The method you describe sounds extremely similar to the method described by Aquilante et al (2004). There is no official name for it. The articles I've found referred to it as "soil grain technique," "soil grains-sowing technique," or similar.

----------------------------------------------------------------------------------
https://annalsmicrobiology.biomedcentral.com/articles/10.100...

Azotobacter species were isolated using the soil grains-sowing technique as described previously (Aquilanti et al. 2004; Jiménez et al. 2011). Briefly, 30 g fresh soil was saturated with sterile water, and 3 g mannitol was added to the soil paste. The soil paste was then transferred and pressed into about 3 mm-diameter grains in a Petri dish using a sterile spatula. A total of 16 soil grains was placed on each Ashby-Sucrose agar plate (Jiménez et al. 2011), and each soil sample consisted of three replicates. The plates were incubated at 28°C for 7 days, after which sticky and glistening colonies around the grains were streaked and incubated on Ashby-Sucrose agar three times for purification.

-----------------------------------------------------------------------------
http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1517...

The primary isolation was made using the grain soil technique (Figure 1A) according to the methodology previously described by Aquilanti et al. (1) and Becking (4) in Ashby-Sucrose agar (Agar 1.5%, Sucrose 0.5%, CaCO3 0.5%, MgSO4 0.02%, NaCl 0.02%, KH2PO4 0.02%, FeSO4 0.0005%). Plates were incubated at 28ºC for 7 days until observing sticky and glistening colonies around grains (Figure 1F). Counts were made for each cultivated soil (% Recovery = Number of grains with sticky and glistening colonies/total x 100).

--------------------------------------------------------------------------------
The original technique is described here, if you can find the article:

Aquilanti L, Favilli F, Clementi F (2004) Comparison of different strategies for isolation and preliminary identification of Azotobacter from soil samples. Soil Biol Biochem 36:1475–1483