How to care for irrigated soils?

Do you know the main problems of irrigated soils? Do you monitor your irrigated soils and implement management strategies to care for them?

Finding answers to these questions is essential since, according to the UN: “Agricultural production will have to increase by 601 TP3T to meet global food demand in 2050, yet 331 TP3T of the planet's soils are degraded.”

In the framework of World Soil Day and under the motto “Soils, the origin of food”, in this article you will find monitoring tools for irrigated soils and sustainable management strategies for their care.

Two common problems in irrigated soils
When we irrigate, we are introducing water into the soil that has a different quality than that of rain. This produces changes in the physical and chemical properties of the soil.
The most frequent problems in irrigated soils are:
● Compaction
● Salinization

1. The effect of soil compaction under irrigation
   Compaction is the decrease in soil porosity, which impacts physical, chemical and biological fertility. This problem ends up affecting the development of
   crops, as it makes it difficult for the roots to penetrate.Soil compaction under irrigation can occur due to:
   ● Inadequate irrigation management
   ● Poor quality of irrigation water
   ● Passage of machinery on wet ground
   ● Trampling of animals

● Tillage
● Soil sodification
● Loss of fertility
The 5 effects of compaction of irrigated soils are:
● Change in soil structure
● Difficulty absorbing nutrients
● Decrease in crop productivity
● Decrease in water infiltration into the profile
● Water loss due to runoff

2. The effect of salinization of irrigated soils
   Soil salinity is the presence of soluble salts in water, which affects
   mainly microbial activity and nutrient uptake by plants.
   A soil is considered saline when the electrical conductivity is greater than 4 ds/m (USDA
   Agricultural Handbook 60, 1954).
   The main causes include:
   ● Inadequate irrigation management
   ● Poor quality of irrigation water
   ● Natural causes
   ● Improper management of fertilizers and wastewater
   Consequently, the effects produced by salinity are:
   ● Difficulty in absorbing nutrients
   ● Reduction in water availability
   ● Toxicity
   ● Variation in pH

Irrigation management involves providing water in the amount and at the times when the crop needs it. We must do this efficiently because, on the one hand, we can affect the physical condition of the soil by generating surface compaction due to excess water, as well as affecting the chemical composition with the contribution of soluble salts from the irrigation water.

Three irrigation management methods to prevent and minimize the impact on the soil
Irrigation management is also a key tool to prevent and minimize the effects of soil compaction and salinization (Adapted from Peralta G, 2021).

1. Maintain optimal soil moisture levels, especially in the early stages of cultivation when the roots must pass through hardened or thickened layers.
   compacted, and in critical periods of high environmental water demand.
2. Managing moisture levels at harvest is also key to prevent the soil from having a low load-bearing capacity at supra-optimal moisture contents, which make it susceptible to compaction.
3. Water and salt contributions through irrigation: proper irrigation management is necessary, avoiding unnecessary contributions of water and salts, which contribute to modifying the physical-chemical state of the soil and promote compaction and salinization.
   

It will therefore be essential to know the characteristics of the environments to be irrigated (their total water storage capacity and useful water; their infiltration capacity), as well as to have an irrigation and monitoring strategy that allows us to manage the frequency and intensity of irrigation.

4 tools to monitor irrigated soils
Before establishing any management strategy, it will be essential to know essential aspects such as the characteristics of the soils we are irrigating, their useful water retention capacity, rooting depth, infiltration capacity, the presence of physical and chemical limitations to root growth, and the quality of the water with which we are irrigating, among other factors. Knowing what environments we are irrigating, and how our management may be influencing our soils, should be the starting point for the development of management strategies aimed at increasing water use efficiency and yields in our production systems (Peralta, G. 2021).

1. Test pit: NDVI images can be used to identify different environments and georeference sampling points. Test pits are made at these points where the development and depth of roots, moisture distribution, presence of compacted layers and salinity can be observed. Secondly, it is important to take advantage of the opportunity and extract soil samples.
2. Physical and chemical analysis of the soil: the result of this procedure will determine the physical characteristics such as texture, structure and apparent density, and the chemical characteristics such as nutrient availability, pH, electrical conductivity (EC), cation exchange capacity (CEC) and exchangeable sodium percentage (ESP).
3. Analysis of water salinity and sodicity: When we irrigate, we introduce a water quality that is different from that of rain, so it is important to measure the RAS, which is the possibility of sodification that sodium can exert on the soil (compaction). We also obtain parameters such as pH and electrical conductivity. This analysis should be carried out before the irrigation season begins, and, as far as possible, repeated as frequently as possible.
4. Measuring the mechanical resistance of the soil: to know the physical environment where the roots develop, a conical penetrometer can be used to measure the resistance of the soil to being crossed by a conical tip.

Two key aspects in irrigation management
In conclusion, we can say that irrigation can generate physical-chemical changes that alter soil functioning, irrigation efficiency and crop productivity.
Once compaction and/or salinization problems have been established, it is more difficult to correct or mitigate them. It is important to prevent them and to do so we must take into account two key aspects:

1. Knowing the soil and characterizing its environments
2. Knowing the quality of irrigation water

Knowing which environments we are irrigating, and how our management may be influencing our soils, should be the starting point for developing management strategies aimed at increasing water use efficiency and yields in our production systems.