Water is a scarce agricultural resource. Some European regions already today repeatedly suffer from water shortage. Climate change might worsen this situation in thefuture. How to achieve optimum yield while reducing water consumption? What is the role of plant nutrition on water consumption?
Yara has conducted research on plant water uptake and nutrition status and this Pure Nutrient fact shares our findings.
Plants need water to grow. Most water is consumed by transpiration. In addition, water from precipitation or irrigation can be lost by surface run-off, drainage and evaporation from the soil. Efficient use of water determines crop yield. Water demand for crop yield varies depending on the crop and the growing conditions.
How can plant nutrition contribute to make best use of available water for food production?
Table 1 summarizes the water demand for key crops under good and bad conditions. In this context, "good conditions" means sufficient nutrient supply and efficient use of water, whereas "bad conditions" stands for nutrient insufficiency, high evaporation losses, high temperature and low air humidity.
|Crop||Water Consumption (mm/T harvest *ha)|
|Table 1: Water consumption per ton of harvested product varies according to meteorological and nutritive conditions. Under good conditions, significantly less water is needed to grow the same amount of crop than under bad conditions.|
How Crops Use Water
Water leaves the field by four main pathways: transpiration, evaporation, run-off and drainage. Contour cropping and zero-tillage are best agricultural practice to improve water retention. Under good farming practice, evaporation and transpiration are the main causes for water consumption.
Evaporation from the soil is an unproductive loss. Reducing evaporation losses by a well-developed crop canopy makes more water available for biomass growth.
Transpiration is necessary for plant growth. As the plant takesup carbon dioxide (CO2) for photosynthesis through the respiration openings in the leaves (stomata), water is transpiredand leaves the crop tissue. As soon as water is in short supply, stomata close and biomass production declines because of a lack of CO2 inside the plant tissue. Transpiration is also essential to cool the crop tissue and to prevent heat stress. It triggers the uptake and flow of water in the plant and thereby the uptake and movement of nutrients.
Yield for Water
The objective of good crop management is to get as much CO2 as possible into the plant tissue per unit of water transpired. The water demand per unit of yield depends on whether crops are grown under good or unfavorable conditions like heat, low humidity or lack of nutrients.
There is a widespread belief that water demand would increase with intensive agriculture while, on the other hand, low input farming would save water. In fact, however, the opposite is true and water demand per unit of production decreases with yield.
What has Yara research revealed about the relation between nutrient supply, yield and water uptake?
The Role of Key Nutrients
Potassium is essential for controlling stomata function. In case of deficiency, stomata don’t close appropriately and water is wasted.
Sufficient phosphorus availability supports early root and shoot development, contributing to early canopy closure and access to deep soil moisture.
Nitrogen is essential for chlorophyll and enzyme formation, both required to metabolize CO2 into biomass. Lack of nitrogen reduces photosynthesis, resulting in wasted water.
Nitrogen and Water Use
Under laboratory conditions, winter wheat was grown under different nutrient conditions in individual pots. Water consumption was measured daily and the resulting grain yield at harvest was weighted.
Grain yield increased with nitrogen supply. Water consumption also increased with nitrogen supply, but not to the same extent. As a result water demand per unit of grain decreases with increasing nitrogen supply (figure 4). In other words, the better the nitrogen supply, the less water is required per kilogram of grain grown!
Nitrogen Supply and Transpiration
Reducing evaporation (figure 6) makes more water available for transpiration. Sufficient nitrogen availability ensures that the available water is converted into growth and yield.
How Much Water?
How much water is needed to grow a ton of grain? Findings from pot trials have been confirmed under real farming conditions. The relation between nitrogen supply and water use was evaluated in 200 field trials under rain fed conditions. All plots received the same amount of water, but different nitrogen supply. The corresponding response curves (figure 7) demonstrate that water demand decreases from 100 mm to 60 mm with increasing nitrogen supply up to the economic optimum.
- Under optimum nitrogen supply, producing 10 tons of grain requires 600 mm of water.
- Under nitrogen deficiency, producing 10 tons of grain requires 700 mm of water and more surface.
Optimum nutrient supply improves Water Use Efficiency. But which nitrogen form is best suited if water supply is the limiting growing factor?
Many studies have shown the superior performance of nitrate-based fertilizers over urea and UAN under dry conditions.
N Availability in Dry Soils
In a pot trial performed by Yara , columns of 5 cm height were filled with arable topsoil (silty loam or sandy soil, respectively). Nitrogen was applied on the surface of the soil column as CAN or urea. Moisture content was kept constantly low at 7.5% and 5.0%, respectively, for three weeks at 25°C. At the end of the trial, soil columns were cut into slices of 1 cm. The content of nitrate, ammonium and urea was measured for each slice.
More than 80% of nitrogen from CAN has been recovered, compared to only 40% with urea, indicating significant better N availability even in deeper soil layers as compared to urea. Though residual soil moisture was sufficient to convert urea to ammonium, more than 60% of the nitrogen applied as urea was not recovered mainly because of volatile ammonia losses. Furthermore, with urea hardly any nitrate nitrogen became available in deeper soil layers, holding potentially still more available water.
Urea or Ammonium Nitrate?
With limited water availability, choosing the right N-form is important. Dry soil conditions boost nitrogen volatilization losses as ammonia from urea. Volatilization losses are unpredictable and thus hamper precise nutrition plans for high yield and protein content.
Ammonium nitrate dissolves fast without any losses and nitrate is more mobile and penetrates deeper into the soil under dry conditions. Readily available nitrate nitrogen can be rapidly taken up and metabolized and therefore offers the best features to ensure targeted nitrogen supply when water becomes a limiting factor.
Higher Yield with Nitrate Fertilizer Under Drought
Independent field studies conducted in France compared yield for different nitrogen sources under adequate water supply and drought stress. 130 kg/ha of nitrogen were applied at stem elongation (GS 30) as AN or UAN. Part of the surface was covered by movable glass roofs and received no rain for 23 consecutive days after spreading, while the other part received rain on day 1 and 7 after spreading. Yield was lower for UAN than for AN under both, normal and dry conditions, but the difference was more marked under dry conditions.
Overall this shows that crops can produce more grain based on a limited amount of water when ammonium nitrate is applied instead of UAN. This is an accumulated effect of less losses,higher mobility in the soil, better root growth and a more efficient use of water with nitrate nutrition.
Water availability has an important impact on yield and fertilization requirements. Adequate plant nutrition is key to make good use of water resulting from precipitation. If irrigation is an economically viable option, water management based on sensors to determine optimal irrigation rate and timing is a prerequisite to make good use of the scarce resource.
Yara Research and Development
Agronomic research in Yara investigates the interaction between water supply and nutrient demand. New knowledge is used to optimize fertilizer programs and to help crops to cope better with drought stress.
For irrigated crops, tools and recommendations are developed allowing a more efficient use of water. Yara’s water technology comprises leaf clamp sensors, soil humidity sensors and real time data acquisition and transmission equipment. Data is collected wirelessly and can be accessed via the Internet. This technology will enable farmers to further optimize irrigation.
Research has shown that maximum use efficiency can be achieved by combining water and fertilizer application, a technique called fertigation. Yara has developed fertigation recommendations for crops and regions where water and nutrient management is required.