Water soluble mycorrhizae doesn't sound too effective. Once they're in solution they'll need a constant supply of sugar/cellulose/protein and oxygen to stay alive until they can get to their next sugar source which is the root of the plant. Mushroom liquid cultures can be kept alive in the fridge to slow down metabolism for longer but that's under controlled conditions.
Digging for roots and rubbing the powder into the roots and surrounding soil feels more effective to me.
The water soluble is what one would use on plants already in the ground or containers, as long as some reach the roots, they will attach themselves and multiply.
another method is using a dibber or stick try to make a small whole down to the root zone, and use a granular myco.
Of course the best way is to apply it directly to your seeds and roots when planting.
The small little envelopes I got from Fungi Perfecti 1oz cost only 6 bucks and go a long way.
this is the product I am using now, but any similar product like White Shark or dozens of others I am sure would be just as good if they are fresh.
http://www.fungi.com/product-detail/product/mycogrow-soluble-1-oz.html1 teaspoon in a gallon of water goes a long way, I have even did foliar sprays with this, since this formula has Trchoderm, it will attack other fungi on your plants, its being used commercially in some plantations for this purpose, and I have noticed a reduction in powdery mildew when I use it that way.
also the beneficial bacteria help as a foliar also.
You can grow your own, one method I saw that someone was using, he would grow bahia grass in wooden trays, the seeds and soil were inoculated with beneficials,which will multiply with the roots, and before the grass reaches to seeding stage, you removes it, and feeds it to the worms, but the roots and soil is used and mixed with the compost, the roots will keep the fungi happy for a while.
Not all fungi die if they do not reach a root right away, the spores can remain dormant. its things like heat and sunlight that can kill them. Just 1 spore reaching the root can over time multiply into the thousands and millions.
The beneficial bacteria in these products are even more hardy.
The more I research Mychorizzae and beneficial bacteria, the more I love these things, I believe it is the pro bios that will revolutionize agriculture in the future. even corn fields have increase in production and become more drought tolerant.
But probably the ones with most experience with this stuff, are the indoor gardeners growing their "Medicinal" herbs.
One reason I picked the above product is because it has Gigaspora margarita, which has been shown in research to have a symbiotic relationship with Mango trees.
Abstract
An experiment was conducted to study the effect of AM fungi on germination, growth and graft-take in mango. The inoculation of Gigaspora margarita and Glomus fasciculatum had resulted in highest germination (51.01 and 49.20%, respectively). The vegetative parameters (rootstock height, stock diameter and number of leaves) and root parameters (primary root length, number of secondary roots and vigour of the stock) were found to be highest in the rootstocks treated with Gigaspora margarita followed by Glomus fasciculatum. Spore count and percent root colonization were higher in the AM fungi treated rootstocks. Significantly maximum graft success was recorded by Glomus fasciculatum and Gigaspora margarita (80.30 and 83.22%, respectively). On the other hand, survival of grafts was maximum in Gigaspora margarita (95.36%) followed by Glomus fasciculatum (88.33%).
Interaction of 10-month-old rootstocks with Gigaspora margarita recorded maximum vegetative parameters. Significantly maximum graft-take, per cent survival of grafts, sprout height, number of leaves and graft diameter were exhibited in different aged rootstocks inoculated with AM fungi when compared to non-AM rootstocks.
arbuscular mycorrhizal (AM) inoculation will increase nutrient uptake, reduce watering needs, and help keep roots healthy, even repel/kill harmful nematodes and other critters.
Mycorrhizal Inoculant - Fungi You Want In Your Organic GardenMycorrhizal fungi: Central Texas Gardener William Glenn