Soil Moisture

Water contained in soil is called soil moisture. The water is held within the soil pores. Soil water is the major component of the soil in relation to plant growth. If the moisture content of a soil is optimum for plant growth, plants can readily absorb soil water. Not all the water, held in soil, is available to plants. Much of water remains in the soil as a thin film. Soil water dissolves salts and makes up the soil solution, which is important as medium for supply of nutrients to growing plants.

Importance of Soil Water

1. Soil water serves as a solvent and carrier of food nutrients for plant growth
2. 
   
   Yield of crop is more often determined by the amount of water available rather than the deficiency of other food nutrients
3. 
   
   Soil water acts as a nutrient itself
4. 
   
   Soil water regulates soil temperature
5. 
   
   Soil forming processes and weathering depend on water
6. 
   
   Microorganisms require water for their metabolic activities
7. 
   
   Soil water helps in chemical and biological activities of soil
8. 
   
   It is a principal constituent of the growing plant
9. 
   
   Water is essential for photosynthesis

Retention of Water by Soil: The soils hold water (moisture) due to their colloidal properties and aggregation qualities. The water is held on the surface of the colloids and other particles and in the pores. The forces responsible for retention of water in the soil after the drainage has stopped are due to surface tension and surface attraction and are called surface moisture tension. This refers to the energy concept in moisture retention relationships. The force with which water is held is also termed as suction. 

Measurement Of Soil Moisture

There are two ways to measure soil moisture:

1 Soil Moisture Tension and
2 Soil Moisture Content

After rain or irrigation, the pores in soil fill up with water.

Imagine you have a container of soil. Add water to it until it is full.

Now imagine you can take all the pieces of soil out, leaving just the water. This is the maximum amount of water that this soil can ever hold, this is a super saturated or waterlogged sample of soil.

Waterlogged soil is not good for plant growth as there is no air or oxygen in the pore spaces for the plant roots to extract and use. Roots will rapidly die in waterlogged soils.

Thankfully, gravity causes excess water to slowly drain away through the profile and end up in the water table. As the water drains, air (including oxygen) is pulled into the pore spaces, creating a good environment for roots.

Not all of the water will drain away, a large amount of it will stay in the soil:

• In the pore spaces, and
• As a thin coating on the outside of soil particles

After the excess water has drained away, the soil is said to be at FIELD CAPACITY. This is usually 2-3 days after the wetting event.

The water in the pores is easy for the plant roots to get to and use. The water that coats the soil particles may be tightly bound and the plant roots may not be able to extract it. So once the pore water is used up, even though the soil is still wet, plants may not be able to access the water and will start to wilt. The amount of water the plant can successfully extract from the soil is called the PLANT AVAILABLE WATER.

So how do you go about working out how much moisture is in your soil and how much the plant can actually use? There are two ways to go about measuring soil moisture:

• Soil Moisture Content - the amount of water in the soil (usually described as a percentage)
• Soil Moisture Tension - how hard the plant root has to work to extract water from the soil

How do you measure soil moisture?

There are two ways to measure soil moisture:

1 Soil Moisture Tension and
2 Soil Moisture Content

Soil moisture tension
Soil moisture tension tells you how easy it is to extract water from soil. When a soil is saturated, there is plenty of water in the pore spaces and plenty of water coating the soil particles. All this moisture makes it very easy for plant roots to get water and the soil moisture tension is low.

Imagine sucking on an ice slushy - when you first start slurping, it's easy to get a drink. The more you drink the harder it is to get liquid from the ice and the harder you have to suck - it's the same for the plant root, as water in the pores decreases, the suction or tension that the roots need to apply to get moisture increases.

When soil tension reaches a certain threshold, the plant can no longer extract water from the soil even if there is water present, it is stuck to the soil particles and they won't let it go. This water is unavailable to the roots. At this point the plant will become stressed, begin to wilt and will eventually die if water is not replenished.

Soil moisture content

Soil moisture content tells you how much water is in the soil - usually as a percentage - representing what percentage of total 'volume' of soil is moisture.

Imagine a cubic metre of topsoil. Pull out all the soil particles and compact them to remove all gaps between them (suppose it squashes down to about 40% of the original volume). Do the same for the organic matter - this would occupy about 5% of the volume. What is left? The rest of the volume is made up of pore spaces which can be occupied by either air or water.

So, in a totally saturated sample of this soil, the water component would be 55% of the original cubic meter - the rest is the soil.

Given that the soil holds onto a layer of water that is inaccessible to plants, the value of "dry" soil when roots can not get any more moisture and plants become stressed, wilt and die will not be 0% but something slightly more.

Soil Moisture Tension in the Field

Measurement UnitsYou will hear both k Pa and negative 'something' k Pa bandied around in reference to Soil Moisture Tension.

Kilo Pascals (k Pa) are units of pressure measurements. Suction is a negative pressure which is also referred to as a tension.

Soil Moisture Tension is a measure of suction, and the correct way to refer to it is minus or negative X k Pa. However, it is quite common for the minus to be dropped.

It can be a little counter intuitive when thinking of wet soil vs dry soil with soil moisture tension as smaller numbers that are closer to zero don't mean less water. In fact these low numbers indicate more water, wetter soils. They show how much suction is needed to extract water from the soil. So small numbers, mean it's easy for the plant roots to get a drink. As a soil dries out the kPa values becomes larger (and more negative) and the hard it is to extract water.

Effect of Soil Type
One great thing about measuring in Soil Moisture Tension is that soil type is largely irrelevant. -25kPa in clay is the same as -25kPa in sand. Plants in these conditions in either of these soils are working exactly the same to extra moisture from the soil.

Irrigation Scheduling
Permanent crops, such as tree crops and vines are relatively forgiving to irrigation practices - they have extensive root zones, which can access large volumes of stored water; and episodes of water stress may damage the current year's crop or reduce the following season's fruit set but rarely lead to loss of the plants.

The picture for seasonal crops is much more critical - a single episode of water stress can lead to the immediate and complete loss of the plants.

Whilst tree crops may be happy extracting water to -60 k Pa, most of the annuals can only extract to levels of -20 or -30 k Pa.

A bit of research on the internet or a chat to your agronomists can tell you which k Pa level you should be stressing your crop type to, before irrigation is required. You can also do this by ground truthing, digging in the soil and watching crop water stress responses.

Which sensors measure Soil Moisture Tension?
1 Gypsum blocks
2 Tensiometers

Soil Moisture content in The field

Measurement Units
Volumetric Soil Moisture Content refers to the volume of water in a given volume of soil as discussed on previous pages. This type of Soil moisture Content is measured in m3.m-3 (so how much of a cubic metre is water out of the entire cubic meter of soil sample). Alternately, the Volumetric Soil Moisture content can be referred to as a % of volume (which is much easier).

Effect of Soil Type
Soil moisture content is very dependent on soil type. A saturated coarse, sandy soil can hold far less water than a saturated heavy silty clay. Sand has large particles which take up a lot of physical space. Also, as sand particles do not bind water, a lot of water will drain out of the sand due to gravity before field capacity is reached. For these two reasons, sand has a much lower maximum and minimum water content than a clay soil does.

What this means for you is that if you are monitoring a number of sites across your property, the soil moisture content values should not be compared between paddocks with different types of soil. Reading 20% moisture in one paddock and 20% in another paddock does not mean that the plants will be equally happy.

In sand, plants in 20% moisture will be very happy as sand readily releases its moisture and the most sand can hold is around 30%.

However, in clay, a plant in 20% moisture will be extremely stressed. Clay soils often have a maximum moisture reading of 50% or more, so 20% is very dry. The clay particles also bind water to themselves and at low moisture contents like 20%, the clay will not give the water up for the roots to use.

It is therefore important, for each site need to be considered individually!

Irrigation Scheduling
When you start out, you have to learn what the various content values mean for your soil. What is the maximum reading you are getting after heavy rain? As the soil dries out, compare the values you are getting to how the soil looks when you dig around a bit and to how water stressed the plants are. What is the minimum reading you are getting in very dry soils at the end of summer when you've not irrigated for awhile? All of this information will give you a good start on working out at which soil moisture % you need to start irrigating and when you should stop.

Which sensors measure Soil Moisture Content?
Measures soil moisture using Capacitance (Frequency Domain Reflectometry):

• EnviroPro Probe

Measures soil moisture using TDR (Time Domain Reflectrometry) and TDR-like technology:

• Aqua flex
• Theta Probe
• Profile Probe
• WET Sensor