The Irrigation System is the Lifeline of Agriculture
Irrigation Management

Proper irrigation management is essential in agriculture as droughts increase in severity and frequency. Irrigation affects the soil’s physical characteristics as well as the soil microbiota. Both the physical characteristics and microorganisms play a significant role in soil and plant health, ultimately affecting the consumer’s health. While each crop variety and soil type has its own specific needs, there are general principles that one can apply to irrigation management to promote soil health:

  1. Irrigation management, which achieves a mild water deficit in the soil, can benefit the soil’s and crops’ long-term health.
  2. Alternating between moist and dry conditions prevents the domination of denitrifying bacteria and slows the decomposition rate in the soil, leading to the accumulation of organic matter. 
  3. Delivering the irrigation water to reduce soil disturbance preserves soil aggregates and will help retain pore space and organic matter content. 

A healthy soil consists of abiotic factors such as proper physical structure and organic matter content and biotic factors like soil bacteria and fungi. Water influences almost every one of these soil characteristics: heavy rainfall or irrigation can break apart soil aggregates and leach nutrients from the soil, and too little water can inhibit the growth of crops as well as bacteria, fungi, and earthworms that improve soil health. Proper irrigation management provides the soil with the right amount of water, in the right way, at the right time to build the soil’s physical structure and promote beneficial microorganisms.

A seemingly endless stream of research has shown that plant growth-promoting rhizobacteria (PGPR) and arbuscular mycorrhizal fungi (AMF) can significantly boost plant growth, increase plant tolerance to droughts and pests, and improve the roots’ ability to uptake water and nutrients. AMF form networks throughout the soil with their hyphae, connecting the roots of neighboring plants. These fungi can redistribute water between plants during drought, reducing water stress. Soil microbes cycle carbon, nitrogen, and other nutrients throughout the soil, acting as a slow-release reservoir of these nutrients for plants. The composition of the soil microbiome is determined by soil moisture, organic matter content, and soil nitrogen content.

Source: Naylor & Coleman-Der

Figure 1. The plant root is the interface between the plant and its environment. It is responsible for absorbing water and nutrients and mediating interactions with the soil microbial community. The microbial community plays a vital role in maintaining plant health. Bacterial growth is favored in moist soils, whereas fungi dominate in drier soils. Irrigation management which alternates the soil through periods of wet and dry, will prevent any one group of microorganisms from dominating and will also reduce rates of decomposition and denitrification. When soil alternates between wet and dry, swelling and shrinking in the soil can create more pore space, improving soil aeration and moisture capacity.

Source: Martin et al., 2010

Figure 2. The water balance in agriculture is the difference between the total amount of water that enters an agricultural system and the total amount of water that exits. Water enters an agricultural system through precipitation, irrigation, or runoff. Water exits an agricultural system through evaporation and plant uptake. A positive value for water balance indicates that more water is exiting than entering. A negative value indicates the opposite.

How Much Water?

It may seem counterintuitive, but too much water is much worse for your soil (and plants) than too little water. Increasing soil moisture has been correlated with increased bacterial activity and mineralization of organic matter–excessive irrigation can accelerate the decomposition of organic matter in your soil. However, in arid regions with low soil organic carbon and low moisture, irrigation may favor the accumulation of organic matter by stimulating plant growth and carbon inputs via root exudation and harvest residues. When soil is saturated above 70% water-filled pore volume, reduced soil oxygen can also lead to the development of anaerobic conditions, which favors denitrification (the release of N as N2O gas). Heavy irrigation can also cause N losses through leaching. Therefore, excessive irrigation may lead to soil nitrogen and carbon loss.

Irrigation Rate Affects Soil Structure

The irrigation/water infiltration rate significantly affects the soil structure. Slow water infiltration, such as drip irrigation, has little effect on soil aggregate stability, whereas rapid infiltration, such as flooding or furrow irrigation, can displace trapped air and cause disaggregation. These types of heavy irrigation can also lead to soil compaction by compressing pore space and reducing the soil’s ability to retain water. A 2008 study compared the effects of mild water deficit irrigation on maize. The study’s authors compared conventional irrigation, alternate partial root-zone irrigation (APRI; one side of the plant irrigated, alternating sides with each irrigation event), and fixed partial root-zone irrigation (FPRI; one side of the plant irrigated, same side each time). The authors found that the soils in mild water deficit had more than 2x the number of soil microorganisms in conventional irrigation, with the alternate partial root-zone irrigation having the most significant number of soil microbes. The plants in the APRI treatment also had the highest irrigation water use efficiency. In a similar study of tomatoes, the authors found that deficit irrigation and partial root-zone drying significantly increased the water use efficiency for fruit production and allocated a more significant proportion of N to fruit production than vegetative growth.

Further Reading

Drenovsky, R. E., Vo, D., Graham, K. J., and Scow, K. M. (2004). Soil water content and organic carbon availability are major determinants of soil microbial community composition. Microbial Ecology.

Li, G., Niu, W., Sun, J., Zhang, W., Zhang, E., and Wang, J. (2021). Soil moisture and nitrogen content influence wheat yield through their effects on the root system and soil bacterial diversity under drip irrigation.. Land Degradation & Development.

Li, X., Liu, F., Li., G., Lin, Q., and Jensen, C. R. (2010). Soil microbial response, water and nitrogen use by tomato under different irrigation regimes. Agricultural Water Management.

Qi, Y., Li, J., Deng, S., Wang, J., Zhang, Y., Pei, H., Shen, Y., Hui, D., Lambers, H., Sardans, J., Penuelas, J., and Liu, Z. (2021). Long-term irrigation reduces soil carbon sequestration by affecting soil microbial communities in agricultural ecosystems of northern China. Soil Science.

Trost, B., Prochnow, A., Drastic, K., Meyer-Aurich, A., Ellmer, F., and Baumecker, M.(2013). Irrigation, soil organic carbon and N2O emissions. A review. HAL Open Science.

Wang, J., Kang, S., Li, F., Zhang, F., Li, Z., and Zhang, J. (2008). Effects of alternate partial root-zone irrigation on soil microorganism and maize growth. Plant and Soil.

Weisskopf, P., Reiser, R., Rek, J., and Oberholzer, H.-R. (2010). Effect of different compaction impacts and varying subsequent management practices on soil structure, air regime and microbiological parameters. Soil & Tillage Research.