Manufacturing Top Soil

We all know that you can’t believe everything you read on the internet.  But when I first started to look for articles for this month’s library (about making topsoil using biosolids), the first ‘hit’ I got was for a report that claims that the market for manufactured topsoil will be $7.8 billion dollars by 2021.  The asking price for the report was well over the budget for the library (hundreds or thousands of $$s), and so I haven’t read it, and it isn’t in the library.  But even if the authors are off by 50%, they are not talking chump change.

More and more, programs that produce Class A are realizing that they can vastly expand their customer base and their revenue by using the biosolids as part of a mixture.  Biosolids compost has long been the classic take on this approach.  However, there are other options besides woody materials and time and temperature to get to a nice product.  What works best depends on what amendments are available in your region.  Very often some type of inorganic or mineral material is used as a based.  This can be sand or dredged materials.  Biosolids, often with some higher carbon material, are mixed in.  If you get the proportions and the ingredients right, you have hit the jackpot.  But doing so isn’t always so easy.  In addition, the approach for testing your mixture requires a very different focus and set of tools than for calculating nitrogen availability for wheat or for calculating the risks posed by pharmaceuticals.  You want your mixture to grow pretty petunias, and tomatoes, and lettuces…. You want your customers to grow and smell the roses, not the biosolids. 

There aren’t that many papers on this topic, and, in my opinion, the few papers out there haven’t all gotten everything right.  Take the first paper.  The authors composted green waste, with and without biosolids, and then mixed the compost with river sand.  They tested a range of agronomic parameters on the material, including total nitrogen, electrical conductivity (EC), available nutrients and physical properties, such as water holding capacity.  This part is great.  There are likely a range of standard soil tests that can help you figure out if your mix is a good one.  But here, instead of planting flowers, they tested the microbial species richness of the different mixtures.  That parameter, while it gets you in a peer reviewed journal, does not sell topsoil.  The researchers did find that adding the biosolids to the green waste got the compost piles hotter, made the mixes more nutrient-rich (good) and caused higher conductivity (bad).  They also found that the biosolids made the mixtures more acidic and so increased fungal population. 

The same authors also wrote the second paper in the library.  Here they tested green waste compost alone, with poultry manure or with a mixture of grease trap waste and septage.  Just thinking about the last option is enough to make my watercress wilt.  It turned out that the EC in that treatment was high enough to limit germination of watercress.  Note here that the germination assay was not done using the actual soil mixtures, but rather an extract from the soil mixtures.  The extracts were added to Petri dishes with de-ionized water as the control.  Adding soil or soil plus coal ash to the mixtures and then getting the extracts helped to increase germination in the green waste alone and with poultry manure treatments.  Two lessons here: first is that this assay is not necessarily representative of what would happen if you put seedlings in a pot with the mixtures and watered them.  Second is that EC is a really important consideration.   If you live in an area where rain is not limiting, high EC will pass with a few storms.  However, in a more arid environment, high EC can be a real problem for your mixtures.  Woody material typically has low EC and is one option to consider for mixtures. 

The next two papers are by the real pros. This is the group that developed the potting soil mix for the City of Tacoma which has helped its biosolids program increase revenue.  I always grab a few bags when given the opportunity.  Rita Hummel is a true horticulturist, not someone that biosolids people typically worked with.  Her methods are industry standards and so are worth looking at.  The first paper focuses on chrysanthemums.  The mums are grown as container plants; they are typically grown and are watered with fertigation (meaning, injection of fertilizers, soil amendments, and other water-soluble products into an irrigation system) at both high and low nitrogen levels.  In addition to standard soil parameters (porosity, water holding capacity, EC, fertilizer value, and particle size), the range of plant performance variables measured merits attention.  We are transitioning here from a focus on assuring safety to a focus on growing lovely flowers.  Dry weight, shoot quality, width to height ratio and flower bud numbers were all measured.  Peat-perlite, the standard potting mix, was used as a control.  If you can do as well as or better than the industry control, you are doing just fine.  The Tagro did better.  The second study looked at marigolds and peppers using the same approach.  It is nice to read about ‘Little Hero Flame’ marigolds instead of little quantities of flame retardants. 

The final paper in the library takes this to the next level, in my mind if not the authors.  They grew a range of garden vegetables in regular soils amended either with biosolids, biochar or a combination of the two.  Now this paper doesn’t fit into this library, as they weren’t manufacturing soils with biosolids.  However, the basis of this paper can be used as a selling point for biosolids-based topsoils.  The authors note that one third of the people on this planet are zinc deficient.  By adding the biosolids or biosolids + biochar to the soils, they got bigger vegetables that had higher zinc.  This is a selling point.  The authors refer to this as ‘biofortification’.  Use biosolids or biosolids-based manufactured soils and not only do you get bigger vegetables, you get more nutritious ones! 


Sally Brown, University of Washington