The Effects of Four Organic Soil Conditioners on Aggregate Stability, Pore Size Distribution, and Respiration Activity in a Sandy Loam Soil

In coarse-textured soils, low water and nutrient holding capacity due to the high number of macropores limits crop growth. To minimize these limitations the application of various soil conditioners (SCs) has been extensively used in recent decades. The objective of this study was to investigate the influences of anionic polyacrylamide (PAM), cattle manure (M), vermicompost (VC), and biological sludge (BS) as organic SCs on mean weight diameter (MWD) of aggregates, water aggregate stability (WAS), pore size distribution (PRSD), and respiration activity (RA) of a sandy loam soil. PAM (0.25 and 0.5 g kg.^-1 of air-dried soil), M (12.5 and 25 g kg.1 of air-dried soil), VC (2.5 and 5 g kg.^-1 of air-dried soil), and BS (1.7 and 3.4 g kg.^-1 of air-dried soil) were mixed with soil and uniformly packed into large and small pots, and then incubated in a greenhouse with FC moisture content of 0.7-0.8 and temperature of 22 plus-minus 4 degree C for 6 months. Large pots were used for soil sampling in order to measure MWD, WAS, and PRSD at 7, 30, 60, 120, and 180 days. Respiration activity was measured at the same times in small pots. All SCs, irrespective of their concentrations, significantly (P. 0.05) increased RA.Manure (25 g kg.^-1) significantly (P . 0.05) decreased the number of macropores ( 75 mu m) and increased the number of micropores ( 30 mu m). Both PAM rates significantly (P . 0.05) increased MWD, WAS, and the number of mesopores (30-75 mu m); the increase in the number of mesopores was accompanied by a decrease in the number of macropores. BS (3.4 g kg.^-1) significantly (P . 0.05) decreased the number of mesopores by increasing the number of micropores. Consequently, application of M (25 g kg.1), PAM (0.25 and 0.5 g kg.^-1), and BS (3.4 g kg.^-1) each improved the water holding capacity of the sandy loam soil by modifying PRSD.