Mycorrhizal inoculants for cotton: Doing less with more

This study investigates the value of mycorrhizal inoculants for cotton production in southern NSW. Cotton seeds were inoculated with one of seven commercially available mycorrhizal products at three times the recommended rate (RR) and at twenty times RR for comparison with non-inoculated controls. Seedlings were grown under two nutrient regimes. Inoculated cotton plants were inspected for mycorrhizal colonisation after 6 weeks growth. However, although there was some colonisation of roots in some products at the higher rate, the extent of that colonisation was very low and inadequate to make a production difference in the seedlings. Additionally, soils were collected from adjacent field plots growing wheat, barley, canola, field peas and along the fence line to provide further assessment of the background mycorrhizal levels. An ex rice soil from Coleambally, NSW, was included as a known positive control. The colonisation of seedling cotton was significant in all cases and well above any colonisation achieved with commercial AMF inoculants. Of the winter crop species backgrounds, wheat was clearly superior and field peas least effective. Our assessment of the commercial inoculant products showed them to be ineffectual at both three times RR and twenty times RR outlined on the label. Management of crop rotations provides an alternative and more reliable means for boosting background mycorrhizal levels in the soil.