I don't blend my own soil, when I do grow in soil I use Biobizz composts, currently I am in coco.
So I went to the interweb and had a little look. Check out the Fertilize Efectively bit which talks about the importance of Super Phosphate for seedlings.
Sorry I can't offer any personal advice in this matter, maybe another of our members can help.
When to Apply Super Phosphate to a Garden
by Joseph West, Demand Media
All synthetic phosphorus fertilizers are made from mined rock phosphate.
Phosphorus is one of the three primary plant nutrients. Fertilizers often supply significant amounts of phosphorus, along with nitrogen and potassium, the other two primary nutrients. Super phosphate is a common synthetic phosphorus fertilizer. It contains a large percentage of phosphate, but you need to apply it carefully to ensure that your plants can use the phosphorus.
The Importance of Phosphorus
Phosphorus is considered a primary nutrient because it is essential for plant growth and because the quantity required by crops is large relative to the supply of available phosphorus in many soils. Fruiting crops such as tomatoes (Lycopersicon esculentum) and squash (Cucurbita spp.) are particularly sensitive to phosphorus because this element is directly involved in the formation of reproductive structures. An abundant supply of available phosphorus also encourages proper root development and vigorous early growth.
Super and Triple Super
Super phosphate fertilizers come in two common forms: single super phosphate and triple super phosphate. Both are manufactured by reacting insoluble mineral phosphate with acid to create a soluble, readily available form of phosphate. Single super phosphate is about 20 percent phosphate (this percentage corresponds to the second number in a standard N-P-K fertilizer analysis), and it also contains significant quantities of calcium and sulfur. A different type of acid is used to manufacture triple super phosphate, which contains about 48 percent phosphate with less calcium and sulfur.
Minimize the Effects of Fixation
Soluble phosphorus fertilizers are subject to a troublesome phenomenon that does not apply to nitrogen and potassium fertilizers. This phenomenon is known as fixation. When you apply soluble phosphorus fertilizers, such as super phosphate, to the soil, chemical reactions cause this readily available phosphorus to revert to forms that are not available to growing plants. This means the benefit of phosphorus fertilizer decreases steadily after application. Consequently, apply phosphorus fertilizers at the proper time and during certain seasons to minimize losses caused by fixation.
Fertilize Effectively
The most important time to apply super phosphate is before planting -- phosphorus is critical for seedling root development and early growth. Perform this application very soon before planting to ensure that fixation does not render the phosphate unusable by the time it is needed by seedlings. Side-dressing may be beneficial if a soil test indicates low phosphorus reserves. The best time for side-dressing super phosphate is when plants enter a stage of high nutrient demand, such as when tomatoes begin to bear or ripen fruit. For perennial crops, apply in early spring when vigorous growth ensures that plant roots can take up the phosphate before it is immobilized by fixation.
Copied and pasted from -
http://homeguides.sfgate.com/apply-super-phosphate-garden-86815.html
to get you started?
!!!
grow 
Karma your way.
