Wrongly, fertilisers like superphosphate, MAP and urea carry derogative labels and are not acknowledged with achieving goals like improving soil quality, as well as high crop and pasture yields.
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Terms like synthetic and artificial are commonly associated with these products, perhaps to overemphasise their so-called undesirable features.
Research supports using these products in crop and pasture systems to correct soil deficiencies such as nitrogen, phosphorus and sulphur.
Where soil deficiencies are corrected, plants grow more grain in not only the case of crops but also more extensive root systems and greater above-ground biomass.
These contribute to more soil organic matter and more soil carbon. Provided good quality fertiliser products are used, there are no adverse residues left in the soil.
Superphosphate first evolved around 180 years ago in Europe.
Phosphorus in rock phosphate is largely plant unavailable, mainly because its phosphorus is in an unavailable form (citrate insoluble).
Treating it with sulfuric acid (like rock phosphate, it is also a natural product), almost all its phosphorus converts to plant available water soluble phosphorus.
Also available in this product is sulphur, likewise a common soil deficiency.
In many respects, superphosphate could be termed a natural product!
Other products can supply phosphorus and sulphur, for example, poultry and feedlot manure.
The level of nutrients in these can be highly variable and depend on aspects such as storage, litter levels, animal type and how they were fed.
Phosphorus in poultry manure, for example, can vary from 0.7 to 2 per cent, and some of it may be in an unavailable form.
Provided similar amounts of available missing elements, like phosphorus, are added via these products, their benefit can be similar to super.
No product has been shown to substitute for fertilisers like superphosphate or urea when they supply less missing soil nutrients.
For example, if 40kg/ha nitrogen is required, it is important to check how much of a given product is needed to supply that amount. Urea at 46pc nitrogen supplies 40kg of nitrogen at 87kg/ha.
The form of phosphorus is shown as important when single superphosphate is compared against rock phosphate.
Most research shows single super, or in the case of cropping products like DAP or MAP, (with balanced sulphur in the case of comparison with superphosphate plus nitrogen in the case of comparison with MAP or DAP), perform far better as phosphorus in rock phosphate is mainly non-available.
A considerable amount of research has shown that soil carbon, a major part of soil organic matter, builds in pastures if soil deficiencies are corrected.
A classic example is the PhD studies conducted at Boggabri by Dr Robert Banks.
A degraded sandy loam soil with poor-quality native pasture converted to high-quality tropical grass plus winter legumes resulted in soil carbon build-up of over 30t/ha.
Superphosphate used to correct soil deficiencies was a vital part of the program.
Other studies, including by NSW DPI, have recorded good build-up of soil carbon where superphosphate is used to correct soil deficiencies.
Good pasture management that includes appropriate species for a given environment and grazing management are also part of the soil carbon story.
Commonly fertilisers, when denigrated, focus on their detriment to soil quality.
These views are mainly not supported by any science and also commonly as part of promoting alternative products that, at recommended rates, supply insufficient levels of deficient elements. Sometimes these claims are supported by competitions, often with at least some government funding.
Often unverifiable claims against fertilisers like superphosphate and herbicides like glyphosate are not supported by any science.
It is wise to check the science behind a given philosophy or product (often incorrectly advocated as science-based).
An example of how devastating wrong information can be is when conventional fertilisers and herbicides were banned by a government (Sri Lanka).
Overnight the country changed from a food exporter to a food importer.
Next week: Winter legumes for next autumn sowing.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact (0428) 752 149.
