Soil-Water
When it comes to water, soils act like batteries, storing water between rain or irrigation events. This allows plants to have access to water during extended periods of no precipitation. Soil water is reduced over time by evaporation, transpiration, and deep percolation (draining through deeper soil layers). This means that the amount of water available in soils depends on how much water is received by the soil, what the soil's capacity to hold on to the water is, and how fast the water is lost from the soil.
The amount of water that soil can hold is known as the field capacity, while the amount of water in the soil that is bound by soil particles and unavailable to plants is known as the wilting point. The difference between the field capacity and wilting point is the amount of water available to crops. There are several forces that influence the amount of water that a soil can store and how fast it is lost from the soil:
- The solid phase (rock and mineral fragments, secondary minerals, and organic materials)
- Dissolved salts
- External gas pressure
- Gravitation
The majority of factors influencing the water holding capacity of soils are out of a farmer's control. One of these factors is the underlying soil type of his fields. Finer soils, like clays, can hold more water than coarser soils (like sand) due to their smaller particle sizes, which result in a greater particle surface area for water to cling onto. The graph below shows the effect that soil types (and their associated particle sizes) have on their ability to hold water.
A farmer needs to find ways to farm as profitably and productively as possible on the soil types found on his farm. A farmer can influence this to a certain degree and improve the water holding capacity of soils by increasing the soil organic matter (SOM) levels. Soil organic matter and water holding capacity show a strong positive correlation. An increase in SOM is associated with an increase in the ability of a soil to store moisture. This is due to water's high affinity to stick to decomposed organic matter. Increased SOM, not only helps the soil store more water, but it also increases the infiltration rate. Increased infiltration allows for better absorption of rains and irrigation water into the soil leading to reduced runoff and erosion. The figure below illustrates the relationship between soil organic matter and the available water holding capacity of a soil (Blignaut et al., 2015).
One sure-fire way of increasing soil organic matter is by implementing regenerative agricultural practices. The following practices have been shown to have a positive effect on SOM in South Africa:
- Summer and winter cover crops
- No-till or minimal tillage
- Manure and compost applications
Each one of these practices contribute to increased SOM levels in different ways. Practices like cover cropping and organic matter applications increase the SOM by introducing more organic matter to the soil. A portion of the introduced organic matter will oxidize, while the rests will be incorporated into the soil as humus or will become food for soil microbes and critters.
On the other hand, practices like no-till or reduced tillage reduce the rate at which organic matter is lost from the soil. Conventional tillage practices not only incorporate crop residue into the soil, but also expose organic matter to oxygen, leading to oxidation and loss of soil organic matter. During tillage, the top soil layers (where the majority of SOM is found), gets disturbed the most. By reducing tillage intensities, less oxygen is incorporated into the soil, leading to lower rates of SOM oxidation.
By implementing these practices, farmers can increase the water-holding capacity of their soils and the infiltration rate. This is beneficial in both wet and dry seasons. In wet seasons, the increased infiltration rate will ensure that the top soil is not washed away, while in dry seasons, the improved water holding capacity will lead to more water being available to crops between rain events.