Fertasa soil fertility and plant nutrition symposium

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Several authoritative overseas and local speakers addressed modern trends in soil fertility and plant nutrition at the Fertasa (Fertilizer Association of Southern Africa) soil fertility and plant nutrition symposium in Pretoria.

Nitrogen use efficiency

Dr Richard Ferguson, professor of soil science, University of Nebraska – Lincoln, USA, discussed fertilizer management approaches to improve Nitrogen Use Efficiency (NUE).

He said one approach to improve fertilizer NUE is the use of enhanced efficiency fertilizers – using inhibitors or slow and controlled release formulations which reduce the potential for nitrogen (N) loss to the environment. Generally these mechanisms are helpful with urea or ammonia-based fertilizers. Inhibitors are available which temporarily block urease activity (a soil enzyme), or nitrification through suppression of nitrifying bacteria in soil. Urease inhibitors help protect against volatile loss of N to the atmosphere as ammonia. Nitrification inhibitors protect against both denitrification (gaseous loss to the atmosphere) and leaching as nitrate.

Another approach is to formulate fertilizers, or coat fertilizer particles, in such a way that these products can increase N uptake in plants. These products are a temporary insurance against N loss, but not yield boosters.

A more recent approach is the use of canopy sensors for in-season application. Applying N fertilizer to the growing crop is a recognised way to increase fertilizer use efficiency. Canopy sensors can accurately adjust N rates within fields according to soil variation as well as adjust N rates based on weather and growing conditions up to the time of sensing.

Nitrogen and phosphorous losses

Dr Catherine Watson, head of the Agri Environment branch, Agri-Food and Biosciences Institute, Belfast, Northern Ireland, discussed nitrogen and phosphorous (P) losses from grassland soils and said that loss of phosphorus to water from diffuse agricultural sources is a major threat to water quality. This leads to eutrophication of surface water bodies. Loss of nitrogen also has environmental implications for water quality, ammonia and greenhouse gas emissions.

The EU Water Framework Directive commits member states to restoring all water bodies to good ecological and chemical status by at least 2027.

Nitrous oxide (N2O)  is the most potent global warming gas, with a potential of 298 times that of carbon dioxide (CO2). Over 80% of nitrous oxide emissions come from agricultural sources, soils and fertilizers.

Stabilised N fertilizers (using urease and nitrification inhibitors) are potentially useful tools for reducing N losses to the environment and increasing crop yields. Globally, urea dominates the world nitrogen market because of its high N content of 46% and relatively low production costs. However it can be an inefficient N source due to ammonia volatilisation.

Humic and fulvic acids

Dr Christo Malan, MD AgriLibriumCape, discussed the use of humic and fulvic acids in practical farming.

He explained that humic substances (HS) are the unknown and unidentifiable components that are found in extracts of organic sources like compost, peat, lignite etc.  Commercial humic acid products are usually a salt of potassium, sodium or ammonium. A large portion of the humic acids occur as precipitated particles that cannot be taken up by the plant roots. It only assists in enhancing the uptake of nutrient minerals from the soil by binding the charged minerals (2+ cations) on its ion exchange sites to prevent it from reacting with phosphate anions to form insoluble phosphate compounds.

Fulvic acids are soluble in water at all pH conditions. Fulvic acids can also be described as being “humic acids” of lower molecular weight and higher oxygen content. Mineral nutrient fulvate complexes can be taken up by the roots and utilized by the plant. Fulvic acids are also taken up by the leaves. Thus fulvic acids in comparison with humic acids, contribute directly towards efficient nutrient uptake and utilization from soils and as foliar sprays.

The effective regulation of the use of HS and specifically humic and fulvic acids within certain guidelines and norms has become essential in order to control the quality of these compounds.

Sulphur deficiency

Dr Lawrence van Rensburg, director of Nulandis, emphasised sulphur’s role in crop nutrition as the fourth major agricultural nutrient. He warned that a sulphur deficiency in agricultural soils is widespread worldwide with the potential to threaten future food supply if left unchanged.

“Sulphur deficiency must be addressed at multiple levels. Effective and efficient nutrition is apparent and highlights the key role that sulphur can play in factors such as enhanced nitrogen use efficiency and crop disease resistance,” he said.

He added that according to the Sulphur Institute, sulphur is required in approximately the same quantities as phosphorous. Some 10 million tons per annum of sulphur is currently applied in fertilizers. The net deficit of applied sulphur in agriculture is expected to reach 12.5 million tons per annum by 2015. Sulphur deficient diets and the subsequent shortage of downstream metabolites of methionine and cysteine have been linked to cardiovascular disease, Alzheimer’s disease and diabetes in adults.

The Asian region, including China, faces the biggest deficit – with Africa not far behind. “Fertilizer programmes which incorporate sulphur can contribute significantly to the challenges of enhanced agricultural productivity and improved crop health,” Van Rensburg emphasised.

Potassium cost savings

Dr Neil Miles, senior scientist, South African Sugar Association, said massive cost savings in potassium (K) can be realised through the utilisation of significant amounts of ‘slow release’ reserves of potassium in soils. Results from long term sugarcane field trials conducted in KwaZulu-Natal and Mpumalanga confirmed that on soils with significant amounts of ‘slow release’ potassium reserves, there is no requirement for potassium fertilizers in the medium term.       

   “These findings reflect a potential for massive savings in fertilizer potassium in Southern Africa. However, for such savings to be realized, it is imperative that a test for ‘slow release’ potassium reserves be included in soil testing packages. Current annual expenditure on this nutrient in the South African sugar industry is estimated at half a billion rand,” said Dr Miles.

Seaweed benefits

Mr Thomas Mason, MD of Metson Manufacturing, outlined the benefits of seaweed extract products that should be considered to be more than just N, P and K. These products, known for decades as a soil enhancer, have many beneficial effects horticulturally and agriculturally, namely increased resistance to frost, increased uptake  of inorganic constituents from soil, more resistance to stress conditions and reductions in storage losses of fruit, thereby boosting farmer profits.

Seaweed extract products can be applied in three different ways: root application, seed treatment and foliar application.

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