In these tables, we have also included how much you’ll need of each component to manufacture sufficient quantities for your chosen planter model, according to the composition of the soil-based growing media given in Tables 3 and 4. In each case, this is shown for each of the three soil types and the five standard planter models. Allowance has been made for producing 125 litres of soil-based growing media per containers. This allows 25% extra for settlement, which you’ll use over the course of the first growing season to maintain the soil level relative to the drip irrigation pipes.
It will be noted (possibly with some consternation) that each of our soil-based growing medias contains five or six components. This might appear a little excessive as Table 2 does show that the contribution of each of the medias may overlap! This, at face value, does suggest that the requirements identified in Table 1 could be achieved with fewer components… and the short answer is that they probably could(!), particularly if the soil used could be inspected first-hand and a certificate of analysis was available, but we think that (in most cases) ‘doubling down’ is the right strategy, here.
It should also be stressed that Table 2 is a simplification, and masks differences between the media and the ‘added value’ that each offer. While green-waste compost, bracken compost and wool compost all provide organic matter, there are important differences between these, as the following demonstrates. The rate of decomposition of organic matter and mineralisation of plant nutrients is dependent on the proportion of three important constituents in organic matter, these being: lignin, hemi-cellulose and cellulose. The proportion of these constituents would be expected to differ between the composts. The green-waste compost should contain the most lignin, the bracken compost the most hemicellulose, and the wool compost the most cellulose. Corresponding differences between carbon to nitrogen ratios would also be expected ("plant nutrition: explained" may be help to revisit, here), with the green-waste compost being the highest at 20-25:1, followed by the bracken compost at 20-15:1, and finally the wool compost at 15-10:1. Given that the organic matter, containing the greatest amount of cellulose and lowest carbon to nitrogen ratio would also be subject to most rapid decomposition and mineralisation, it is necessary to balance this with the greatest amount of lignin and highest carbon to nitrogen ratio. The benefits of including all three composts is in ensuring mineralisation, as well as providing a ready supply of nitrogen, phosphorus, potassium and sulphur to your soil-based growing media; the benefits of this over an extended period soon become apparent!
Likewise, the role of green-waste compost and bracken compost as sources of potassium appears to be one-in-the same, but here again there is good justification for including both! While these materials would be expected to have a comparatively high potassium content, this will still only be of the order 1%. Composts are natural materials and feature considerable variation in content of all nutrients, including potassium, due to seasonal effects. Reliance on a single compost to supply potassium presents a risk if the batch obtained contains a relatively low potassium content. However, the risk of this is obviously diminished if two sources are used. Given the absence of information on the properties of the compost actually supplied, we find that this pragmatic approach is sensible, and provides something of a fail-safe. The risks of oversupply of nutrients are not considered significant, and certainly very much less than the consequences of the plant nutrients required, being deficient!