Plant nutrition remains the key to successful agriculture. Correcting soil deficiencies, like phosphorus, sulphur, nitrogen and potassium are essential for highest possible yields of pasture and crop for any environment. More fertile soils, as a consequence of correcting soil deficiencies, is also an important part of having the highest soil quality.
Fertilisers like urea, superphosphate, MAP and DAP, often referred to as conventional products, are frequently by far the cheapest way to correct deficiencies. Conventional fertilisers commonly contribute to adding to soil organic matter via their effect on greater plant biomass, including root systems. Animal manure, such as poultry or feed lot, can equally correct soil deficiencies plus supply some organic matter.
One sometimes sees/hears totally unproven statements about conventional fertilisers as being harmful to soil. Sometimes these are espoused by so called “experts”, commonly associated with a sales host. The ultimate test of any product, conventional or so called “natural” “biological” or the like is “where is the scientific published research (especially in peer reviewed journals) to support such claims”?
Over many years research in Australia has aimed to provide the most reliable soil tests to correlate with soil deficiencies and with what rates of nutrient correction are most economical for a given area. Overseas and Australian developed tests have been assessed for best accuracy. An Australian accreditation scheme exists to ensure reliability of testing procedure across laboratories. Accredited laboratories generally use tests proven best for given crops and environments.
Research has shown that correcting soil deficiencies can lead to regular upgrading of organic matter levels over a long period. For example phosphorus and sulphur deficiency correction, combined with legumes that build nitrogen, correct main deficiencies resulting in greater biomass which leads to improved soil organic matter.
Where soil deficiencies are corrected greater biological biomass and biological respiration (established protocols for measuring soil health) also commonly occurs.
Perennial pastures may be better for building soil carbon (which is part of soil organic matter) than annuals however some research has shown they can both be effective builders. Rather than fertiliser product the key issue is correcting soil deficiencies.
It is harder to build soil organic matter in cropping systems that don’t include pasture phases. Zero till/no-till combined with high soil fertility has been shown to arrest further declines in organic matter than full cultivation. In better rainfall areas some research has shown zero till cropping can gradually increase in organic matter.
The odd cultivation in zero till or no-till cropping appears to have little impact on soil organic matter levels. Sometimes an occasional cultivation is required as part of an integrated weed control program.
Many other soil nutritional aspects, such as pH, are important in cropping and pasture management. Incorrect use of superphosphate has been blamed for increasing soil acidity. Soil acidity has increased for three main reasons; increased product removal, build up of organic matter via legumes, and farming systems that for most of the year only have shallow root systems (e.g. annual crops and annual legumes). Remedies include lime and/or perennial based pastures.
Many other aspects contribute to good soil quality including grazing and stubble management. Be especially wary of claims about soil quality and how to improve it that are not science based.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email firstname.lastname@example.org or contact (0428) 752 149.