Too much of a good thing?

Inadequate, imbalanced, poorly timed, excessive or otherwise inappropriate use of crop nutrients can cause unwanted environmental, economic and social impacts.

Insufficient crop nutrition leads to soil degradation

Nutrient mining (the ongoing removal of soil nutrients from successive crops with little or no replenishment) is a major cause of low crop yields in parts of the developing world, particularly in Sub-Saharan Africa. This negatively affects food supplies and farmers’ profitability. The most extreme form of this land degradation in arid and semi-arid zones is called desertification .

Furthermore, low fertility prevents a robust crop cover from growing, leaving land exposed to erosion. As soils are depleted, farmers shift to other fields, often clearing forests and exposing new land to erosion.

“On a global scale, soil fertility depletion is far more widespread than is soil fertility improvement. Nutrient depletion destroys the productive capital of the valuable soil resource. Depletion of soil nutrients is caused primarily by negative nutrient balances, faulty nutrient management strategies and a lack of resources for investment in soil-fertility-enhancing inputs.” (Roy et al. 2006)

Wrong product(s), wrong timing and wrong rate can all trigger a number of unwanted impacts

Nutrients may be absorbed by crops, immobilized by the soil or lost from the soil system. Depending on the nutrient and various conditions, these can be lost to the atmosphere by volatilization or denitrification, through soil erosion, or by leaching.

Both nitrogen and phosphorus can be implicated in the overenrichment or eutrophication of surface waters . This may upset the balance of the local ecosystem, with negative impacts on biodiversity. For example, such enrichment may trigger algal blooms. When the algae die off, their decomposition can lead to a dramatic decrease in the water’s oxygen content, a phenomenon called hypoxia. The hypoxia may, in turn, cause other organisms to flee or to perish. The nutrients involved may come from improper use of animal manure and fertilizers, or from the release into the environment of untreated or poorly treated industrial and urban wastes.

Nitrogen losses from agricultural lands can also affect air and water quality. One of the nitrogen gases lost from the field is nitrous oxide, which is a potent greenhouse gas.

All nutrients can be lost by surface runoff as well as through water and wind erosion of soil particles. Although industrial sulphur has been associated with environmental impacts such as acid rain, the levels used for crop nutrition are not known to cause negative environmental impacts. Losses of K, Ca and Mg to the environment are considered benign. The primary environmental concern regarding micronutrients is toxicity above certain thresholds.

Fertilizer Best Management Practices Optimize Nutrient Use

The best outcomes occur when farmers match their nutrient use to local conditions. This means managing organic and inorganic sources of nutrients in an integrated fashion and using the right products at the right time and rate in the right place. Good management increases the amount of nutrient absorbed by the crop or stored in the soil, making less available to be lost to the environment. IFA is currently working to create a global framework for the elaboration and wider adoption of site- and crop-specific best management practices.