FERTILIZER: an ASSET for the plants, a PLAGUE for the soil

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Fertilizers stimulate plant growth, but they would alter soil stability. This is shown by research, based on 50 years of study of a cornfield subjected to different rates of inorganic fertilizers. Explanations on this result.

The use of  fertilizers  has significantly increased crop yield. For example, it has quadrupled the profitability of wheat since 1950. Intensive agriculture mainly uses inorganic fertilizers, such as nitrogen and phosphate products. The nitrogen is one of the essential nutrients for good growth and development of plants. Absorbed by the plant in mineral form ( ammonia or nitrate), it comes either from the  mineralization of organic matter or fertilizers. While inorganic fertilizers are used worldwide and have proved effective, their impacts on the soil are not well known. The latest study provides very contrasting results. Researchers Humberto Blanco-Canqui and Alan Schlegel studied for 50 years the influence of inorganic fertilizers on plots cultivated in Kansas. They show that in these soils, the storage of  organic carbon has increased, but that in contrast, the stability of the aggregates has deteriorated.

The  fertilization of plants  by fertilizers improves their growth and increases the rate of organic matter in the soil. Fertilization is the main determinant of biological activity and influences the physical and chemical properties of the soil. Particles bind more easily with each other thanks to carbon: the aggregation and stability of the soil structure should then increase, the higher the organic carbon content of the soil. However, against all odds, this new study suggests that under the effect of fertilizers, no improvement in the stability of soil aggregates was observed, despite the increase in organic carbon. The results were published in the Journal of Environmental Quality .

Do fertilizers dissolve soil aggregates?

The study was conducted continuously for 50 years on maize plots . They were irrigated and plowed without interruption. At random, portions of land were selected, and received six different levels of ammonium nitrate and three different phosphate levels over the period. The soils of the plots have seen their organic carbon concentrations gradually increase. The increase in nitrogensupply  induced an increase in organic carbon to depths ranging from 0 to 15 cm. On the other hand, by adding phosphorus , the organic carbon concentration in the soil has increased up to 30 cm deep.

As for the stability of aggregates in the soil, on the contrary, it decreases with the addition of these substances. And this especially when nitrogen and  phosphorus  have been applied at high levels. The addition of 90 kg of nitrogen per hectare reduced the number of stable aggregates by 1.5 times in the absence of phosphorus. This effect is even more pronounced when nitrogen combines with phosphorus. Thus, when added to 90 kg of nitrogen per hectare, 20 kg and 40 kg of phosphorus, the number of stable aggregates decreased by 2.1 and 2.5 times, respectively. Of course, more studies are needed to accurately assess the role of land application . It is certain that the consequences on  soil stability also depend on climatic factors, tillage and type of crop. Scientists suggest, however, that adding ammonia-rich fertilizers could cause soil particles to disperse rather than aggregate. This would then offset the positive effects of increasing the  carbon content caused by fertilizers. In addition, tillage disrupts seasonally, and may even negate the benefits of fertilization. In any case, it is certain that it is necessary to determine the influence of fertilizers on the stability of the soil, in order to better adapt the ratio between the different minerals. Otherwise, in the long term, fertilizer or not, the land will not be more fertile anyway.

Dr. Kimberly Seltzer

Postdoctoral Scholar, UC Berkeley Research Assistant, MIT

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