined by collecting random samples from the soil and citrus plant roots, extracting and enumerating the number of second stage juveniles, males/100 g of soil and females/5 g of root. The relationships between nematode population, and the factors of: organic matter content, Nitrogen (N), Phosphorus (P), Potassium (K), organic carbon, Calcium Carbonate, soil texture, Electrical Conductivity (EC) and pH were determined. Increase in nematode population density was observed by increasing soil saturation percentage (up to 43%), and by an increase in soil silt, sand, P, K and organic carbon, but by an increase in soil salinity, Calcium Carbonate, total Nitrogen and the amount of clay in soil, the nematode population decreased. Maximum nematode population density was recorded in a loamy soil texture. The most suitable soil pH for nematode activity was found almost seven while either an increase or decrease in soil pH, resulted in a decrease in nematode population. The number of second stage juveniles ranged from 58 to 2,730/100 g of soil while females were present by 11 to 331 individuals/g of root respectively. About 62% of the studied orchards were infested with T. semipenetrans.
