Compaction and Moisture Issues By Robert Steinkamp

Improper or careless mix handling can result in various problems in the greenhouse.

A very wise grower once told the young assistant “avoid compacting the mix at all cost,” and then proceeded to tell her the various ways that compaction can occur. In fact, improper or careless mix handling can result in many problems.

In a mix, the components form into a sort of skeleton in which air spaces occur. When the mix is watered, the smaller air spaces fill with water while the larger air spaces drain out and fill with air. A fine mix, containing mostly small air spaces, will hold a lot of water. A coarse mix, having mostly large airspaces, will have a low water holding capacity. With fresh, unused mix, properties of water retention and aeration (physical properties) can be measured. If the mix is handled correctly, these properties will not be greatly altered after the mix is potted. Let’s discuss ways that the mix’s properties can be changed.


We all know that growing mixes and peat moss can be compacted because compressed bales have been used for many years. Loose-filled bags are also subject to a degree of compression during packaging and shipping. These products must be fluffed out to attain the maximum yield. This is the point when the physical properties should be measured.
Now let’s start filling pots. How much mix actually gets into a pot? Some growers pack it in and some gently fill. Because the packed pots contain more mix, the mix is compacted as compared to the gently filled pots. By compaction, the air spaces become squeezed shut. Aeration is reduced and water retention is increased as compared to the gently filled pots. Now consider two workers hand potting, and one packs more than the other. Both workers’ pots get placed on the same bench. There is a good chance that the grower will notice pot-to-pot watering inconsistencies because of the altered physical properties in the packed pots. With potting machines and flat fillers, changes in adjustment or the usage of several machines can also result in packing variations and inconsistencies.


The mix is easiest to work with when it is about 45 to 50 percent moisture. Here it will feel barely damp and when squeezed into a fist and released, it will fall apart and not form a clod. It will easily flow through potting machines and flat fillers at this moisture level. Sometimes the mix might be too dry, or the grower might just want to add additional water before potting. Often, a hose is run to the potting machine soil hopper. Water is added at intervals to moisten the mix before filling. Depending on how this is done, compaction can occur. Sometimes the water is added until the mix looks wet enough. Sometimes the mix is gently turned to facilitate uniform wetting. Sometimes the mix is vigorously stirred. If the mix is not uniformly wet, or if it becomes too wet, and especially if it is over-stirred, compaction problems can occur.

If the mix is too wet, stirring or the tumbling and pouring action of the pot filler can cause the mix to form clods. If this wet mix were to be squeezed into a fist and released it would not spring back as it would if it were drier. If the mix in the hopper is inconsistently wet, some pots might receive the wet, compacted mix and others might receive mix drier and less compacted. The operator might uniformly wet the mix in the hopper but apply varying amounts of water each time the moisture is adjusted. In this case, the mix from one hopper-full could be more compacted than the next. Each of these conditions would result in pot to pot watering inconsistencies.

The key is to uniformly add the correct amount of water. First, a target moisture level must be determined. The operators need to be trained to estimate the percent moisture of the mix before the decision to add water is made. This can be done by feel. If water is to be added, the operator must know how much water to add per bag of mix to get the desired moisture level. If the operator is going to stir the mix, it should be done the same way each time.

Other Compaction-Related Issues

Some growers buy mix and add fertilizer using a mixer. When doing so, only run the machine long enough to blend in the fertilizer. The tumbling or stirring action in a mixer can have a grinding effect on the mix and make it finer, altering the original physical properties. Increased water retention and decreased aeration may result. Perlite and vermiculite are particularly fragile and are easily damaged by over-mixing. The same concerns about adding water, discussed previously, apply to mixers.

The storage of pre-filled flats can cause issues. Often the flats are stacked on a pallet in a way that the cells can nest together. The result is that there is a gradient of compaction with the bottom flats affected the most. Flat-to-flat watering inconsistencies can result.

It is important to understand and manage compaction of growing media. There are cases where some compaction is good. Very fluffy, lightweight potting mixes should be lightly or moderately compacted during potting to avoid excessive settling during the initial watering. Water-holding properties of a high porosity mix can be improved by increasing soil compaction when using drip irrigation or sub-irrigation systems that require the lateral or upward movement of water. But as a rule, avoid compacting the growing mix.

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Robert Steinkamp

Robert Steinkamp is technical specialist with SunGro Horticulture. He can be reached at [email protected] or 800.833.7645.

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