Do ammendments, the materials added to growing media in an attempt to change its physical structure, work?

Pumice, for example, is added to bark to improve 'drainage' or more accurately to increase the amount of air filled pore space (AFPS) or the amount of air in the medium when drainage has ceased.

AFPS is not constant. As water is removed from the medium the space it occupies fills with air so that the only time we have a constant measurement is when the container is freshly drained.

The practice of adding amendments to growing media goes back years. The effectiveness of these materials does not seem to have been studied but some information has been published about AFPS using different amendments. These studies have been measured at 10cm of water tension and bear no relationship to the situation in a freshly drained container.

Theoretical studies on amendments suggesting they may be of limited use have not attracted much grower attention.

To understand the value of amendments measurements were recently made of five levels of three pumice grades on the AFPS and freely available water, the two major physical properties of a growing media, on a good quality peat.

The three grades of pumice were: (1) coarse 3-5mm, (2) medium 1-2mm and (3) fine 0.2-0.5mm. The first two were obtained from commercial sources the third sieved from a mixed grade.

The five amendment levels -0 (100 percent peat), 12.5 per-cent, 25 percent. 37.5 percent and 50 percent, were selected arbitrarily to illustrate the effect of increasing the amount of amendment.

The results are shown in Table 1 below.

Coarse and medium pumice grades had virtually no effect on AFPS until the level of amendment reached neatly 50 percent of the total. This is in line with the calculation mentioned earlier. The reason lies in the manner in which the amendment affects the AFPS.

The edge effect

There are two ways this effect can occur - (1) either the amendment has a much higher AFPS than the primary material (ie peat) or (2) because the particles do not fit neatly together and created spaces, the so called 'edge effect'.

To create an edge effect there must be enough amendment particles to contact each other. This only happens when the amendment is about 50 percent of the volume because at lower rates most of the particles are surrounded by the primary material which being inflexible does not permit the edge effect to occur.

Any fine material moving into the larger pore spaces in the primary material reduces the amount of AFPS in the mix. This may make both the drainage and the AFPS very poor and the mediurn worthless. The extent to which increasing amounts of fine material bring the AFPS down to unacceptable levels is a functioin of the amount of AFPS in the primary material.

Looking at the effect of pumice on freely available water we see a less favourable picture. As coarse and medium pumice increases, water availability reduces but not at the same rate because some water is available from the pumice.

If we know the amount of water available from each of the constituents we can reasonably accurately predict the amount of freely available water in the mix.

Fine pumice

Fine pumice. for example. creates a small increase in freely available water because the air filled pores in the peat become filled with fine pumice. The fine pumice packs fairly tightly so the amount of water held in the pores is much less than the volume of air it has displaced.

These results indicate adding amendments such as pumice to growing medium may not necessarilv improve drainage or

AFPS and may decrease the amount of freely available water in the rnedium. In other woords, far from improving the desirable qualities the medium's performance may deteriorate.

It might be argued that these results apply only to some peat/pumice mixes. Those wishing to differ should very carefully check whether the mix they are using is producing the desired results. They may very well find it is not!

Table 1: Air filled pore space.

Grade            0%      12.5%     25%    37.5%        50%

3.5mm  23.0    22.8          22.8          21.6              24.7

1.2mm  22.3    22.6          22.5          21.7              23.7

0.2-0.5mm   23.1    13.0          10.1           8.4               4.5

Table 2: Freely available water.

Grade   0%       12.5%         25%       37.5%            50%

3.5mm  25.9      24.1          20.5           18.6            17.0

1.2mm  30.0      25.0          21.6            19.1           17.5

0.2-0.5mm  29.8      28.5          29.0            30.7           31.3

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