Currently, corn has a growth rate of 50% in sparse jungles.
If the aim of sf crops growth rates is to be more accurate (as is seen by the newest overhaul), I believe this current statistic is inaccurate and should be changed to 100%, if not more.

In this suggestion, I will attempt to give a detailed but brief explanation of soil quality and climate, in an attempt to prove my point of view.


pH levels:

According to the university of New Hampshire, corn thrives in soil with a pH level of 6-6.8
Other universities echo similiar desired values of pH (University of Minnesota Extension 5.8-7, University of Georgia extension 6.0-6.5)
Sparse jungles in game are found north eastern Asia, southern Europe, and scattered throughout the inland of the north east and southern Americas. All these areas have close to ideal pH levels (sourced from National Library of Medicine)
North East Asia 5.01-8.41 (National Library of Medicine or NLM)
Southern Europe 5.5-7 (European Soil Data Center or ESDAC)
North East America 4.5-8.2 (Cornell University)
Southern America 6.5-8.4 (J.B.J Van Ryssen Maize Research Samples)

Sources:
(New Hampshire)
https://extension.unh.edu/resource/growing-sweet-corn-fact-sheet#:~:text=The%20ideal%20soil%20for%20corn,so%20seed%20germinates%20more%20readily.
(Minnesota)
https://extension.umn.edu/vegetables/growing-sweet-corn
(Amherst)
https://ag.umass.edu/vegetable/fact-sheets/corn-sweet
(Georgia)
https://extension.uga.edu/publications/detail.html?number=C905&title=growing-home-garden-sweet-corn
(NLM)
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9779465/
(ESDAC)
https://esdac.jrc.ec.europa.eu/public_path//ph2.png
(Cornell Competency Area 5)
https://nrcca.cals.cornell.edu/nutrient/CA5/CA0536.php#:~:text=Northeastern%20mineral%20soils%20generally%20have,to%20fall%20between%203.5%20%E2%80%93%208.2.
(Van Ryssen Maize Study)
https://www.researchgate.net/figure/Map-of-the-soil-pH-of-South-African-soils-with-permission-of-the-Institute-for-Soil_fig3_262749727#:~:text=Although%20the%20majority%20of%20South,Ryssen%20%26%20Oelofse%2C%202012)%20.

Soil Composition+Temperature

Michigan state university states that corn germinates best in soil temperatures above 70 degrees fahrenheit, grows best in well-drained soils, heavily feeding off nitrogen.
While there are no real life biomes known as "Sparse jungles" to directly compare soil type too, there are similiar ones. [Seasonally] Dry tropical forests (found in south America, Asia, and parts of Africa), Secondary forests, and Transitional forests bear the most resemblance. However, since secondary forests and transitional forests are inconsitent in appearance/can be considered temporary states of being for the ecosystems they are involved in, we will be exclusively considering Dry tropical forests.

NASA's earth conservatory documents an example of such a forest in Manaus, Brasil from 1970 - 2000, where the average monthly temperature ranged from 27 to 28 degrees celsius (80.6 to 82.4 degrees fahrenheit).
Another example, called the Mato Grosso tropical dry forests, documented by the enviromental organization One Earth, averaged 23.5 celsius in recent years, or 74.3 fahrenheit. They also remarked that
"the rest of the year is dry, and because the soil is porous, the wetlands and pools dry quickly to form massive flats and dry forest patches, as well as patches of savanna and moist forest."
which fits the description of well-drained soil.
A peer reviewed research paper by Aber JD, Nadelhoffer KJ, Steudler P, and Melillo JM also notes that these forests consit of "70% of global terrestrial N fixation and 62% of global nitrous oxide (N2O) emissions".

Sources:

(Michigan)
https://www.canr.msu.edu/resources/how_to_grow_corn#:~:text=Corn%20grows%20best%20in%20loam,especially%20of%20nitrogen%20(fertilizer).
(NASA Earth Observatory)
https://earthobservatory.nasa.gov/biome/biorainforest.php
(One Earth)
https://www.oneearth.org/ecoregions/mato-grosso-tropical-dry-forests/
(Aber JD)
https://link.springer.com/article/10.1007/s10533-021-00788-6#:~:text=The%20tropical%20forest%20biome%20is,)%20emissions%20(Hedin%20et%20al.

In conclusion, sparse jungles ingame should have near perfect conditions for Maize cultivation, and their 50% growth rate should be adjusted to a much higher value.