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Soil nitrogen levels, soil fertility are entwined

It is these rhizobia bacteria, unique to the legume species, with specific strains for each pasture or crop species, that are responsible for supplying legumes with nitrogen from air and for building soil nitrogen fertility.

It’s not just a simple job of throwing in packet inoculum with seed. Rhizobia is a living, specific plant fungus easily killed if not property stored and applied.

All too commonly poor root nodulation occurs and legumes fail to thrive and/or build soil nitrogen.

One relatively new innovative way of rhizobia introduction with new legume sowings or into existing poorly nodulated pastures is via a granular (eg. manufactured and marketed via Alosca) based on bentonite clay. Granules are impregnated with appropriate strains of root nodule bacteria. Granular applied inoculants are especially popular in WA and interest is rapidly increasing in NSW.

Research supports that granular inoculants such as the Alosca ones provide long life if correctly stored and handled. For example in WA they are commonly applied with seed sown into the soil in January or February, well ahead of pasture germination. Soil can be dry and hot with long rhizobia survival. WA farmers commonly sow species like serradella and biserrula in summer where they are using their own unprocessed seed (that is “hard” seed and needs fluctuating hot and cold temperatures to “soften”).

Such early sowing requires a medium (clay bentonite) where rhizobia can survive long term and under hot, dry conditions. Normally inoculated and inoculated plus lime pelleted or many other commercially available pelleted seed is far less suited to such adversity and best suits sowing into moist soil at time of expected germination.

Research indicates it is best to direct drill clay pellets into soil with seed in new establishments and either with some seed or on its own to existing, poorly nodulated pastures.

Physical separation of rhizobia via pellets from seed also allows insecticides and fungicides to be applied with the seed, which would otherwise damage rhizobia.

Clay-based granules are normally applied at eight to 10kg/ha when sowing on 18cm row spacing. Granules should preferably be mixed with seed at sowing, or distributed through a third box and drilled with the seed. They can also be mixed with fertiliser as it is delivered to the furrow, in close proximity to seed. They can be stored for over six months without refrigeration. Store in a dry cool area away from direct sunlight.

Soil fertility is the third key aspect (after variety/species and nodulation) for productive legume pastures. Most important elements for vast areas of NSW (and other states and territories) are phosphorus and sulphur and possibly periodically molybdenum. If deficiencies are not corrected nodulation will suffer, plant growth will be severely compromised and pastures will fail to build soil nitrogen. Correcting soil fertility, while a necessary cost, is commonly not that exorbitant and pays for itself relatively quickly. In our case we reckon its use is the difference between a gross margin of around $350/ha versus less than $100/ha. That’s on mainly sandy to medium loam country. And commonly soil fertility in pastures builds up following a few fertiliser applications and less frequent applications are required. Appropriate soil testing (nationally accredited laboratories) is a sound way to monitor soil fertility build-up and deficiency levels. the land.


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