When using solution culture rather than sand culture, for which the original Hoagland and Arnon solution was devised, it is advisable to reduce the concentration of PO4to low levels to avoid P toxicity due to excessive uptake of certain species such as lupin and wheat. This means that P uptake should be monitored and P added regularly. - See more at: http://prometheuswiki.publish.csiro.au/tiki-index.php?page=Hoagland's+nutrient+solution#sthash.9CIrg1Y8.dpuf