Media Nutrient Levels.
Published guidelines for results of 1:1. 5 extract analyses for fertilised peat and bark modules (Prasad et al, 1986) may not be correct for bag cultures in peat or bark and seem unlikely to be correct for pumice. Conversion of results by multiplication using the factor of extract volume over known water holding capacity yields values much higher than those recommended for rockwool retained solution analyses. The bag sampling technique and analysis of the 1:1.5 water extract may well provide a more reliable and simple management method than the more usual drain water analysis.
Comparison of nutrient concentrations in the applied liquid feed and in the water extract (feed composition calculated at the same EC as the water extract) may indicate which nutrients are being supplied at greater or lesser rates than they are being taken up by the crop. The quality of the nutrient solution, in terms of the balance of nutrients relative to plant uptake, is a key factor in nutritional management with all of these soilless systems. Internal buffering in fertilised peat, bark or sawdust modules allows more tolerance in nutrient solution quality than do other systems.
Careful interpretation of analytical results allows a near
perfect balance between supply and uptake for NFT systems, but there is
no obvious method of reaching this balance with once through nutrient
solution for planter bag systems. The need for excessive supply and
regular excess drainage is a consequence of this lack of balance. An
ability to provide more balanced solutions for planter bag growing systems
might improve crop nutrition and yield, reduce costs and discharge to the
environment. Control of low pH in planter bag media can be difficult at
times when N supply needs to be restricted.