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Integrated nutrient management to optimise water use efficiency during drought

A plant’s needs for water and nutrients are interdependent. Water is not only required for plant growth, but it is also the medium through which nutrients are absorbed by roots and transported through the plant. Water is necessary for all chemical reactions in the soil to take place, and also to transport nutrients to the roots. Therefore, an adequate water supply improves the nutritional status of crops, whereas a balanced nutrient supply saves water. With a proper nutrient management strategy, the farmer will optimise the use of water and substantially increase crop productivity.

How soil water influences crop nutrition

Figure 1 illustrates the positive effects that an adequate supply of soil water has on many processes such as the accessibility, availability, uptake and efficient use of soil nutrients. Excess water, on the other hand, can cause nutrient losses through leaching and denitrification under anaerobic conditions. A shortage of soil water will result in poorer nutrient solubility, transport and uptake by plant roots.

Crop nutrition influences water demand

The amount of water required for crop production is both plant specific and climate dependent. It is largely determined by nutrient supply and the size of the crop canopy (or leaf surface area). The parallel between water use efficiency (WUE) and the specific (relative) growth rate means that WUE will improve with increasing nutrient availability. A typical response to nutrient supply is shown in Table 1. Adequate nutrient supply creates higher osmotic pressure in plant cells, which results in a better resistance to drought. Nitrogen (N), being an integral part of plant DNA, chlorophyll and proteins, plays an important role in cell metabolism, photosynthetic capacity and yield. Potassium (K) plays an important role in regulating the functioning of stomata on the leaves that control water loss and therefore a good supply of K can conserve water. Phosphate (P) promotes early root growth, which allows for better access to water from deeper soil layers.

Fertilizer images

Fertilizer images

Nitrogen source affects water use efficiency

Crop yields can vary in response to N management and it is widely published that the N source has an effect on crop water use. Table 2 focuses on the impact of different N sources in the root zone on WUE. The general finding is that nitrate (NO3–) gives higher water use efficiencies than ammonium (NH4+), especially in dicotyledons, with similar trends for other crops and grasses.

Fertilizer images

Omnia’s Strategic Agricultural Services department conducted a series of greenhouse and field trials over the last two summer seasons in order to determine the effect of different N products on the efficient water use of maize grown under dryland conditions. From the results, it was evident that N applied as either nitrate, or in combination with calcium and potassium, significantly increased WUE under drought soil moisture regimes when compared to ammonium. Figure 2 clearly indicates that there is a probability of 95% to increase WUE by 16% when ANO (21) is used as top-dress fertilizer on maize when compared to Urea (46).

Fertilizer images

Conclusion

Given the ever-increasing risk associated with dryland farming in semi-arid environments, the time has come to revisit current nutrient management practices and their effect on sustainable production. Soil chemical and physical properties, for example cation exchange capacity, soil pH and available water in the soil profile, in addition to the microbial biomass and the dynamics of soil organic matter, are all important determinants of water and nutrient use by crops. Therefore, every producer needs to accept their responsibility in fully embracing technology and in adopting scientific farming practices in order to ensure the efficient use of water and nutrients, while also sustaining soil health.

By Louis Ehlers
Manager: Agronomic Services