Water is critical to the lives of both humans and plants. However, access to water is just as important as its presence. The Rime of the Ancient Mariner said it best when stating that having water, water, every where, nor any drop to drink is just as bad as having no water at all.
So how can you ensure that your plants have access to the water you are giving them? Knowing about plant available water, proper irrigation, and moisture content are all important in this regard. Let’s explore these topics so you can better maintain your plants.Â
Key Soil Water Concepts
In order to properly maintain water content for your plants, it's critical to understand a few key concepts and how they are determined. Knowing this will allow you to measure and control your water content and have more success growing your cannabis plants.
Field capacity is sometimes also called the drained upper limit. Water naturally drains downward due to gravity. If you thoroughly saturate the soil, then allow it to drain freely, the water that remains held in the soils is the field capacity.
Determining the field capacity of your soil will take about one to two days to allow for full drainage.
If a plant wilts, but you later give it enough water, it will sometimes make a full recovery. However, sometimes the soil holds so little water that the plant will wilt and never recover, even if you give it sufficient water.
The amount of moisture content in the soil when the plant cannot recover is known as the permanent wilting point. At this point, the plant cannot extract any more water from the soil.
Bulk density is the density of soil. You can find it by dividing the dry weight of the soil by the total volume. The volume includes all of the soil particles as well as the pores in between. Bulk density is usually expressed in grams per cubic centimeters.
Bulk density is important to soil water content as it indicates how compacted the soil is. This measurement is also used to convert between weights and volumes in soil and convert gravimetric water content to volumetric water content.
What Is Plant Available Water?
Plant available water is the amount of water stored in the soil that a plant can actually access. This amount is the difference between field capacity and the permanent wilting point. Having a higher plant available water capacity allows your plants to access the water they need.
A number of physical and chemical characteristics affect plant available water. One of the most important is the soil texture and type.
In general, as the fineness of a soil increases, so does its available water capacity. This is because these soils have additional pores and surface area that hold water against drainage. Well-aggregated loams and silt loams tend to have high plant available water capacities.
It is important to note that even though clay soils are finer, they usually have a lower available water capacity. This is because they have a higher permanent wilting point as plants have a hard time extracting some of the water held by clays.
Compaction, which can be measured by the bulk density, negatively impacts both field capacity and the permanent wilting point. As a result, plant available water decreases as a soil becomes more compacted.
This is mainly because the pore volume is reduced, making less storage space available for the water. In addition, larger soil pores are reduced into smaller micropores, which hold onto water more, increasing the permanent wilting point.
On the other hand, organic matter increases how much water soil can hold in a number of ways. It has a higher water capacity than mineral soil of a similar volume. The amount of water that is directly available to plants also increases.
In addition, organic matter improves other soil characteristics that improve soil quality and increase available water capacity. This includes increased pore size and volume, better soil structure, and improved aggregate stability.
Why Does It Matter to Cannabis?
Unless you are growing cannabis using hydroponics or aeroponics, you'll need soil to grow your plants. Whether you choose to grow your plants in pots or beds, this means you'll want to pay attention to the water that your plants have available to them.
The amount of water that is available to plants is a key factor in the yield the crops will produce. As a grower, determining the plant available water in your setup will allow you to make more informed decisions regarding the soils you choose and how you irrigate.
In addition, knowing the plant available water in your soil can help you time planting and can sometimes even affect variety choices. It can also help you make decisions about resource input, such as how much nitrogen fertilizer to use.
How Do You Determine Water Content Measurements?
You could take a soil core, dry it out and determine the bulk density of your soil. The difference between the wet soil and the dry soil will give you the gravimetric moisture content. Then, you can multiply these two measurements to determine the volumetric water content of your soil.
If you take these measurements using soil that has been thoroughly saturated, then left to drain for a couple of days, you'll be left with a fairly accurate field capacity for your specific soil.
The permanent wilting point of a soil is a little harder to determine. Usually, it is identified in a lab using the retention curve method. You could estimate it by determining the moisture content when the soil's matric potential is -1.5 MPa or 15 bar.
Due to the difficulty in determining these methods on your own, you may want to consider sending your soil samples to your local soil lab. Many extension offices offer these services. In addition, you can also use tools that make the job a little easier.
If you know the type of soil you have, you can use a database that shows the typical value for each of these factors for your specific type of soil. Just note, this method works best if you are working with natural soil, as imported topsoil will not hold to the characteristics.
A soil moisture sensor is another tool that can make it easier to estimate the water content of your soil. These simple and easy-to-use tools measure the volumetric water content in the soil, which is the ratio of the volume of water to the volume of soil.
A benefit of using a sensor is that it gives you real-time data that you can use to make decisions about timing irrigation.
How to Improve Plant Available Water
If you determine that your soil has lower than ideal plant available water, it's not too much cause for concern. You can make some changes that can improve the water holding capacity of your soil.
If you are growing your plants in pots, you'll usually have less of an issue with water holding capacity than in fields because you are able to choose and more directly control the type of soil you are working with.
Raised beds also offer a way to potentially improve plant available water. Many soils have physical barriers below the surface that can affect water storage capacity and create a barrier to growth.
By creating additional soil depth, you'll be able to decrease this issue. In addition, raised beds make it easier to choose the types of soils you are using, as many require additional soil to build.
If problems persist, there are other ways to increase available water. One option with the biggest payoff is to incorporate more organic matter in the soil. Organic matter increases the water holding capacity of soil, but also offers a number of other benefits as discussed earlier.
In addition, you can leave crop residue on the soil surface if you are growing in beds or a field. This reduces evaporation and helps water infiltration.
How to Ensure Your Cannabis Plants Get Enough Water
Improving your soils plant available water is one way to help your plants get enough water. However, there are additional steps you can take.
Using an irrigation system can help you control various aspects of your plants access to water. Most importantly, this includes the amount of water they receive and when.
Irrigation scheduling can also be used to deliver nutrients in a highly controlled manner. This ensures they get enough of each nutrient while reducing unnecessary waste.
Managing the humidity of your growing area can also help keep your plant water content at a good range. In general, cannabis plants prefer a relative humidity between 65-80%, though many growers decrease this to 45-65%% during the bloom period in order to prevent bud rot.
Keep Your Plants Healthy with Proper Water Content
Identifying the plant available water, field capacity, and permanent wilting point of your soil can help you ensure that you are maintaining proper water content. From here, you can use irrigation and other tactics to keep your plants healthy and fruitful.
Contact the experts at FloraFlex to make sure you have all the tools you need for your cannabis grow system.
