Effect of biosolids compost on two NSW coastal soils used to grow vegetables

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The study compares the effects of biosolids compost (BC) and inorganic fertiliser (IF) on the nutrient status of a clay loam at Rydalmere in Western Sydney, and an earthy sand at Somersby, 85 km north of Sydney. The soils represent the textural range used for vegetable production in the Sydney Basin of New South Wales. The soils had been under permanent sod (Rydalmere) or native vegetation (Somersby) for at 40 years prior to the experiment. There were 3 treatments during February 1994: IF, fallow (F), and BC. The same plots were treated again during September 1994 to give the combinations: IF followed by IF (IF + IF); F by BC (F + BC); BC by BC (BC + BC). The 4 replicates of each treatment were randomised in blocks. Treatments were incorporated into raised beds to a depth of 15 cm and vegetables were planted in March and again in October 1994. The total amounts of C and N initially present in the soil in the beds (0-15 cm) at Rydalmere were at least 3 times greater than at Somersby and the ratio was greater for S, P, Ca, Mg, K, and Na. By April 1995, the quantities of the 8 monitored nutrients had decreased at both sites for the IF + IF treatment; increased at Somersby, but not at Rydalmere for F + BC; and increased at both sites for the BC + BC treatment. The changes caused by the BC + BC treatment were greater in relative terms at Somersby, because of the initial difference in fertility between the 2 soils. For example, the C and N content increased by an approximate factor of 2 at Somersby, and by greater factors for the other elements. BC + BC was also the only treatment to increase other indices of fertility, such as effective cation exchange capacity (ECEC) and Bray-P, in the surface 15 cm. The treatments did not affect pH in the soil profile (0-50 cm) and increases in EC were ephemeral. The nutrient composition of the 20-30 cm zone was affected at both sites. By the end of the experiment the concentration of C, N, P, Ca, and Bray-P increased and Mg decreased. For C and N between-treatment differences were small, indicating that the observed effects were mostly caused by leaching of the products of mineralisation of soil organic matter. Treatment effects were larger for P, Ca, Mg, and Bray-P, and the final concentrations for the F + BC and BC + BC treatments exceeded those for the IF + IF treatment. At Somersby, both BC treatments increased ECEC, consistent with the importance of leaching as a process of nutrient loss at this site. Consequently, intensive vegetable cropping may pose serious environmental risks particularly on sandy sites.