Welcome to Hydrogold's Soil Moisture Concepts page. Others may call this Soil-Water-Plant Relationships.
This basic knowledge of Soil Moisture Concepts is essential to managing the water in the soil. Soil Moisture is poorly understood since it occurs underground, away from our eyes. However, its impact is seen on the surface with poor quality plants and water-logged soils. Understanding Soil Moisture Concepts allows us to irrigate efficiently and have effective drainage systems with healthy turf/plants. Misconceptions result in failed projects and wasted effort ($$$).
I acknowledge the time and effort that Laszlo Vranyak has put in producing most of the great graphics for this article.
|TABLE OF CONTENTS|
|Basic Terms||These are the terms you need to understand to learn about Soil Moisture.|
BASIC TERMS AND CONCEPTS
These terms and an intuitive understanding of them are fundamental to learning about Soil Moisture. Without knowing these terms, it is difficult to communicate.
Soil consists of solid particles and pores (voids). These pores can be filled with air or water. When discussing Soil Moisture, we often refer to the following three defined levels of Soil Moisture:
|Saturated||Field Capacity (FC)||Permanent Wilting Point(PWP)|
Water fills all the pore space (Large, Medium and Small Pores)
This is the maximum water holding capacity of the soil. Water movement can be relatively quick in saturated soils (normally downward by gravity unless constrained).
Field Capacity (FC)
The water held in the Large Pores (Gravitational Water) has drained away.
Water is held in the Medium and Small Pores.
This is the Soil Moisture level that is left after draining by gravity (say 30 minutes to 24 hours depending on the soil). The water remaining has sufficiently bonded (adhered) to the soil that it no longer drains.
The water in the soil does not drain (in practical terms) when the Soil Moisture is at or below Field Capacity. Water remains in the soil and is available to the plants.
Permanent Wilting Point(PWP)
The water in the Medium Pores (Plant Available Water) has gone. Typically this is by Evaporation from the ground's surface and Transpiration by the Plants (Evapotranspiration).
Water is held in the Small Pores.
This is the Soil Moisture level at which the plant cannot extract any water from the soil. The water tightly adheres to the soil particles. The plant will wilt (lose turgidity) and not recover (die). This level is different for different species of plants.
This is the water that drains (from the Large Pores) from the Saturated soil until the soil is at Field Capacity.
It is the difference between Saturation and Field Capacity.
Plant Available Water (PAW)
This is the water (from the Medium Pores) that is available to the plant.
It is the difference between Field Capacity and Permanent Wilting Point.
Plant Unavailable Water
This is the water (in the Small Pores) that is unavailable to the plant.
It is the water content at the Permanent Wilting Point.
In soils, this is the movement of water through the pores in the soil.
So the terms you should now be familiar with are:
- Saturated Soil
- Field Capacity (FC)
- Permanent Wilting Point (PWP)
- Gravitational Water
- Plant Available Water (PAW)
- Plant Unavailable Water
MEASURING SOIL MOISTURE
For the purposes of this article, we only consider Volumetric Water Content (%). There is another way of measuring Soil Moisture (potential) but Volumetic Water Content (%) is the most appropriate for the purposes of this article.
Again, Soil consists of solid particles and pores (voids). These pores can be filled with air or water
|Example No. 1. Let's look at a Saturated soil:|
|Saturated Soil||=||Dry Soil||+||Water||Equivalent Depth
The equivalent depth of water (40 mm)
is divided by the depth of soil (100 mm)
then multiplied by 100%
to calculate the Volumetric Water Content of 40% (= 40 mm / 100 mm * 100%)
|Example No. 2. Let's look at Field Capacity:|
|Field Capacity||=||Dry Soil||+||Water||Equivalent Depth
The equivalent depth of water (20 mm)
is divided by the depth of soil (100 mm)
then multiplied by 100%
to calculate the Volumetric Water Content of 20% (= 20 mm / 100 mm * 100%)
|Example No. 3. Let's look at Field Capacity again but only 50 mm deep:|
|Field Capacity||=||Dry Soil||+||Water||Equivalent Depth
The equivalent depth of water (10 mm)
is divided by the depth of soil (50 mm)
then multiplied by 100%
to calculate the Volumetric Water Content of 20% (= 10 mm / 50 mm * 100%)
An important note. While the soil moisture content (20%) is the same as Example No. 2, the equivalent depth of water is only 10 mm (compared to 20 mm in Example No. 2). This demonstrates the importance of a deep root zone to provide access to water for the plant. It allows for less frequent irrigation and helps to convserve water.
Here we learn about Sand, Clay and Silt and properties of soil in relation to water.
Soil Particle Classification
According to ISO 14688-1, soil particles are classified as:
|SOIL PARTICLE CLASSIFICATION|
|Very Coarse Soil||Large Boulder||630|
|Coarse Soil||Gravel||Course Gravel||20--63|
|Fine Soil||Silt||Course Silt||0.020--0.063|
Soil Texture Classification
Soils are not homogeneous; they are mixtures of the above classes of soil particles. The following Soil Texture Triangle defines the 12 major textural classes of soil:
Getting an Indication of Soil Texture
Technically, to know the soil texture you submit soil samples to a laboratory for a soil particle analysis. This is the reliable and accurate way of doing it.
But you can get some idea but using the following technique:
- Mix a soil sample with water in a clear jar
- After a minute, a layer of solids will have formed. Mark the top of that layer, the sand layer.
- After 2 hours, another layer of solids will have formed. Mark the top of that layer, the silt layer.
- After 24 hours, the final layer of solids will have formed (with clear water above). Mark the top of that layer, the clay layer.
- Measure the depth of each layer and you can then determine the soil texture.
Typical Water Holding Capacity of Soils used for Turf
For turf, we normally have a soil that ranges from Sandy Loam to Clay Loam.
* If the soil has more sand than Sandy Loam, the soil will not retain sufficient water (fast drainage - high percolation)
* If the soil has more clay or silt than Clay Loam, it will not drain properly (slow drainage - low percolation)
The following diagram show the typical range of water holding capacity of soils used for turf. Soils outside these ranges typically present problems with either too fast to too slow drainage of water through the soil profile.
Or by Soil Texture...
Typical Water Holding Capacity of Soils
First off, you will find many of these tables, usually within the indicative range of the following table (from the Australian Department of Agriculture bulletin No. 462 in 1960). You may also notice a discrepancy between the following table and the preceding diagram (if you are still awake). My point: these table are indicative and useful for education. For real-life applications, you need to have your particular soil tested. Organic content in the soil will also have an impact.
We must teach the simple before we can understand the more complex. If we keep teaching the simple, it never becomes complex. Lao Tsu: "A journey of a thousand miles began with a single step."
|TYPICAL WATER HOLDING CAPACITY OF SOILS|
|Soil Texture||Field Capacity||Permanent Wilting Point (PWP)||Plant Available Water (PAW)|
|Light Sandy Clay Loam||23%||10%||13%|
|Sandy Clay Loam||28%||13%||15%|
Typical Basic Infiltration Rates of Soils
Soils normally absorb water quickly (Initial Infiltration Rate) but the infiltration rate slows with time to the Basic Infiltration Rate. See the graph in the next section. The following table provides indicative rates. Actual rates may be affected by organics and the chemical composition of the soils (e.g. "Hydrophobic Soils"). The infiltration rates for your particular soils should be measured (in situ) using an infiltrometer.
|TYPICAL BASIC INFILTRATION RATES OF SOILS|
|Texture||Basic Infiltration Rate
Range in mm/h
Infiltration Rates and Irrigation
The rate at which water infiltrates the soil is particularly important for irrigation. For irrigation efficiency, the application rate of the irrigation should not exceed the infiltration rate of the soil (to avoid ponding or run-off). The table below shows a sprinkler (with a constant 32.5 mm/h application rate) along with the infiltration curves for 3 different soils.
- The sprinkler can irrigate for 1.25 hours before it exceeds the Infiltration Rate of the Sandy Loam. That is, an application of 40.63 mm (= 32.5 mm/h * 1.25 h).
- The sprinkler can irrigate for 0.30 hours before it exceeds the Infiltration Rate of the Loam. That is, an application of 9.75 mm (= 32.5 mm/h * 0.30 h). If we needed to apply more than 9.75 mm, we would need to allow that water to soak in and then re-run the cycle (cycle and soak).
- The precipitation rate of the spinkler (32.5 mm/h) exceeds the Initial (Maximum) Infiltration Rate of the Clay Loam (20 mm/h). That is, this sprinkler is totally unsuitable for irrigating Clay Loam.
Slopes should be irrigated at a lower application rate (or using cycle and soak programming) to avoid run-off. e.g. A 1:8 (12%) slope should be irrigated at half the infiltration rated in the preceding section. If there is no vegetation coverage (e.g. at planting), the application rate should be halved again.
These are based on a good draining soil profile.
|The "Perfect" Irrigation Cycle||Over Irrigation
The "Perfect" Irrigation Cycle
The irrigation water applied is just sufficient to leave the root zone at field capacity. The wetting front does not pass the root zone. All irrigation water is available to the plant. No water is lost "wasted" below the root zone. While this may be considered "Perfect", excess salts may build in the soil profile. Salt levels should be tested regularly.
Too much water is applied. The wetting front moves past the root zone. The water below the root zone is not available to the plant and is "wasted".
An excessive volume is deliberately applied. The thick wetting front moves past the root zone carrying the solubles (e.g. some salts and some fertilisers) below the root zone. This lowers the salt content in the soil within the root zone. It can also leach some fertilisers into the underlying water table.
The Importance of Deep Root Zones
Deep root zones require less frequent but deeper irrigation cycles. This increases the oxygen into the soil, reduces evaporation from the surface and in turn promotes deeper root zones. The less frequent the irrigation, the more likely it is that rainfall will save some irrigation water (and the electricity to pump it). Deeper root zones also promote healthier plants.
The following table shows that the deeper the root zone, the longer the interval between irrigation cycles. We look at 3 different depths of root zone. This is based on a Maximum Allowable Depletion (MAD) of 50%.
Maximum Allowable Depletion
... is the maximum amount of depletion of Plant Available Water (PAW) that can occur without stress to the plant. In classical irrigation education, this is taken to be 50% (as per the example below). In practice, it is dependent on the type of plant and soil.
Management Allowable Depletion
... is the fraction of Plant Available Water(PAW) that is to be depleted from the root zone before irrigation is applied. It is dependent on the type of plant and soil. Typically in the range:
- 30% (for weak or shallow-rooted plants) - Less oxygen in the soil - More water lost to evaporation
- to 70% (for healthy or deep-rooted plants) - Better water/oxygen mix in the soil - Less water lost to evaporation
What is the difference between Maximum Allowable Depletion, the historical term, and the more modern Management Allowable Depletion? Largely semantics. Management Allowable Depletion more correctly reflects the fact that the depletion of Plant Available Water (PAW) is a managment decision (based on plant, soil, aesthetics, cultural practices, water supply, budgets...) and not an arbitrarily determined 50%.
Classical EvapoTranspiration Irrigation
Traditionally, if 6 mm a day of water is lost to EvapoTranspiration, then a 6 mm application of water would be applied that night to replace the water lost. This approach (daily irrigation) is keeping the water in the root zone at Field Capacity most of the time reducing the amount of oxygen in the soil and maximising the water lost to evaporation from the soil surface.
Classical Soil Moisture Irrigation
Referring to the above table and using a 50% Maximum Allowable Depletion (MAD), we irrigate each second day. This provides a better oxygen mix in the soil, reduces water lost to evaporation from the soil surface. It also encourages the plant to develop a deeper root system.
Over-Watering (with leaching)
If we do not know what the Soil Moisture Content of the soil is, we may over-irrigate and loose water by deep percolation (leaching below the root zone). Sometimes this may be done deliberately to flush salts and other solubles from the root zone.
Stressing / Deep Watering
In this example, we allow the water to be depleted more than 50% Maximum Allowable Depletion. This is where the term Management Allowable Depletion is more appropriate. It reflects a deliberate management decision based on factors such as plant, soil, aesthetics, cultural practices, water supply and budgets.
Implementing a Deep Watering Program - An Example
Assuming we have an 18-hole golf course and want to water every second night during the dry season. It would be logical to irrigate the front 9 holes one night and the back 9 holes the next night. But this option may not be safe...
|How to Schedule a Deep-Watering Program for Your 18-Hole Golf Course|
|1. How the Irrigation System was Designed This is probably how the irrigation system was designed to operate - watering distributed around the golf course with a balanced load on the irrigation system.||2. How NOT to Implement Deep-Watering With implementing a "Every Second Day" Deep-Watering program, it might seem easy to water the front 9 holes one night and the back 9 holes the next. This places an unbalanced load on the irrigation system. It is unlikely that the irrigation system was designed for this situation due to budget and hydraulic limitations. This would result in low operating pressure at the sprinklers and potentially damaging the irrigation system with high velocity water in the mainline pipes.|
|The Right Way to Implement a Deep-Watering Program|
|3. Water the Odd-Numbered Holes One Night By watering the odd-numbered holes on the first night, we still have a balanced load on the irrigation system.||4. Water the Even-Numbered Holes the Next Night The even-numbered holes are irrigated on the second night with a balanced load on the irrigation system.|
WEATHER STATIONS versus SOIL MOISUTRE
Traditionally we have used Weather Stations to determine the amount of water we apply in an irrigation cycle. There is now a strong swing towards using Soil Moisture Sensors.
What is Wrong with Weather Stations?
Weather stations tell you what is going on above ground while the Soil Moisture Sensor tells you what is going on underground. Weather stations calculate the water that has (potentially) been lost by EvapoTranspiration from a range of weather parameters (typically, Temperature, Relative Humidity, Wind Speed and Solar Radiation). It is a theoretical calculation and does not tell you how much water is left in the root zone (underground water tank). To properly manage irrigation, we need to know the actual Soil Moisture Content (underground).
Weather stations are often not installed in a location that is representative of the turf. Then there is the high cost of regular maintenance (including the cost of transport to/from the nearest service centre) and damage by lightning. The end result is that after 2 years, 50% of weather stations in Asia are not operating. Too often an expensive white elephant.
Woud You Drive Your Car Without a Fuel Guage?
Why Would You Irrigate Your Plants Without a Soil Moisture Sensor?
|Your Speedometer is like the Weather Station (WS) - Telling you how fast you are using Petrol (Water).
Your Fuel Guage is like the Soil Moisture Sensor (SMS) - Telling you how much Petrol (Water) is in the tank.
Speedo (WS) says add a 1/2 tank (overfill).
Fuel Gauge (SMS) says not to.
Speedo (WS) says add a 1/2 tank.
Fuel Gauge (SMS) says not to.
Speedo (WS) says add a 1/2 tank.
Fuel Gauge (SMS) says add 3/4 tank.
Speedo (WS) says add a 1/4 tank (half the speed).
Fuel Gauge (SMS) says add 3/4 tank.
If Soil Moisture Sensors are So Good, Why Haven't We Been Using Them?
Because Soil Moisture Sensors have been:
- Short Life
- Too Expensive
- High Maintenance
- Not Integrated with the Irrigation Control System
We have been able to get by using weather stations and the Superintendent's eyes.
So What has Changed?
- Rapid development of Soil Moisture Technology - Sensors are now reliable, longer life, cheaper and lower maintenance.
- Sensors now can measure salinity and temperature as well which is important in Soil Moisture management.
- With sloth-like speed, the monolithic irrigation manufacturers have integrated Soil Moisture Sensors with their Irrigation Control Systems (finally!).
- Higher water demand and limited water resources in the community
- Government regulations and water quotas
- More focus on sustainability (both environmental and financial) means we must do better
- It is reasonable to expect a 20 to 25% saving of water and electricity (for most projects) along with more consistent turf quality
MOVEMENT OF WATER
An intuitive understanding of these fundamentals is the key to predicting the movement of water. The failure to understand these concepts results in the widespred misconceptions in the industry and the many failures we see in drainage systems, horticulture, irrigation, etc.
The biggest misconception with water movement in soil is that water will move sideways in soil at a reasonable rate. The truth is water moves sideways in soil extremely slowly and can be ignored in most (but not all) situations. Understanding the balance between each of these 4 forces will help you understand that.
|The 4 Forces that Move Water:
It is the balance of these 4 forces (more technically, potential) that determines the direction that water will move (or not move if they are in balance).
|Water moves from high to lower elevations.||Water moves from high to lower pressure areas.||
Water moves from areas of low concentration to areas of high concentration (to dilute the higher concentrated water).
The reversal of this process (reverse osmosis) is used to purify water.
Water moves from large pores to small pores (Capillary).
Water moves from wet areas to dry areas.
PERCHED WATER TABLE
The Perched Water Table is a poorly understood concept since it occurs underground, away from our eyes. And it defies our normal perception that water will move downwards with gravity.
What is a Perched Water Table?
Definition:A Perched Water Table is unconfined ground water separated from an underlying body of ground water by unsaturated soil or impervious material (e.g. rock).
In sports turf, our main focus is on the first part of the defintion. That is, "Unconfined ground water separated from an underlying body of ground water by unsaturated soil".
This is created by having a soil layer (small pores) overlaying a gravel layer (large pores).
The most well-known specification is produced by the United States Green Association (USGA). This is a good solution but not the only one. However, when straying from their specifications, you really need to understand the fundamentals of Soil Moisture.
This graphic is of a typical Perched Water Table used in sports turf. Note the Perched Water Table sitting above the (unsaturated) gravel layer. The underlying body of ("natural") ground water is beneath the consolidated base and not shown. This situation can occur naturally but in this article we are focussed on the man-made structures (though the same concepts apply in both cases).
It also occurs in situtations normally not associated with Perched Water Tables such as drainage systems and planter boxes. Failure to understand this results in the many failed drainage systems we see.
The Perched Water Table does not move down by gravity because it is opposed by the "Matric Potential"; refer to the "Forces that Move Water" above. That is, the water wants to move from large pores (gravel) to small pores (sand), in opposition to gravity. This is commonly called "Capillary Action".
|Soil Moisture Distribution in a Perched Water Table|
You may expect that after irrigation the entire profile is left at Field Capacity.
But this is not the case
(in all but an exceptional situation)!
The water in the saturated zone (the "Perched Water Table") is subject to two opposing forces (refer to the "Forces that Move Water" above). That is, Gravity pulling the water down and the Matric Potential holding the water up (the water wants to move from the large pores of the gravel to the small pores of the sand). Above the saturated layer, the Soil Moisture Content decreases so that the Soil Moisture at the suface (ideally) is at Field Capacity (or just below).
The selection of the soil (and its depth) is most important:
Too fine (or too shallow) and the saturated layer will extend to the surface creating soggy ground; or planter boxes that never drain because the water never reaches the drainage pipe.
Too coarse (or too deep) and the soil will not retain sufficient water and the plants will remain in a droughty condition.
What are the Benefits of a Perched Water Table?
A properly constructed Perched Water Table retains water for the plants while offering excellent drainage. In most soil situations, we can have either Soil Moisture Retention or Good Drainage but not both. e.g. Clay retains water but provides soggy ground. Sand drains well but does not retain moisture.
The Perched Water Table uniquely offers us Soil Moisture Retention and Excellent Drainage.
What is the Problem with Perched Water Tables?
The primary problem is cost. Construction takes time and sources of the correct sand mix will be expensive in many areas. So the construction of Perched Water Tables is reserved for special situations. Classically this has been most replicated on golf course greens but has been applied to many other sports turf applications. e.g. Racing tracks, bowling greens, crochet lawns and the like. While your local soccer field will not use it, high profile sports fields will spend millions to use it.
How Does it Works?
|The Mechanism of a Perched Water Table|
|1. The soil profile after irrigation.||2. Water is added (say by rainfall). This puts additional pressure on the water at the sand/gravel interface.||3. Water flows down as the gravitational force is greater than the matric force.||4. Water continues to flow down.|
|5. Water stops flowing from the Perched Water Table as the Gravtity and Matric forces again reach balance (equilibrium). The wetting front continues moving down through the gravel layer.||6. Water continues to flow through the gravel.||7. Water has drained from the gravel leaving the water distribution in the sand as it was before the rainfall. With the correct selection of the sand, this occurs rapidly.||8. The Animated Version.|
Problems with Perched Water Tables
Problems commonly arise from selection of the sand for the sand layer. There are ranges of acceptable sands (the USGA has set good standards) but cost considerations often lead to compromises. Sometimes, problems arise just out of plain ignorance; that is, not knowing/understanding the principles explained in this article.
By using the wrong combination of sand type and depth for the sand layer, we can end up with disasters. If the sand is too coarse or too deep, there will be too little water retained in the root zone. The plant will be of droughty appearance even though it is likely we will be using more water. If the soil is too fine or too shallow, we will end up with a water-logged profile to the surface; highly undesirable for the plants (anearobic environment) and for people using the turf (golfers, players).
Maintenance is also an issue. If organics are allowed to build up in the soil, this will increase the height of the water above the gravel to the point the surface is saturated. Chemicals introduced to the soil may also have an impact.
Soil Moisture Rentention Curves
Now the article is getting quite technical. So for readers who don't want to take the time to understand this fully, perhaps it is time to call in an expert.
One of the prime factors to consider for the sand is the "Soil Moisture Retention Curve" (alternatively called the "Soil Moisture Characteristic"). Technically there is a Soil Moisture Retention Curve and a Soil Moisture Release Curve but this distinction is beyond the scope of this article.
These are typical Soil Moisture Retention Curves for some different soil types.
The horizontal axis shows the Volumetric Soil Moisture Content (as a %).
The vertical axis on the left shows the Soil Matric Potential in Bars. This is another way (more technical and useful for scientists) of measuring Soil Moisture.
The vertical axis on the right is converted into metres (m) of water which is more relevant to our world.
Logarithmic Scale. An important aspect of both vertical axes is that they are logarithmic, something that scientists do for reasons of scaling but we do not see regularly. So you see the scale on the right axis goes from 0.01 m to 1.00 to 100.00 to 10,000.00 m. Scientists may be interested in the 10,000 m (10 km) part but as turf and landscape people, we are mostly interested in the first metre. So we take a look at that in the next graphic.
Side Note on logarithmic scales: Our most commonly seen logarithmic scale is the Richter scale for measuring earthquakes where a Richter 7 earthquake is 10 times more severe that a Richter 6 earthquake. Another is the pH scale for water. A pH of 3 is 10 times more acidic than a pH of 4. pH 7 is neutral. A pH of 9 is 10 times more alkaline that a pH of 8.
Now we look at the range that most interests the turf and landscape industries; from 0.01 to 1.00 m (10 to 1,000 mm).
Looking at the orange Sand curve, we see that 0.10 m (100 mm) above the gravel, the sand is saturated (37% Soil Moisture). At 0.20 m (200 mm) above the gravel, the Soil Moisture is 27%. At 0.30 m (300 mm) the Soil Moisture is 22%. 300 mm is the typical depth of the sand layer in a USGA specification golf green. This sand on the borderline of being acceptable or it may be acceptable if used with a 400 mm sand layer (at which the surface Soil Moisture would be 18%).
Looking at the red Clay curve, we see that at 0.10 m (100 mm) above the gravel, the clay is saturated (45% Soil Moisture). At 0.3 m (300 mm), the Soil Moisture is 43%. Even at 0.5 m (500 mm), the Soil Moisture is 42%, almost saturated. Therefore a planter box 0.5 m (500 mm) deep with a gravel layer underneath will not effectivly drain. This is a common problem with planter boxes, particularly in Asia with it heavy soils and scarcity of sand.
In summary, you can see the assessment of the sand used (and the its depth) is critical to the Soil Moisture at the surface. You really need a professional assessment of this but it is also good to understand these concepts yourself.
Perched Water Tables Created by an Impervious Layer
Refer back to the definition of a Perched Water Table:
"A Perched Water Table is unconfined ground water separated from an underlying body of ground water by unsaturated soil or impervious material."
While the focus of this article is on the first part of the definition (that is, a Perched Water Table created by an unsaturated layer), the second part is also important.
A perched water table may be created by a thin layer of organic matter or clay. Until the soil above the layer is saturated, the water may not move deeper into the soil. All of the salts may build above this layer and cause problems for the plants unless excess irrigation water is periodically applied or a saving rainfall occurs.
IMPACT OF SALT ON SOIL MOISTURE
Salt has a significant impact on the movement of water in the soil and the health of the plant. It should be tested regularly in the field (few do) and core samples examined. While the plants may look healthy, a problem may be developing underground. Often testing is not done until a problem is seen. Regular testing will "head off" damage while it can be avoided.
Proverb: "A stitch in time saves nine".
The Wilting Plant
When the Soil Moisture is low, plants wilt. That is, the water is depleted in their cells and the plants lose their turgidity (vigour). This is the plants "gasping" for the last bit of water from the soil.
Review the 4 forces that move water:
What causes the water to move (against gravity) from the soil up the stem of the plant? It is the matric potential, the water wanting to move from large to small pores ("Capillary Action" as taught in primary school).
Remember, it is the balance of the 4 forces that will determine the movement of the water.
- When the plants wilt, water has been depleted from the cells.
- The salts in the plant remain within the cells.
- So the salts within the plant's cells become a higher concentration as the plant wilts.
- Therefore the Osmotic potential is also increasing the potential of the water to move from the soil to the plant
- This is the plant's way of "gasping" for the last bit of water it can get from the soil
- This often happens on a daily basis during hot, dry afternoons - "Transient Wilting"
- If the Soil Moisture drops too low, the plant will not be able to recover - the "Permanent Wilting Point"
However, if salts have accumulated in the soil, then this will increase the potential of the water to move from the plant to the soil. That is, when you have high salt content in the soil, the plants cannot extract as much water. That is, there is less water available to the plant than if the soil had a low salt content. This emphasises the importance of managing the salt content of your soil. Before you can manage the salt in your soil, you need to know what it is.
So if you are watering with high-salt water and/or not leaching the salt from the soil profile, this is likely to be part of your turf management problems.
Where to start? The best way is to get a portable Soil Moisture Sensor and start using it.
In 2007, 10 years after Hydrogold designed the irrigation system at the Singapore Turf Club at Kranji, John Pryor is back on site demonstrating a portable Soil Moisture sensor to Jayaraju.
This type of sensor is a great start. You can quickly sample Soil Moisture and start understanding what levels occur at different points and times. One drawback with this is you are only sampling the Soil Moisture in the first 75 mm (3"). But it is a start! You will learn things you may not have expected about your Soil Moisture.
Our advice is that you purchase a high quality Soil Moisture Sensor for accurate, reliable and consistent readings. Nowadays (2013), that means about US$ 2,000 (indicative figure only). Get one that you can take the reading while standing; bending down is a pain and discourages using it). One that logs with a GPS location is really nice.
Then we can start planning to install sensors at permanent locations and integrating them with the Irrigation Control System... (contact Hydrogold).
At the end of the day, the plant is the ultimate Soil Moisture indicator. The Irrigation Superintendent must use not only his eyes, but the Soil Moisture technology to extend his view below the surface. The age-old method of taking core samples still provides valuable information. Using these inputs and the understanding of the Soil Moisture Concepts presented here, we will achieve better plant health with optimum use of resources (including water, labour, electricity, fertiliser, etc). Irrigation and drainage system will work better.
I have not created this knowledge (I present it) and have relied on my education by more knowledgble people (as always). There are a few good texts written on the subject. They take longer to read, time to understand and more time to apply to see the results (successes and failures). I recommend the following books for your further education:
- Drainage for Sportsturf and Horticulture by Keith McIntyre (BS. Hons Msc) and Bent Jakobsen (PhD). ISBN 0-646-34971-6. In my opinion, the most useful book written on the subject. A really great bed-time read (or fill-in-time on the plane). 162 pages of relatively light reading, concise and useful facts.
- Soil - An Introduction by Michael J. Singer and Donald N. Munns. ISBN 0-13-449174-2. At 480 pages, a heavier, more text-book read. But Chapters 2 (Solids and Pores) and 5 (Water in the Soil-Plant System) are particularly worthwhile reading.