So, here's a chart for the bonuses. At rank 1 Compost, after the first 3 kills you are at Level 1 of the chart. Add your additional ranks in Compost to whatever your current temporary buff is to get your Level. So, for example, if you have rank 2 Compost and 6 things die then you'll be at Level 3 on the chart.
Level 1 - Uproot Bonus Per Tick: 20 - Shamblers Dmg: 4 - Shamblers HP: 60
Level 2 - Uproot Bonus Per Tick: 40 - Shamblers Dmg: 6 - Shamblers HP: 120
Level 3 - Uproot Bonus Per Tick: 60 - Shamblers Dmg: 8 - Shamblers HP: 180
Level 4 - Uproot Bonus Per Tick: 80 - Shamblers Dmg: 10 - Shamblers HP: 240
Level 5 - Uproot Bonus Per Tick: 100 - Shamblers Dmg: 12 - Shamblers HP: 300
From a purely minion buffing perspective, Morale is superior to Compost. If you are using Uproot then obviously Compost has a big impact.