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Water capacity of soil, what it depends on, the division of soil pores, from which pores water is available to plants
Water capacity of soil
Soil consists of three phases: solid, liquid, and gas. Maintaining a balance of 50% solid phase and 25% each of liquid and gas ensures optimal plant growth. The water capacity of specific soil is determined by the ratio between the liquid and gas phases; increased air content in soil reduces its water retention capability. The agronomic category of soils is also significant. Soils with higher proportions of fine particles, such as dust and clay, can store more water, which is crucial during increasingly frequent dry spells in spring. The content of these fine soil fractions determines the type of pores present.
Division of soil pores
Generally, we distinguish three types or sizes of soil pores: macropores, mesopores, and micropores. The first and last may contain water, but it is not available to plants. Water drains quickly from macropores, especially in lighter soils, and plants do not have time to utilize it, a process known as gravity drainage. In micropores, water is so strongly bound that roots are unable to extract it.
The percentage share of individual pores varies across different soils. Generally, in most soils found in Poland, the share of the solid phase is similar, ranging from 50 to 60%. The remainder consists of pores filled with air or water. For example, in sandy soil, the share of small pores, crucial for effective water retention and availability to plants (mesopores), is approximately 5%. In contrast, soil with a high clay content has around 20% mesopores. An increase in medium pores correlates with a decrease in water availability in the largest pores, which allow for easier water flow. Sands contain the most large pores, usually filled with air, while heavy soils with higher dust and clay content have the fewest large pores.
From an agricultural perspective, the most important soil parameter regarding water capacity is field water capacity. It refers to the condition in which the soil is saturated with water after gravitational water, or weakly bound water, has drained from it. This water either flows from the field or percolates through the ground to drains. Such a state typically occurs after the spring thaw, assuming the winter was snowy.
Water capacity of various soils in liters per square meter.
Water availability is influenced by soil structure. In the presence of compaction at a depth of 15 cm, clayey sand provides plants with up to 60 liters of water. If compaction occurs at a depth of 30 cm, up to 90 liters of water are available. On heavier soils, such as black earth, a compaction depth of 15 cm allows up to 120 liters of water, while a depth of 30 cm permits up to 170 liters.
Enhanced soil water capacity
Improving soil water-holding capacity is a crucial aspect of agricultural land management. The following methods can enhance the soil’s ability to retain water for plants:
Enhance organic matter content
Compost:Regularly adding compost to the soil introduces organic matter, enhancing the soil's structure and its water retention ability.
Green fertilizers: Crops such as clover, alfalfa, or mustard, when cultivated and then incorporated into the soil, contribute organic material to it.
Vegetation cover: Leaving plant debris on the soil surface increases the organic matter content of the soil, thereby enhancing its water retention capability.
Soil management
Avoiding congestion: frequent fieldwork and operating heavy equipment on the soil can cause compaction, reducing water capacity. Minimize driving over the soil, particularly when it is wet.
Deep cultivation: can help break up hardened soil layers, increasing water permeability and benefiting root development.
Promoting earthworm activity: introduction of reduced tillage methods, enabling earthworms to naturally aerate the soil and enhance its water accessibility.
Summary
Soil water capacity refers to the amount of water that soil can store and make available to plants. It is a key parameter that determines the soil's ability to support vegetation. Enhancing soil water capacity not only increases the ability to support plants during droughts but also improves soil quality, plant health, and potential productivity.
Terminology
Soil pores - are voids between soil particles, channels, and cracks.
Macropores - defined as large pores, are those exceeding 30 micrometers (μm).
Mesopores – medium-sized pores - 0.2 - 30 micrometers (μm).
Micropores - the smallest pores, are less than 0.2 micrometers (μm).
μm – a unit of measurement, micrometer, one thousandth of a millimeter.
