After drought advisory 'VAM' watch by John Cameron
GroundCover™ Issue: 44
What is VAM?
At a glance
- VAM helps a crop to grow. Soils naturally contain beneficial fungi that help a crop to access nutrients like phosphorus and zinc. The combination of the fungus and the crop root is known as VAM (vesicular-arbuscular mycorrhizae).
- The lack of VAM shows up as 'long fallow disorder' — the failure of crops to thrive despite adequate moisture. When VAM levels have dropped, it is difficult for the crop to access nutrients.
- Optimising the benefits of VAM depends on knowing:
- the cropping history of the paddock including length of fallow periods and the ability of previous crops to produce inoculum. From this you can estimate the VAM inoculum level in the soil as high or low
- the nutrient status of the soil in that paddock, particularly the available phosphorus and zinc levels
- the VAM dependency of the crop you wish to grow.
BE AWARE when planting your first crop after the drought that your VAM population may be depleted. This could lead to yield loss if soil phosphorus (P) and zinc (Zn) levels are not addressed.
VAM (vesicular-arbuscular mycorrhizae) are beneficial associations between crop roots and soil fungi that occur naturally in cropping systems. VAM can improve the growth and yield of most grain, oilseed, pulse and fibre crops through improving the uptake of nutrients, particularly P and Zn.
At the Goondiwindi Grains Research Update in March, Senior Principal Scientist at the Queensland Department of Primary Industries (QDPI), John Thompson told the assembled advisers and growers that "having VAM present and working for you leads to more efficient uptake of P and Zn, as well as facilitating improved N fixation in pulses, greater drought tolerance, improved soil structure and better disease tolerance. The absence of a host crop during the drought is likely to have led to a decline in VAM inoculum".
Long fallow disorder
In northern growing regions the impact of VAM on crop nutrition can be a very important factor. Some growers will know the symptoms of VAM deficiency by the name 'long fallow disorder'. Such is the importance of VAM that it has made it to the agendas of two Northern Updates this year, with presentations at Goondiwindi by Dr Thompson, and at Roma by Nikki Seymour, soil microbiologist with QDPI.
"Different crop species and varieties have different levels of dependency on VAM. The P and Zn status of the soil will also have an impact on whether crop growth is affected when VAM levels are low", said Dr Seymour.
"Most pulses and oilseed crops (except lupins and canola) have high to very high dependency and will therefore suffer more than a winter cereal if VAM levels are low. Canola and lupins are not hosts of the VAM fungi and thus do not breed up inoculum and are unaffected by VAM levels. As these non-host crops do not contribute to building up VAM levels, they are not as beneficial as say wheat or sorghum in the rotation for a future VAM dependent crop."
|Very high||Maize, Pigeon pea, Lablab, Cotton, Linseed, Faba bean,|
|high||Sunflower, Soybean, Navy bean, Mungbean, Sorghum, Chickpea|
|Low||Field pea, Oats, Wheat, Triticale|
|Independent (non-hosts)||Canola, Lupins|
Due to this variation in the amount of inoculum that different crops produce, cropping history and length of fallow are very important for estimating if VAM inoculum levels in soil might be high or low.
"Predicting the VAM status of a particular crop is not straightforward, nor is estimating the optimum P and/or Zn fertiliser requirements of crops that you suspect will be low in VAM," said Dr Seymour. "There is still much we do not know about how VAM survival is influenced by soil temperatures and moisture levels both in-crop and during the fallow periods. Soil disturbance can reduce VAM and so minimising tillage is more conducive to optimising VAM levels."
If you suspect low VAM in a particular paddock — for example, if you had a long fallow without vegetative matter in the paddock — there are some management tools you can use to minimise the impact on your cropping system:
- grow crops with low VAM dependency, e.g. wheat or barley — they won't suffer as much yield loss, providing P and Zn are well supplied in soil or fertiliser, but will still increase the VAM inoculum for following crops
- avoid non-VAM crops, as they will not increase VAM inoculum status
- if you wish to grow a crop of high VAM dependency for reasons such as good price, apply high rates of P and Zn fertilisers
- adopt zero- or reduced-tillage practices during fallow periods, as this is less harmful to VAM fungi than frequent tillage.