THE USE OF PAPER-MILL SLUDGES ON AGRICULTURAL LAND

A 3-year experimental study was conducted on the factors influencing t he use of paper-mill sludges on agricultural land. An array of 6 m x 6 m plots was used, allowing spreading to be carried out by full-size a gricultural machinery. The factors investigated were crop type (grass and wheat), soil type (sandy loam and clay), sludge type (sludges from two different paper mills, designated A and D), method of sludge appl ication (surface spreading of dewatered sludge, surface Spreading of u nderwatered sludge and subsurface injection of undewatered sludge) and rate of sludge application (5 and 10 tDM/ha in the first season and 1 0 and 20 t DM/ha in the second and third seasons: bz all cases applied each September). Soil condition was assessed in detail both at the be ginning of the experiment before any sludge was spread and after the e nd of the third season. All plots received mineral nitrogen fertiliser in amounts according to normal fanning practice, i.e. no 'credit' was given for any N, P or K content of the sludge. Several statistically significant differences in crop yield were found. The most pronounced difference was that Sludge A gave greater yields than did Sludge D. Th is effect was found for two seasons (1992 and 1993) for grass on clay soil as well as for one season (1992) for winter wheat on sandy loam s oil (although in this last case yields were abnormally low). The cause of this effect was believed to be chemical differences between Sludge s A and D, bur they must be subtle ones because the only major chemica l differences evident from the analyses were in the CIN ratios, the po tassium and the lead contents, and in all three cases there was eviden ce that these particular chemical differences were not the cause of di fferences ill. crop yield. The second most pronounced effect was found with the wheat on clay sail. In all three seasons, there was a signif icant effect on yield of method of sludge application. The level of si gnificance differed from year to year: in 1991 and 1993 it was weak, b ut in 1992 it was very strong. In all three seasons, surface spreading of undewatered sludge gave the highest yields. The physical reason be hind this effect may be differences in the distribution of sludge in t he topsoil after sludge spread by the three different methods had been ploughed in prior to sowing the wheat. In mast cases, topsoil conditi on, as assessed by the percentage of organic carbon, was significantly improved as a result of mill-sludge applications over 3 years. For ex ample, grass plots on clay soil which received the higher rates of slu dge application by injection had at the end of the experiment 2.99% or ganic carbon (Sludge D) and 3.15% organic carbon (Sludge A), while for the corresponding control plots, duplicate measurements gave 2.60 and 263%. In other; physical, tests to assess topsoil condition (e.g. mea suring vol% air capacity, vol% total pore space and vol% total availab le water), it was found that, on the plots selected (all of which were wheat plots), spreading mill sludge over 3 years significantly improv ed the topsoil condition for the clay soil, but hardly did so on the s andy loam soil. The general conclusions front this work were that alth ough a number of statistically significant differences in crop yield w ere found, none of these was large in absolute terms, and that signifi cant improvements in topsoil condition were found in most cases, parti cularly when this was assessed by measuring the percent organic carbon . Therefore, when del?eloping any strategy for persuading farmers to a ccept paper-mill sludges on their land the case should be based mainly on the potential improvements in soil condition perhaps arguing that increases in crop yield should follow from successive improvements in soil condition over a number of years. Such a benefit ought in due cou rse to be particularly evident in very dry seasons. (C) 1997 Elsevier Science Ltd.