Potato Review

38 POTATO REVIEW JANUARY/FEBRUARY 2023 FERTILISATION B IOLOGICAL nitrogen xation is a more economical, ecological and pro table option to using nitrogen fertilisers according to Helena Estiveira of Alltech Crop Science. As crops rely on a continuous supply of nitrogen, many growers resort to nitrogen fertilisers to ensure increased soil fertility and crop production but, over the last few decades, excessive use of inorganic nitrogen fertilisers has been shown to a ect soil health, causing unbalances in soil and inhabitant microbial communities and impacting marine, freshwater and terrestrial ecosystems. Helena stresses that fertilisation is not the only solution to ensure crops can utilise nitrogen in the soil and says biological nitrogen xation instead uses nitrogen- xing soil microbes. “Our atmosphere is abundant in nitrogen — 79% of nitrogen is in the form of N2 gas. Nonetheless, this is unavailable to crops unless it is “ xed” (combined) in the form of ammonium (NH4) or nitrate (NO) ions to be used for plant development. In this sense, nitrogen is often a limiting factor for optimal crop growth, even under ideal climate and water supply conditions,” Helena states. In theory, fertiliser use would be less problematic if the crops used all that was supplied but the ratio of nitrogen in harvested crops compared to nitrogen inputs (through fertilisers or manure), shows an unbalanced output. “Our crops take up less than half of the nitrogen we apply,” said Helena. “ e comparison between nitrogen input and output is called “nitrogen use e ciency” (NUE). e higher this number is, the better the plants are uptaking and utilising the nutrient provided. An NUE of 80%means that the quantity of nitrogen in crops equals 80% of the nitrogen supplied as inputs, with the plants not utilising the remaining 20% of nitrogen. “A low NUE is undesirable, as it means that very little of the nitrogen applied gets absorbed by crops, and the rest becomes a pollutant. Since 1980, global NUE has remained at a low level of 40-50%. e remaining nitrogen is waste that seeps into the natural environment, owing o the soils and polluting rivers and lakes, disturbing ecosystems and causing biodiversity loss. Consequences of a lowNUE groundwater pollution, algal growth, nitrogen deposition and nitrous oxide being formed through denitri cation (greenhouse e ect). Meanwhile, lack of uptake means more and more nitrogen fertilisers need to be applied to continue growing crops at an increasing rate while the soil is depleted and crop prices rise. It now takes signi cantly higher doses of nitrogen to produce the same amount of crops as in the 1960s. “We seem to be closed in this continuous cycle, where many people think that additional fertiliser use for crop yields is an unquestionable trade-o . Yet, we are not limited to this compromise, and microorganisms play a key role in the emerging solution,” said Helena. Soil nitrogen-fixing bacteria and nitrogen cycle Bacteria are the only known microorganisms capable of converting nitrogen gas into the plant- available organic compound ammonia. Before commercial nitrogen xation methods were developed, plants relied only on microorganisms to provide useable nitrogen, said Helena. “Soil bacteria play a crucial role in practically all elements of nitrogen availability, supporting the formation and growth of both underground ecosystems and plants,” she states, adding: “Soil bacteria are also responsible for the major conversion of N2 into ammonia and subsequently into proteins in the process called nitrogen xation (or dinitrogen xation).” Nitrogen xation is the process of converting relatively non-reactive atmospheric N2 into more reactive molecules (nitrates, nitrites or ammonia). ese reactive forms are essential for crops, helping them thrive. Nitrogen shortage, on the other hand, stunts crop growth and healthy development. Nitrogen- xing bacteria, such as Rhizobium, Azospirillum or Rhodobacter, manufacture a unique enzyme responsible for nitrogen xation, which accounts for about 90% of natural nitrogen xation on our planet. In short, nitrogen- xing bacteria transform atmospheric nitrogen into inorganic chemicals. Nitrogen- xing bacteria accomplish what crops cannot themselves: they get assimilative nitrogen. Bacteria absorb it as a gas from the air and release it to the soil, typically as ammonia. Helena Estiveira of Alltech Crop Science discusses soil fertility. Biofertilisation: Increased nitrogen availability