Soil Considerations for Water Management

By Romeo Dragan

We intend to delve a little deeper into the importance of water management in today’s agriculture and to explain concepts like soil type and water infiltration. Farmers need to correlate the soil’s infiltration rate and storage capabilities with the irrigation system they use and plant water needs.

Farmers do their best to have a bountiful harvest; they plant the best seed at the optimum time of sowing, fertilize, and apply herbicides and insecticides. Although all this has an impact on plant growth and production maximization, nothing influences the growth of the plant throughout all the stages of vegetative development more than the well-managed soil water management.

Soil types

Agronomists classify soils in 3 categories depending on the size of the particles that compose the soil. Small particles are considered those under 0.019 mm (0.00007 inch), average particles between 0.002 mm and 0.02 (0.00007 – 0.0007 inch) and large particles are considered particles bigger than 0.02 mm (0.07 inch).

Depending on the percentage of particles of a certain size that compose the soil we can catalogue:

• Clay soils – contain at least 40% small particles;
• Loam soils – contain a mixture of small and medium particles ranging between 15% and 39%;
• Sandy soils – contain small and medium particles up to a maximum of 14%.

Soil capability in water and air retention

 Besides nutrients, the soil acts also as a water and air reservoir. Soil porosity retains water and air in the spaces between the particles that compose the soil. Each type of soil has different capabilities of retaining water, air and nutrients that plants need.

Sandy soils are well drained and aerated due to large particles as well as large spaces between the particles that compose these soils. Sandy soils usually drain very well being able to retain a small amount of water and because of this sandy soil has a reduced capacity to retain water and nutrients.

Clay soils exhibit diametrically opposite characteristics  from sandy soils. Clay soils suffer from lack of drainage and aeration. The lack of aeration and drainage is caused by the small size of particles that compose these types of soil as well as the small spaces between particles which allow them to retain large amounts of water and nutrients at the same time.

Loamy soils have the right mix of incorporating both the advantages of sandy soils and clay soils. The loam absorbs water, retains water and drains very well but at the same time aerates very well and contains better concentrations of micro and macro elements. This is precisely why on these types of soil farmers can obtain the best productions.

Water infiltration

Water infiltration rates vary depending on soil type:

• Sandy soils have a high infiltration rate of 19 to 25 mm/h (0.74 – 1 inch/h);
• Clay soils have a low infiltration rate of 3 to 8 mm/h (0.11 – 0.31 inch/h);
• The loamy soils have an infiltration rate of 9 to 13 mm/h (0.35 – 0.51 inch/h).

Given the rate of infiltration we can understand that clay soils need a high-water application rate with reduced frequency due to its capacity to store, sandy soils need low application rates with high frequency due to low capacity to store and the loamy soils are the most flexible on the rate of application and frequency with which we apply water.

The irrigation method also influences the way the water moves. Irrigation by aspersion (sprinkler) results in infiltration vertically because the whole surface of the soil is irrigated and lateral movement is not necessary; whereas water applied by drip irrigation will infiltrate both vertically and horizontally.

Conclusion

In order to practice good water management, farmers need to use an irrigation system that has an application rate correlated to the soil infiltration rate and plant needs. Once we know the plant needs and the soil’s infiltration and storage capabilities, we can easily choose an irrigation system capable of covering the plant needs in that specific soil conditions.

How do we calculate the irrigation system’s application rate? Very simple.

Example 1: For a drip irrigation system using a 2 l/h (0.52 gph) dripper, a 0.5 m (19.68 inch) spacing between drippers and a 1 m (39.37 inch) distance between driplines (rows) is easily calculated by dividing the emitters flowrate to the distance between emitters and dividing again to the distance between driplines (rows):

2 l/h / 0.5 m / 1 m = 4 mm/h    (0.52 gph / 19.68 inch / 39.37 inch = 0.0006 inch/h)

Example 2: For a sprinkler irrigation system using a 1500 l/h (396 gph) sprinkler, a 12 m (472 inch) spacing between sprinklers and a 12 m (472 inch) distance between laterals (lines of sprinklers)

1500 l/h / 12 m / 12 m = 10.4 mm/h (396 gph / 472 inch / 472 inch = 0.0017 inch/h)