Will adding compost adversely affect the soil I’m growing vegetables in?
Question:
I grow vegetables in a field via direct seeding or transplants. I have noticed that when I try to plant I have a lot of clumps. When I squeeze the clumps, they break down easily into small granules. I need to add additional soil to to my growing area. My choices are loam, compost or a blend. My most recent soil tests in this area are 2026 and 2027 from April 2021. These reports indicate that I have adequate organic matter. Will adding just compost adversely affect the soil? I’m confused by the presence of the clumps as the CEC indicated a sandy loam which should have less tendency to clump compare to clayey loam. I would appreciate your thoughts.
Answer:
I can understand your confusion on the clumpiness (typically a sign of soil compaction) in a soil with plenty of organic matter and a sandy composition. My first question would be whether you use mulch in your garden? If not, that would be my first suggestion. Even with sufficient organic matter, if you aren’t mulching, the impact of direct rainfall and baking sun can alter the composition of the top layers–mulch should help maintain uniform soil moisture and improve the texture. Excessive tilling can also cause compaction/clumpiness, so we advise against it; conversely, gently working the vegetable bed with a garden fork to loose things up before planting can help break those chunks up.
If you are mulching and you aren’t tilling, my next thought would be to check to see if you have Asian jumping worms, a widespread invasive species Maine is struggling with that converts the top layers of soil into a granular consistency. It sounds like the clumps you’re seeing are larger, but the fact that you mention granules makes me want to rule this out. I also mention it because you are planning to bring in additional soil, and we are heavily stressing to Maine gardeners and homeowners that they exercise caution when purchasing mulch, compost, or soil, to prevent bringing the jumping worms onto your property if you don’t already have them. I highly recommend speaking to your supplier first to see if they are aware of the jumping worms and mitigating the problem, as well as “solarizing” organic amendments you bring onto your property (spreading the material out on a flat surface in a relatively thin layer, and covering it for several days under clear plastic in the sun) to kill any worms and cocoons that might be present.
To your specific question, if you have adequate organic matter per a soil report, I wouldn’t add pure compost in large quantities (though if you haven’t added anything since 2021 and are heavily growing, you might want to test it again to confirm). We typically recommend gardeners put down an inch or so of compost annually to active growing beds to maintain nutrition and texture. It’s a rule of thumb and you can get away with adding more, but if too much goes in it can cause problems. Broadly speaking, if you don’t have jumping worms, and you are mulching, watering, and adding small amounts of compost each year, I would expect the clumpiness to recede over time.
Happy gardening.