Soil has seen its place in the sun this year, with the United Nations having declared 2015 the International Year of Soils and continued scientific research showing the importance of healthy soils as a key component of our food system.
Looking forward, the world is faced with a colossal challenge: to produce more food in the next four decades that we have done in the last 10,000 years combined. This cannot happen without healthy soils to nourish the crops we grow.
While fertilizers are a vitally important way to maintain healthy soils, the topic has remained a polemical issue. Are mineral or organic fertilizers better for the environment? What is even the difference between the two?
Many myths have circulated on this topic - but science can set the record straight:
1. Mineral fertilizers can be "natural" too.
It is important to distinguish here what we mean by organic and mineral, natural and synthetic (man-made). Organic fertilizers contain carbon. Some are natural such as manure, and others are synthetic, such as compost. Mineral fertilizers are all inorganic can be synthetic, such as manmade fertilizers like urea, but they can also occur naturally in the environment, such as phosphate rock and potassium chloride.
2. Mineral fertilizers do not poison the soil.
Neither mineral nor organic fertilizers poison the soil when applied at agronomically correct rates, timing, and placement and with the right source (the 4Rs). The important distinction to make is that excessive rates of either mineral or organic fertilizer can cause serious environmental damage. Excessive application rates of either organic or mineral fertilizers can cause increased greenhouse gas emissions of (which is 310 times as powerful as CO2) and methane (which is 20 times as powerful as CO2), nitrate leaching to groundwaters and runoff entering waterways.
3. We do not lose as much of the nitrogen fertilizer applied to crops as commonly thought.
Crops retain between 30 and 70 percent of fertilizers applied, but it has been assumed that the remainder has been lost to leaching, denitrification, N2O gas emissions, runoff and erosion. But science has a different answer. Microbes in the soil take up much of the nitrate from fertilizers before the roots can. They convert it to soil organic matter that later is mineralized and taken up by plants. Far from being a "loss" this, in fact, makes the soil richer.
4. Fertilizers do not deplete soil organic matter.
As a continuation of the point above, science also tells us that mineral fertilizers actually increase crop biomass, if crop residues are incorporated in the soil. You will have more crop residue and roots to decompose if mineral fertilizer has boosted your yield. This in turn increases soil organic matter. Using a combination of mineral and organic fertilizers increases soil organic matter further.
5. Organic fertilizers are not the only route to plant nutrition.
The plant does not care whether the nutrients it takes up comes from fertilizer dissolution, soil organic matter mineralization, or the decomposition of manures, roots or crop residues. But the soil does. We need to recognize that organic and mineral fertilizers offer different benefits to the soils. Organic inputs provide carbon, the energy source for soil microorganisms; mineral fertilizers do not contain carbon. However, organic fertilizers have much lower nutrient content - they are only about 1-3 percent nitrogen, in comparison to around 46 per cent in the commonly used mineral fertilizer urea. Once again, science indicates that combining organic and mineral fertilizers is the best approach.
These five facts dispel the notion that organic farming is somehow superior to conventional methods. Using only organic fertilizers can work in some areas where the soil is already very nutrient rich. But it is important to remember that this is mainly a product of decades of mineral fertilization. However, it is simply not possible to rely on organic farming in areas where land is depleted of nutrients, such as sub-Saharan Africa. Even cattle manure cannot work efficiently in these areas, as the cattle themselves are often undernourished, and therefore their manure absorbs rather than releases vital nutrients.
It is our duty to ensure that farmers all over the world have access to this science, in order to grow the food they need, and make sure the environment is left intact for future generations.