Potential roles of biological amendments for pro fi table grain production – A review

There is increasing interest in use of‘alternative’soil amendments in agriculture, but the wide range of resourcesand products available differ greatly in their potential to overcome soil constraints and improve nutrient useefficiency. The three main types of biological amendments can be categorised as biostimulants, organicamendments and microbial inoculants. Many have potential to influence biological, chemical and physicalconditions of soil, but most are not well researched or easily used in agriculture. The main exception is legumeinoculants, which are very well researched and contribute enormously to agricultural productivity when le-gumes are incorporated into farming systems. Biostimulants include amino acids, chitosan, seaweed extracts andhumic substances. Organic amendments include manures, composts, compost derivatives and biochars.Microbial inoculants include specific bacterial inoculants for legumes, and less specialised rhizosphere bacteria,arbuscular mycorrhizal fungi, ectomycorrhizal fungi and a range of disease suppressing microorganisms. Somebiological amendments applied to soil may be more effective when used in combinations rather than singly.Furthermore, those used over longer periods may have potential for cumulative effects not captured when usedover shorter timeframes. Such differences in effectiveness would occur primarily where benefits involve mi-crobial interactions with chemical and physical soil processes leading to slow transformations within the soilmatrix that influence soil fertility and soil health. Similarly, addition of manures and composts may requireseveral years for any quantifiable increase in soil organic C. Although considerable knowledge of the modes ofaction of many biological amendments is available, their performance underfield conditions is usually less wellunderstood. The wide variety of natural and manufactured products available in most cases precludes adequatepeer-reviewed research to support claims about their effectiveness. This can lead to proliferation of un-substantiated assertions of efficacy. This review highlights the lack offield-scale evidence of benefits for manybiological amendments with potential to be used in agriculture. We propose complementary approaches of (i)laboratory- or glasshouse-scale research to understand modes of action, and (ii) targetedfield-scale participatoryresearch involving groups of farmers using on-farm trials as a forward pathway. Use of biological amendments toovercome soil constraints is expected to expand with intensification of agriculture and as a result of climatechange. Therefore, information that enables farmers to discriminate among products that have different levels ofeffectiveness is necessary, and on-farm participatory research should contribute to addressing this need.