As you know, in mushroom production, any one key area improperly managed, can create detrimental results to our crops. In my opinion, one of the areas not fully understood is the casing layer, and its habitat. What I want to discuss here with you is a scientific term known as Matric Potential. We will learn about what it is, what it means, how to utilize it in our industry, and what the future might hold by going down this path.

Presented at the 30^th Australian Mushroom Grower’s Conference, Melbourne, Australia

When we first rolled out Spawn Mate, as part of our efforts to assist growers with various growing parameters in order to see them achieve the best results with the product, we formed a technical group. Back then, for the most part, the setback-pinning area was fairly straight forward across North America. Get the casing mixture wet, scratch it, and give it a hard flush, with the air at about 14 degrees Celsius, until the beds came down to 19 degrees. Than raise the air to about 16 degrees and maintain a CO₂ of 1200 ppm.

This method, along with the casing we used, worked fine for us when we were producing 25 kilos. But the bench mark has dramatically risen. When we are pursuing yields of 36 kilos, our old means of casing just won’t support the casing program we are attempting to achieve.

Methods and rumours

Over the past 15 years, many operations have adapted specific setback and pinning procedures to enhance yields, suit their market needs, and provide a more stable and uniform production schedule for its personnel.

There are many different procedures being used at this time. Some flush soft, others hard. Some scratch and level, others don’t. Some believe in initiating with cooler beds allowing the air to be run a little warmer during flushing, while others believe in beds approaching 30-31 degrees and driving them down hard. Some operations use a step-down method of utilizing CO₂, others merely drop it down to around 1200 ppm.

For most operations, these procedures are based on what works for them at their farm. For others it is rumour from what others are doing. Very few have a good understanding of what is really going on in this area.

Over the past decade more and more farms appear to be heading in the direction of utilizing a heavy, dense casing, which apparently supports cleaner product, larger size, and heavier tissue weight. This is done through various methods including sugar beet lime, specific types of Black Peat, and combinations therein. Do we really know how and why this system works for us?

I have heard little as to what is really happening in the micro-climate during this sensitive phase of the crop and became more and more interested in the objectivity and physics of it. I couldn’t help but wonder, that with new technology, I couldn’t shed some light on this area.

Soil and water

First, we need to understand a few principles of soil and water science. Water has certain properties associated with it. As an example, water moves from wet to dry soil. Water moves from large to small pores, and from high to low free energy. The difference in the free energy causes the water to move.

Soil particles are of different shapes and sizes, and the spaces between them contain varying amounts of air. In between the air spaces, we can see the capillary water in between the particles, and the adsorbed water attached to it. It is worthy to note that the adsorbed water is the attraction for soil solids to water. The tighter the air spaces, or capillary, the higher the water will rise.

Most soil water movement occurs as unsaturated flow through the capillary pores. The movement is slow due to the irregularity of the pores and the air in the pores. 

The squeeze test

As most growers have experienced, there have been countless times when we have found ourselves in a growing room squeezing casing soil, and compost with others. There is a large difference between a 130 kilo individual squeezing a sample as compared to a small individual. It is very interesting and also very subjective as to how much water we can extract depending on our strength.

What we are really doing is that we are testing for water held at a certain matric potential. With the strength in our hand, we are introducing an overburden pressure to overcome the matric potential of the material. Squeezing is the equivalent to adding pressure to the system.

Mushroom growers should be concerned about matric potential of casing materials because the casing layer is very important in the water management of the crop, and understanding matric potential provides a means of better understanding the relationship between the casing material and water. Different materials hold water in varying manners and the more we can learn about it, the better producers, and more objective we can become.

Matric potential is the only true objective means to define moisture dynamics in our crops. It is not adequate to just obtain a moisture percentage, or do a squeeze test.

Osmotic potential

The conceptual development of water potential has arisen from soil science. Water potential refers to the energy status of water in a system. The three major potentials having a bearing on mushroom growing are the gravitational, osmotic, and the matric potential. The osmotic and matric potential are negative values, while gravitational potential is expressed as a positive reading. Of these, with the exception of gravitational, which, as an example, could lead to leaching on the casing interface, most people have some familiarity with the concept of osmotic potential, but not with matric potential.

In Osmotic potential, it is only dependant on particles in the system, thus making it a physiological process. The matric potential, on the other hand, not only takes into account these physiological processes, but the physical processes as well, such as the movement of water and soluble food substrates, which can have a profound effect on the overall activity, including mushroom growth. Understanding this can help answer some of those questions we have such as when we have pins that do not want to move. Maybe they are ‘stuck’ because our matric potential is off. 

Anything that dissolves in water such as gypsum, salts in dpw, and limestone, will affect the osmotic potential to a higher negative value. People have discussed osmotic potential primarily because of mannitol. This sugar compound can represent up to 19 percent of the dry weight of a mushroom and have a significant effect on size and quality.

As an example, over the past two decades, many growers water their crops with Calcium Chloride. Because it is a highly soluble salt, it raises the osmotic potential. Sugar Beet Lime is another example, most probably because it decreases the capillary spaces.

Although significant, it is not overly important because there are not enough salts in the system to have a significant influence on the total system, therefore not leading to a high enough negative number to effect the Pascal reading, and therefore the matric potential. 

We increase osmotic potential when we add soluble materials such as salts or sugar to water. After we add a soluble material to water, such as these salts or free sugars released from the compost, the water molecules interact with the solute and the free energy status of the water is decreased, thus altering the osmotic potential. If we wish to separate the water and the solute we need to add energy to the system. One way to do this, is to use pressure to push the water through a semi permeable membrane, known as the process of reverse osmosis. 

Properties of water

Going back to what we learned in school, water is a polar molecule because there is a 105^o angle between the two hydrogen atoms bound to the oxygen that makes up water. We can see a number of properties associated with the polarity of water molecules. We can see hydrogen bonding, which is the bonding of hydrogen and oxygen. Because of this, water has an affinity for itself which we refer to as cohesion. Cohesion causes surface tension. Surface tension is caused by water’s attraction to itself. Hydrogen bonding and weaker electrical attractions can also attract water to solid surfaces, this attraction being known as adhesion.

Capillarity is a bridging between two wetted surfaces, as we discussed above with the air spaces and soil particles. This helps us understand why water will rise higher under pressure, and with smaller capillary spaces.

Matric potential is a way to quantify and understand the relationship between water and the surfaces to which it is attracted. The matric potential of a material is defined by the capillary size distribution and the total amount of water in the system. Water held in capillaries has a lower free energy than water associated only with other water. Matric potential is a measure of the potential energy differences between water in a physical matrix of capillaries versus the free energy of water associated only with other water. The potential energy is expressed in the pressure unit of Pascals. 

As I mentioned earlier, matric potential, is always expressed as negative numbers, because water in association will be at a lower energy level than bulk water.

Measuring matric potential

Up until now, we have relied on moisture measurements and squeeze tests. But now-a-days, there are electronic methods of measuring matric potential in the zero to minus one hundred range which are easy to use, allowing growers to easily obtain an objective means of measuring, monitoring, and controlling our soil properties.

A very useful laboratory instrument for investigating matric potential relationships is called a 'pressure plate apparatus'. This consists of a porous ceramic plate inside of a sealable pressure pot, looking a bit like a pressure cooker.  

The sample to be investigated is placed up against the pressure plate, the apparatus is closed up, and then the air pressure above the sample is increased while the water pushed out of the sample is monitored. The relationship between sample water content and the pressure being applied can be used to develop a water release curve, which we will take a look at later.

The water release curve is directly related to the capillary size distribution of a sample. For example, lets say we threw a sand sample into the apparatus. We would expect the water to be pushed out with very little pressure, because the capillary spaces in sand are mostly pretty large. With a clay sample, we would have to have very high pressures to push the water out, because the capillaries in clay are very small and therefore the water is tightly bound.

Water in casing material

In mushroom growing matric potential is important to know about, because osmotic potential is not limiting to the mushroom crop, but matric potential is. As I mentioned earlier, normally, the osmotic potential of the casing is not significant enough to become a limiting factor, because the level of salts is not high enough, while the matric potential is very significant to the mushroom crop, because it also takes into account the movement of water, and the movement of soluble food substrates. These are crucial to success and failure.

Growing mushrooms need a lot of water, and a good deal comes from the available water in the casing layer, which can vary depending on factors such as capillary size distribution, casing depth, and water management practices. 

A point to remember is that not all of the water in the casing is available to the crop. Depending on your farm and growing system and materials, water in very small capillaries is too tightly bound to be available to the mushroom. On the other hand, if it is too available, examples such as no water holding capacity and leaching onto the interface can occur. This becomes even more important when some us are picking breaks up to six days, compared to the three day breaks of times past.

Another example. Remember a decade ago when people tried to increase the water content of the casing using gel materials? There was a great deal of excitement over these products, and people reckoned that water in the casing layer was a good thing, and increasing the water content in the casing using gels would improve yields. Yet, this technology did not take off because it really did not improve the supply of available water. Water became tied up in the gels,  but at a water potential greater than the mushroom crop could get it. We have to remember that what counts for the mushroom is not the total amount of water, but rather what water is actually available.

Water release curve

Using a pressure plate apparatus can tell us the capillary pore distribution in a potential casing material. Correlating the pressure to remove water with the amount of water released, we can develop a characteristic curve for casing material. We can use this curve to compare different casing materials, placing us in a position to accurately replicate our casing mixtures. Lets take a look at a sample water release curve.

Generally, a casing material that holds lots of water in the zero to –30 kPa range would be a better casing in terms of water availability than a casing that holds less moisture in this range, because there is a greater reservoir of available water. On the other hand, it may be too available, which can lead to things such as drying out too readily, or running through the casing and making the interface with the compost wet.

One thing that needs to be mentioned is that matric water potential is directly related to hydrolic conductivity which means how the water, and bacteria move through the system. The wetter the casing layer, the higher the matric potential and the higher the hydrolic conductivity. Water moves more readily through larger capillaries than through smaller capillaries, and this strongly effects water availability. 

Use of a pressure plate can not only be used for studies of desorption, but also for studies of adsorption. For many materials, and especially organic materials, there can be a significant difference between the moisture content and water potential depending on whether water is being desorbed or adsorbed. The difference between the curves for desorption and adsorption is referred to as hysteresis.

Hysteresis

Lets start with the top line, or desorption curve. This tells us, at a given water activity, how much moisture a material will lose, or dry out. The lower line, or adsorption line, tells us that once it dries out, at what rate it can take on water.  An important idea to remember is that at any given potential, you will have more desorption than adsorption, in other words, we lose more water than we gain back.

If you look across the top of the curve, you can see three sections. They are the mono, multiple, and capillary regions.

The mono layer represents a material that is bound up, in essence it can not dry out. You can see the corresponding lines are at a low moisture, and low water activity pressure. Not conducive to successful mushroom crops.

The next section is the multiple layer. You can clearly see that in this range, there is a large spread between the desorption and adsorption lines. These differences are the hysteresis. In this range, you can lose water, but, in looking at the adsorption line, it will be very difficult to rewet it, thus making it harder to handle. This type of situation worked fine when we were producing 25 kilos, but it is a tough deal when we are looking at 36 kilos. Once again, we do not want this type of situation in our growing rooms.

The third section, or capillary region, is where we would like to be for our crops. It is in this region that a lot us are operating at while using these denser casing materials.

You can see, that once we get past –85, up until about –95, we have a low hysteresis, at rather high moisture. This would appear to be an optimum range for our mushroom crops. Generally, if there is great hysteresis for a casing soil, this means that the casing is going to be very hard to wet up if it is allowed to become dry, while a casing exhibiting little hysteresis will be able to be wetted up readily when dry. Generally, peat’s exhibit a great deal of hysteresis, which is why it is a good idea not to let a peat casing material dry out too much. Knowing the hysteresis of various casing materials can provide us with some reasons why one peat mixture might out perform another mixture.

A better understanding

Over the years scientists have studied and looked at water potentials in varying degrees, and there have been countless articles written about moisture in regards to mushroom production.

Ewan Harper a decade ago looked at various casing materials used in Australia using a pressure plate apparatus. He found that water release curves so generated provided good predictive value in how well various casing mixtures would work during actual mushroom growing trials.  Unfortunately Harper's work was not widely disseminated and remains mostly unknown. It is a good time to move further forward on this area.

We have gone from young blond peat moss, to old deep dug bog material. But even more important, we have changed the matric potential of our casing medium, to a finer material with a greater distribution of smaller capillaries, resulting in smaller pore spaces which means your water will be held in a tighter manner. We don’t want it held so tight that it gets bound up like the gels, and we don’t want it held too loose. We need to quickly identify and control the proper control points in this area to provide for more consistency, and understanding in our operations.

So, where does this all lead us to as mushroom growers? Well, it can lead us to understand and evaluate our casing materials in much more quantifiable ways, thus minimizing so much of mushroom growing that has been handled on a subjective basis.  

At this point, the underlying concepts are in place, and the technology and hardware have progressed to the point, that nothing stands in our way of developing a better understanding of our casing materials, and the fundamentals of water availability.

Instead of countless moisture and squeeze tests, a quest should be made to characterize the best casing formulations based on objective information, and how they can provide water to our crops.

This information will enable us to objectively ascertain peat mixtures, properties, and densities. We have enough to be concerned about with how wet the casing is, the CO₂ control points we utilize on our farms, air flow movements, and other factors. With the collection of this information, I am hopeful that the mushroom industry can become more objective in this area of our production, and move further ahead in its development and understanding of the crop.

What have we learned today? I think we have a better understanding of why we have switched to different casing soils. We now have a means in which to monitor, modify, and control our casing mediums.