Effective irrigation depends on sufficient water infiltration into the root zone to sustain the tree until the next irrigation. Regardless of the water applied, poor water infiltration into the root zone results in under-irrigation, which translates into poor growth and yield. Poor infiltration may result in the upper zone may becoming overly saturated, thus more prone to root disease from poor aeration.
The ability of the soil to infiltrate water is related to: soil pores, soil aggregates, and salinity of the water.
Soil pores are the spaces between soil particles. The most effective water infiltration occurs early hours in the irrigation set, through cracks and small pores. As chemical changes occur and the clay particles swell, the cracks and pores begin to close and infiltration slows down. Water infiltration can be improved by treatments which increase pore size and volume. Porosity is the total volume of the pores and cracks per volume of soil. Soil pore size and porosity together determine the water-holding capacity of the soil.
These are electrically charged mineral plates existing in clay soils. Their binding sites hold water, and bind to each other. This creates a stable soil situation, maintaining the small pores and creating larger pores and cracks to enhance water filtration.
- Sandy soils: due to their crystalline nature, these have large pores, allowing sufficient infiltration, but low total porosity, resulting in less stored water in the root zone.
- Fine sandy loams and sandy silts: occurring in the southern San Joaquin Valley are very low in soil aggregates, and have poor water infiltration.
- Clay soils: these have small pores, which allow water to infiltrate slowly and bind to soil aggregates. Soil aggregates form in clay soils.
Salinity is a measure of the salt content, is discussed below in relation to irrigation. For more on salinity: Salinity Management
These are formed when soil aggregates and porosity are lost at the soil surface, and they limit adequate water infiltration. Decades of cultivation, and the use of very low salinity water can worsen the problem, as can the use of herbicides in no-till orchard management, which decreases soil organic matter. Using an auger to check soil moisture is a starting point in asessing soil crusting and water infiltration.
Managing Soil Crusts
- Tillage: If a crust is formed, tillage may precede other management practices. The size of the disc depends on the depth of the crust. Regular tillage to prevent weeds may also aid in avoiding water penetration problems.
- Organic Matter: chipped and shredded tree prunings, and cover crop residues provide biomass which can increase soil porosity when tilled into the orchard floor. Manure and green waste (a mixture of garden waste and lawn clippings), have also been shown to increase soil porosity when applied as mulch. Studies have shown that although these practices often improve water penetration, they need to be regularly applied, as they do not increase the amount of organic matter in the soil over the long term. Some cautions include awareness of the increased salinity associated with manure, and need to thoroughly chip and shred tree prunings.
- Cover Crops: compared to bare ground, soil with a cover crop has increased porosity and aggregate stability. They can be particularly useful in young orchards, to reduce erosion and provide biomass to stabilize soil aggregates. However, they can compete with trees for water and nutrients, and the management practices must be compatible with those for the trees.
- Irrigation Practices: Strategies include using a drip or microsprinkler system to provide flexibility in irrigation and application of chemical amendments, with application rates of chemicals depend on orchard and system design. In flood orchards, increase irrigation frequency from 10 -14 days down to 5 - 7 days in July and August.. In these orchards, a uniform grade insures adequate drainage. Recharge root zone soil moisture by the end of February. Alternate the water supply between well and canal water, specifically in the Shafter- Wasco area, as well water has better EC and SAR values that melted snow water (see below).
Irrigation Water and Soil-Water Factors
pH, EC and SAR are soil-water factors that give us an indication of water infiltration. A soil sample is prepared for analysis as follows: dried and ground, then saturated with distilled water. This water, then vacuumed from the soil, is the soil- water used for lab analyses, and reflects conditions in the soil.
- pH measures acidity or basicity of water, either: soil-water, or irrigation water. Pure water has a neutral pH of 7, acidic water has a pH < 7.0, basic or alkaline water has a pH > 7.0.
- EC (Electrical Conductivity) measures the electrical conductivity of soil-water, a reflection of salinity. This salinity is based on the presence of positive anions: sodium (Na+), calcium (Ca++) magnesium (Mg+), potassium (K +), and negative anions: bicarbonate(HCO3-), carbonate,(CO3=) nitrate(NO3-), sulfate(SO4=), chloride(Cl-). Low EC causes clay swelling, thus reducing pore size. An EC < 0.3 dS/m can be a problem. Rainfall has a very low EC and excessive runoff of rain water may be due to its low infiltration into the soil.
- SAR (Sodium Absorption Ratio) is the ratio of Na+ to Ca+ and Mg+
- Guidelines to diagnose infiltration problems as EC and SAR change have been compiled and are presented in the Table. However, these guidelines are not applicable to all soils, and advice from professional consultants regarding interpretation of EC and SAR values is wise.
|Problem Likely||Problem Unlikely|
||ECe or ECw1||ECe or ECw|
|0.0 - 3.0||<0.3||>0.7|
|3.1 - 6.0||<0.4||>1.0|
|6.1 - 12.0||<0.5||>2.0|
1 EC is measures in deciSieman/meter (dS/m). ECextract is salinity of water that
is removed from a saturated soil-water sample. ECwater is salinity of irrigation water.
Sanden, Blake, Terry L. Prichard and Allan E. Fulton. Improving Water Penetration. (UC Cooperative Exctension Kern County) Retrieved Jan. 26, 2012.