Every time I have the opportunity to pass along nuggets of wisdom from our experts to you I try to do so to benefit all our gardens! Spring is upon us which means many gardeners are bursting at the seams with gardening questions. Soil testing, soil amendments and use of compost are three topics we get lots of questions on this time of year. Recently our organic and sustainable agriculture expert, Dr. Jennifer Reeve, answered a client’s question about use of compost. I thought her advice was excellent so I wanted to make sure her wisdom was available to all of you as well. Here are her recommendations on composting – enjoy!
I would like to start some test sections this year using a no till method. I want to put a slurry compost on the surface each year. I have started experimenting with a slurry of compost tiles that creates a hard layer that should keep weeds down and break down to be soft within a few months to allow plants to become fully established with minimal weeding.
My goals are….
1. Reduce/eliminate weeding
2. Improve soil moisture retention
3. Follow a more natural flow of debris decaying on top
4. Improve soil structure with no tilling
My questions are…
1. How thick does the compost layer need to be to continually add sufficient nutrients to the soil?
2. Would it be better to have a mix of wood chips or leaves in it (like a layer of fallen leaves)?
3. Should I add fertilizer(s) (like nitrogen) into the mix of compost and paper for a thinner layer? (currently my design is ¾” to 1”thick compressed compost slurry.)
You raise some interesting questions. I do not have much direct experience with systems such as you describe. So I can’t really give you answers on how thick your compost layer would need to be to be effective or if it would be better to mix in wood chips or leaves.
I can give you some info on related work we have done so perhaps that will help as you experiment with what works for you.
In dry climates such as Utah growers need to be cautious about applying too much compost as salts can rapidly build up in our soils leading to plant growth problems. I have been contacted by a number of Utah organic farmers who have run in to this problem, especially smaller operations with ready access to large quantities of compost.
For this reason I typically recommend applying compost based on the phosphorus needs of your soil and crop only.
This is typically not much, perhaps an eighth of an inch per year depending on the P concentration of the compost used and your soil test results. Surprisingly our research has shown that even this small amount will dramatically improve soil fertility.
So applying compost as a thick layer as a mulch, while quite a common practice in wetter regions, may quickly result in elevated P and salt content in our climate. Unlike sodium, phosphorus is not toxic to plant growth but it can interfere with trace element uptake which can be a problem in our high pH desert soils anyway. Excess P accumulation is also an environmental concern especially near open water sources or in Urban environments where storm water and irrigation run off enter storm drains.
Plant based composts contain less salt and P than manure or food waste based compost. But the composting process concentrates these elements so you still need to be careful about application rates.
I have found leaf litter works great. Fallen leaves have very low nutrient contents and so don’t contribute to salt or nutrient balance problems. In my yard at home I pile all the fallen leaves I have on my garden in the fall and by the spring they have formed a nice partially decomposed mat on the surface that is very effective at controlling weeds.
At USU we have also experimented with paper slurry used as a mulch and it can be very effective for annual weed control. Perennial weeds such as grasses and dandelions will break through eventually. And you may need annual applications to fill any cracks etc.
All high C:N (carbon to nitrogen) ratio mulches (paper, leaves, sawdust) will tie up nitrogen in the soil if incorporated so it is important to leave it on the surface and not incorporate it. This does make application of solid organic fertilizers problematic. Most organic growers I am aware of use fertigation systems when using mulch and supply soluble plant nutrients such as fish emulsion and compost tea through their drip system. You do need a good filtration system for this to work effectively.
Regarding water conservation, we found no effect of mulch on soil moisture at all in orchard systems in Utah. The trees were such a major source of evapotranspiration that mulch just did not have any effect at slowing this. Vegetable systems in dry hot climates may well be different though.
I hope this helps and good luck!