Creating Soil

I’m building a house on an infill suburban block. The soil as shown in the photo is pretty degraded and is sandy with very little organic material.

When we excavate the first thing is to remove the top layer which will be 50/50 existing organics and this soil. Organics are grass and shrubs and leaf matter from the eucalypts.

I plan to windrow this outside the building zone and use the 12 months or so I’ll be building to improve the soil.

I’ll have access to seaweed, waste organic material such as coffee grounds and fruit and veg. I’m spending the cooler months to make biochar from the tree tops as well.

My thoughts are that I can have a couple of windrows and put the organics and biochar into the space between the rows. I can then mix from either side to make a mix and hopefully hot compost mounds.

Is this a valid plan? Anything else I should be adding, not adding or doing?

Edit: I should have said I’m building a Jean Pain style compost heater for the first 5 years or so and that’s going to produce about 10m3 of humus each year.

You need masses and masses of organic matter. Composting is great but you have to get more than kitchen scraps. Look everywhere for <free> biomass:

Tree trimming services
Ranch waste
Friends with horses?
landscapers
sawmills
Autumn leaves

This is going to take time and effort. Remember that large amounts of biomass can get quite hot piled up into windrows, which is good for composting, but on a large scale is a lot of work without a tractor front bucket. If its not kept wet enough you can risk combustion (specially if your climate is a bit hot and dry). I would go for no more than 12 in deep layer rather than windrows and keep a clear area immediately around the house.
Fortunately I had a friend with horses and brought in around 4-5ton manure each Autumn. The soil was pretty good after ~ 6 years! This was just 1/4 acre in suburbia.

You will notice that the ground will be very "hungry" with all that sand. In a  blink all that biomass will just melt away, but your soil fertility will be much better and you will have much better luck establishing your foundational planting. By around year 3 tree/shrub and other garden waste will be generating substantial amounts of mulch.

Now is a good time to think very carefully about the local flow of water. Since you are on a sandy loam your difficulty will be with having too much drainage and the land will be droughty. A lot of recommendations in books and online often give advice for clay-loams and will not be suitable as the techniques are all aimed at moving water away from plants and improving moisture. Rather than planting on hills go for planting in dips, buried rotting timber (e.g. Hugel), swales to try slowing down run-off (within reason, don't end up directing water into your basement) plus lots and lots of organic matter.
Sandy soils means looking for more drought resistant plants. One trick: if its grey or silver its probably going to work well. Many drought resistant plants have fine hairs or waxes on the leaf surfaces giving them that glaucus look. You will probably get sick of seeing grey everywhere.....but later on you can establish more interesting plants. Work with your environmental conditions, not against them.

Thanks John, excellent advice. I’m looking for a slow and small solution at scale. I maybe also be wildly over optimistic about the amount of time I’ll be able to spend on this project while building a house.

We’ve had some pretty decent storms and I’ve seen the surface runoff is minimal so the droughty advice is so what we want. We’re capturing water with our site works but there’s also a band of clay on one part of the site so it might be elsewhere Planting the tree in holes works too.

I’m contacting local horsey people to see what they have available. I’m not sure about the local zoo which has mixed carnivores and herbivore waste. Biochar feedstock won’t be a problem. We can burn from April to October so that will be making it frequently in a cone kiln.

Probably will get a little loader to do the mixing and I’m looking for a hot pile in sections so we can manage that fast process. I’ll add the char with the manure as adjacent.
Water is a must I agree.

I can get one 70m swale and a couple other shorter ones, plus a mulch detention pond and some Vetiver grass for around the driplines of the 3 trees they allowed me to keep. we’re at 1:4 slope so our terraces are going to be 1500 across.

Start a compost pile as compost makes very rich soil.  Why not check with local restaurants for vegetable scraps.

I like the suggestion by John to watch the local flow of water. Observation is the first principle of permaculture.

The seaweed is a massive advantage if you can get enough of it. Sandy soil eats organic matter fast though so don't expect it to hold onto improvements the way clay does. Honestly the best thing I ever did with poor soil was just keep mulching and let the worms do the work, trying to mix stuff in properly was a waste of time compared to just piling it on top.

Joao Winckler wrote:The seaweed is a massive advantage if you can get enough of it. Sandy soil eats organic matter fast though so don't expect it to hold onto improvements the way clay does. Honestly the best thing I ever did with poor soil was just keep mulching and let the worms do the work, trying to mix stuff in properly was a waste of time compared to just piling it on top.

That’s interesting. I have a seam of clay down on the lowest corner but haven’t looked at the rest of the block. I’d assume that the infiltrated water would follow along that seam.

What can I do with that information? Is there a blend I could make?

What about accessing the water that follows the seam?

Straw mulch and wood chips are good for clay.

With this size of surface area, you really got to get growing your organic matter: alternate cover crops of daikon radish mixed with clover in one rotation, and rye grass or sorghum on the other. This way you will have organic matter building both underneath and on the surface. A few cycles of this and you should have soil building nicely.

Ross,

This soil test did not copy and paste as well as I would have liked but this is from native prairie on our farm. Note the soil composition as expressed by Cornell Soil Lab, mostly sand, silt and some clay. How does this relate to your soil, I don't know, but soils here are comprised of mostly sand. I like oats as a soil building pioneer, does OK on few nutrients and a great nutrient scavenger.


Measured Soil Textural Class: sandy loam
Sand: 79% - Silt: 8% - Clay: 12%
Group Indicator Value Rating Constraints
physical Predicted Available Water Capacity 0.15 64
physical Surface Hardness 250 16 Rooting, Water
Transmission

physical Subsurface Hardness 250 68
physical Aggregate Stability 37.9 65
biological Organic Matter

Soil Organic Carbon: 2.5 / Total Carbon: 2.86 / Total
Nitrogen: 0.2

3.1 91

biological ACE Soil Protein Index 4.6 22
biological Soil Respiration 1.4 100
biological Active Carbon 696 89
chemical Soil pH 7.4 95
chemical Extractable Phosphorus 6.1 100
chemical Extractable Potassium 75.2 100
chemical Additional Nutrients
Ca: 9855.4 / Mg: 684.5 / S: 27.5
Al: 1.9 / B: 1.7 / Cu: 0.04
Fe: 1 / Mn: 13.7 / Zn: 0.2

77

Overall Quality Score: 74 / High

I spoke to the contractor who is doing the earth works on my block prior to building and his estimation is 30m3 of topsoil and vegetation to be stockpiled. So I have to do this at scale.

I’m thinking about three Jean Pain style compost heater piles. Layer branches at the bottom to allow air in. A layer of brown as spoil, layer of green - food waste, manure & coffee. Sprinkle of charged biochar, a layer of seaweed for minerals, layer of soil - repeat except for branches.

Run a few stakes from base to top while building to allow for airflow.

All layers appropriately moist.

I haven’t figured out what to use as an outside buffer, straw isn’t cheap here.

This will run over winter and should be ready to use by next summer.

Any suggestions or critiques welcome