The properties of the soil play a crucial role in where irrigation water goes. Some soil properties, like soil texture, are inherent to the soil type present in the field and cannot be changed by management.
Understanding implications of inherent soil properties on water movement and storage can guide irrigation management. Other soil properties, like soil structure and soil organic matter content, can be improved by adopting practices that build soil health. Improving these dynamic soil properties can help increase the water available to the plant in the long run.
How Do Soil Properties Affect Water Infiltration And Retention?
Soil texture reflects the relative proportion of clay, silt and sand particles. Finer-textured soils have smaller pores but a greater total pore space compared to coarser-textured soils. The small pores exert capillary forces and can hold onto water against gravity. This is known as field capacity, and the volume of water held in the soil is different based on the soil texture. The capillary forces will also cause water to move in all directions in the soil.
When using drip irrigation on a fine-textured soil, the wetting pattern will be wider, and downward water movement will be slower compared to that in a coarse-textured soil. Because of the greater total pore space and the greater percentage of small pores, fine-textured soils can hold onto more water. Therefore, fine-textured soils can be irrigated less frequently with larger amounts of water applied at a time compared to coarse-textured soils. On the other hand, water can move much faster through large pores than through small pores, allowing for higher application rates when irrigating coarse-textured soils.
While management cannot change soil texture, water infiltration and retention can be improved by building soil organic matter and soil structure. Soil structure refers to the three-dimensional units that soil particles are organized in. Sand, silt and clay particles bind together with organic matter in units called aggregates. Aggregation improves both water-holding capacity and infiltration, because small pores are formed inside the aggregates, while larger pores are created between aggregates.
Practices that increase organic matter inputs and promote biotic activity increase aggregation and improve soil structure. Thus, building soil structure through the adoption of soil health practices can help reduce runoff, improve infiltration and increase water-holding capacity, preparing the soil for extreme weather and improving water use efficiency in the long run.
Improve Soil Health With Management Practices
Cover crop plantings, application of organic amendments such as compost and reducing tillage intensity are recommended to improve soil health. Besides benefits for water infiltration and retention, these practices can reduce erosion and dust while increasing nutrient retention, promoting beneficial soil organisms and increasing soil organic matter and soil tilth. Financial incentives for the adoption of soil health practices can be obtained through the Natural Resources Conservation Service’s Environmental Quality Incentives Program and the California Department of Food and Agriculture Healthy Soils Program.
Cover crops are grown during fallow periods in annual crops, while they are planted in the drive rows between the trees or vines in perennial crops. Cover crops add organic matter to the soil, promote soil aggregation and create large soil pores, supporting improved drainage and infiltration. When choosing cover crop mixes with legumes, cover crops also add nitrogen to the soil. To avoid competition for water between the cover crop and the cash crop, cover crops can be terminated through mowing or incorporated at the end of the rainy season.
Compost is one of the most common amendments to improve soil health. Upon soil application, compost serves as a food source for soil organisms and contributes to building aggregates and soil organic matter. The impact of compost application on infiltration and water retention depends on the number of years compost has been applied, the rate of compost application and the feedstock and compost processing techniques. In arid regions, compost quality should be assessed carefully with regard to salt content. Some compost sources can have a relatively high salt content, potentially invoking increased drought stress in the crop.
Reducing tillage intensity to improve infiltration often seems contradictory. Tillage opens up the soil, which in turn promotes fast water infiltration shortly following tillage. However, tillage is known to break up aggregates. Without sufficient aggregation, the openings in the soil surface will quickly close up in response to precipitation and irrigation. Early in the season, tillage may increase infiltration, but this pattern is often reversed as the season progresses.
Completely eliminating tillage may not be possible or suitable for all cropping systems, but strategically planning tillage intensity can help maintain or build a level of soil structure that supports improved infiltration and water retention in the long run. For example, cover cropping or compost application may help mitigate some of the negative impacts of tillage.
What Role Does Gypsum Play?
Gypsum is often seen as the go-to solution to improve infiltration, but how does it work? The chemical makeup of gypsum is calcium sulfate. When gypsum is added to the soil, it slowly dissolves, thereby releasing calcium and sulfate in the soil solution. Calcium holds clay particles together, a process referred to as flocculation. The opposite of flocculation is dispersion, a process that destroys soil structure by pushing clay particles apart.
Dispersion is caused by high concentrations of cations with a low charge and large hydrated radius, such as sodium. Dispersion can manifest itself in the top quarter-inch of the soil and go unnoticed in typical soil testing. Dispersion can be reversed by adding gypsum, followed by leaching the sodium down below the root zone.
Gypsum can also improve infiltration when irrigation water has extremely low salinity or has a high sodium content. In normal soils that contain plenty of calcium, poor soil structure is typically a result of compaction or the loss of organic matter, rather than due to dispersion. In those cases, soil conservation practices such as cover cropping, compost application and reducing tillage intensity are recommended to address the issue.
Charlotte Decock is an assistant professor in soil health and fertility at Cal Poly State University.
Franklin Gaudi is a project manager at the Irrigation Training and Research Center at Cal Poly State University.