Common questions about fertiliser and soil fertility

Do conventional fertilisers damage the soil?

  • No. Where fertilisers increase pasture growth this is often found to increase root growth and soil biology.
  • Conventional fertilisers do not harm soil biology and few directly cause acidification (i.e. a souring of the soil).

 Can soil biology provide all the nutrients required for good pasture growth?

  • Soil microbes are critical for nutrient cycling but there are very few examples of where added microbes increase pasture growth. One notable exception is Rhizobium bacteria which are commonly coated on the surface of legume seeds prior to sowing and form symbiotic relationships with the roots of legumes. These bacteria can fix large amounts of nitrogen from the atmosphere making it available for plants in the soil.
  • Most soils already contain a significant number of microbes. Most microbial products available for sale supply a mass of microbes (eg bacteria and fungi) which amounts to only a tiny fraction (ie much less than one in a million) of that contained in soil. Of the microbes that are applied, it is has been suggested that they disappear at a very fast rate and that few added microbes would remain after 3-4 days.

Why don’t fertilisers seem to work anymore?

  • This might be a sign that the nutrients in the fertiliser are already at high levels in your soil and putting more on won’t give you further responses in pasture growth. Conduct a soil chemical test.
  • Other nutrients (not included in the fertiliser you have been using) or toxicities such as potassium, sulphur or soil acidity may be limiting pasture growth. Conduct a soil chemical test and/or do a test strip to compare alternatives.
  • Pasture composition may be limiting the response. For example, large pasture growth responses to phosphorus rely on the presence of legumes (germinating from seed or as existing plants).

Don’t most of the nutrients in conventional fertilisers lockup in the soil?

  • Lock-up of nutrients is a natural process and is most obvious for phosphorus which can become bound to clays and other chemicals such as iron and aluminium.
  • The binding of phosphorus to clays is an important part of long-term soil fertility.
  • Lock-up of phosphorus applies to all phosphorus products (conventional and alternatives) but can be reduced with products which release plant-available phosphorus at a slower rate.
  • Microbes added to the soil have not been reliably used in the field to increase phosphorus availability to plants. Naturally-occurring soil fungi (Mycorrhiza) form symbiotic associations with plant roots allowing access to a greater volume of soil and increased root absorption of phosphorus, particularly when it is in low supply.

Does the content and availability of nutrients in soil treatments change?

  • Yes, the nutrient content of products such as composts and animal wastes (eg poultry litter, pig manure, feedlot manure) can be highly variable and a nutrient analysis is required to evaluate their cost-effectiveness.
  • Water content in composts and animal wastes can also be highly variable and a nutrient analysis provides this information. For this reason, these products should be purchased and spread on a volumetric (eg cubic metre) rate.
  • Not all nutrients in composts and animal wastes will be available for plants. Nitrogen in animal wastes can be lost to the atmosphere with higher losses (up to 35%) occurring where they are not incorporated into the soil and the gap between application and rain increases. The availability of most nutrients is in the range 25-50% in the first year with continuing mineralisation occurring over time.
  • Composts and animal wastes generally have a lower nutrient content than synthetic fertilisers. When comparing nutrient costs make sure to include the cost of spreading. For example, 1t of superphosphate contains 88kg of phosphorus but 1t of poultry litter or feedlot manure contains in the order of 10-15kg. With this example, you will need to spread the litter/manure at seven times the rate of superphosphate to provide the same amount of phosphorus. Remember that the different products have different rates of plant availability and seek advice in this area.

Should I use a product where reliable field performance has not been demonstrated?

  • It is advisable to be cautious about using products that do not have reliable information about their effect on pasture growth.
  • Verify the claims by seeking evidence of nutrient availability and product performance.
  • If the evidence is unavailable then test the product/s on your farm using test strips (seek advice on how to establish test strips).

Can foliar sprays provide nutrients and increase pasture growth?

  • Foliar sprays are more commonly used in crops (not pastures) for the application of micronutrients.
  • Generally, leaves act as a barrier to keep things out rather than to take them in.
  • Seek evidence of nutrient content and product performance and advice about the role of grazing after foliar sprays in removing (eating) the sprayed vegetation.

Are there any risks with using composts and animal wastes?

  • Possible risks include the presence of antibiotics, heavy metals, salt, microbial pathogens and weed seeds. Seek this information from the supplier.
  • Considering if nutrients which are not required are being added as this has the potential to cause nutrient leaching.

Will composts and animal wastes increase soil carbon?

  • Increasing pasture growth will have a gradual benefit for increasing soil carbon. Therefore, anything that helps to increase pasture growth is likely to increase soil carbon.
  • The level of soil carbon is difficult to change and requires many tonnes of carbon to increase. For example, a soil with 2% carbon may contain 24 tonnes of carbon in the top 10cm. Addition of products with, for example, 20% carbon content at the rate of 5t/ha would add 1t carbon (increasing soil carbon to 2.08%) but much of this carbon may be lost to the atmosphere through microbial degradation.
  • The upper level of soil carbon is limited by soil type and rainfall with higher soil carbon possible with greater clay content.
  • Soil carbon is an indicator of organic matter in the soil. Organic matter also contains other plant nutrients and plays an important role in the physical structure and water holding capacity of the soil.
  • Most productive and stable pasture systems are those with the highest soil carbon.

Is the balance of nutrients more important than the actual level?

  • The balance of nutrients in the soil is important but it should be remembered that roots are not straws, they selectively uptake nutrients.
  • The ratio of calcium to magnesium is frequently discussed due to concern that high levels of magnesium reduce calcium availability and may also lead to dispersion in the soil (ie unstable soil structure). There is little experimental data to support the importance of this ratio.
  • Calcium deficiencies are not common in Australian soils used for pastures.
  • Addition of calcium may have an indirect benefit for pastures in acidic (low pH) soils as it can assist in decreasing acidity and aluminium and manganese toxicities.

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