Effects of Fertilisers on Productivity

Productivity is the rate at which plants produce biomass. Plants need mineral ions to grow: nitrate provides nitrogen for amino acids, proteins, nucleic acids, ATP and chlorophyll, and phosphate provides phosphorus for ATP, nucleic acids and phospholipids. When these ions are in short supply they are limiting factors, and growth is slow.Adding fertiliser removes this limitation. Plants make more proteins and chlorophyll, grow taller and develop a larger leaf area. A larger leaf area absorbs more light, so the rate of photosynthesis increases, net primary production rises and the crop yield increases.The effect has limits. As more fertiliser is added, the yield increases, then levels off, because another factor such as light, carbon dioxide, water or temperature becomes limiting. Very high concentrations can even reduce yield: the fertiliser lowers the water potential of the soil, so less water enters the roots by osmosis. Extra fertiliser beyond the level-off point is wasted money.Worked example
A field yields 6.0 tonnes of wheat per hectare without fertiliser and 8.4 tonnes per hectare with fertiliser. Calculate the percentage increase.percentage increase = (8.4 − 6.0) ÷ 6.0 × 100 = 40%Farmers compare this gain with the cost of the fertiliser to decide how much to apply.