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Using Solomon's worksheets to determine soil additives

Soil prescription

The second page of Solomon's soil worksheet shows how to create a unique prescription of additives to get your soil back in balance.  Once again, I prefer a spreadsheet to do my math for me, but I'll work through the prescription for yesterday's soil sample to help you understand the process.

The first step just consists of copying the numbers from the last column of the other side of the worksheet to the first column of this worksheet.  Anything within 10% of the target amount can be left alone, but we'll need to come up with a plan to decrease larger excesses and fill up larger deficits.  To do so, choose amendments from chapter six of The Intelligent Gardener and add amounts until the final application rate of each element comes out the way you want it to.

Digging in the gardenIt's usually best to start at the bottom of the chart since Solomon recommends using sulfate fertilizers to address any deficiencies in iron, manganese, copper, and zinc.  The sulfur that these fertilizers bring along for the ride will decrease the amount of sulfur you'll need to add at the top of the chart in the form of ag sulfur or gypsum.  Solomon has included a handy reference chart at the bottom of the worksheet showing the percent of each element in various fertilizers, and the last three columns help you keep track of how much sulfur, magnesium, and calcium come along for the ride in fertilizers intended for other purposes.

So, for zinc, you'll notice I'm 17.52 lb/acre short, but Solomon limits applications each year to 14 lb/acre.  To get that 14 lb/acre, I divide 14 by .35 (the percent of elemental zinc in zinc sulfate, shown in the table at the bottom of the worksheet), and come up with an application rate of 40 pounds per acre of zinc sulfate.  I multiply that by .17 to come up with the pounds per acre of sulfur that will come along for the ride, and insert the result (6.8) in the sulfur column as a reminder to myself that I've already met part of the sulfur demand.

If I were going to apply phoshorus to this pasture, I'd hit this row next since most sources of phosphorus also bring along some sulfur.  However, I estimate our ten chickens will add 47 pounds per acre of phosphorus to these pastures each year, and I don't mind a slow-but-steady accumulation of phosphorus, so I'm not going to add any extra.  (All of my vegetable garden areas actually have an excess of phosphorus, suggesting that topdressing with a layer of horse manure would be another good way to increase the phosphorus levels in the chicken pasture while also increasing the low organic matter, so I may consider that if we have extra manure in the spring.)

Zinc in soilNow I head to the top of the chart and decide on the type of sulfur amendment.  This pasture is acidic and low on calcium, so I don't want to add ag sulfur (which would lower the pH further and leach calcium).  Instead, I settle on gypsum.  I've already got 10.3 lb/acre sulfur coming with my zinc and copper fertilizers, so I only need to add another 94 lb/acre elemental sulfur.  Gypsum is 17% sulfur, so I divide 94 by .17 and get 553 lb/acre as my application amount.

Gypsum also contains 20.5% calcium, so I multiply 553 by .205 and get 113 lbs/acre elemental calcium, which I add to the far right column as a note to myself.  Subtract that from the 611 pounds of calcium in which my soil is deficient, and I need 498 more pounds per acre elemental calcium, or 1,277 lb/acre ag lime (assuming a calcium content of 39%).  (Dolomite would be a bad choice for my calcium source since I have too much magnesium in my soil already, but if I had a cheap source of oyster shells, they could work to fill this deficit just as well as ag lime.)

The combination of gypsum and calcium will flush out excess magnesium and potassium from the soil, but I'll want to test again at during the same month (and with the same lab) next year to make sure the soil is getting more in balance.  If I lived in a dry area, I'd also want to water the soil heavily after applying gypsum to ensure the excess cations moved out, but lack of water isn't a problem on our farm.

Meanwhile, Solomon's prescription calls for 114 lb/acre sea salt to deal with my sodium deficiency, but I'm figuring I can just add urine to increase the sodium levels.  (I'm not so sure it makes sense to try to increase these levels right now anyway since sodium is lower on the totem pole than potassium and magnesium and will presumably be washed away at the same time those cations are.)  Finally, 14.2 lb/acre of borax rounds out the soil prescription.

Calculating area of a pastureThe final bit of math is to figure out how large the garden area is and to calculate an actual amount of each fertilizer to add to the pasture.  These two little pastures add up to a twentieth of an acre, so I'd multiply all of my amounts by 0.05. 

Solomon recommends mixing anions (sulfur, phosphorus, and nitrogen) into compost before applying since humus hangs onto these negatively charged particles and releases them slowly into the soil.  Boron is an anion, but it's very easy to accidentally apply too much of this nutrient, so the best option here is to mix the proper amount into water and spray it evenly on the ground.  The cations (including copper and zinc) can be mixed together and spread across the soil surface (or dug in if you till).

Next week, I'm going to continue this lunchtime series with analysis of other parts of our farm, but I do want to add a caveat here.  Solomon recommends adding some fertilizers which aren't organically approved and/or which might cause short-term harm to your soil microorganisms.  Gypsum and lime seem to be relatively innocuous (as long as you check application rates carefully), but I need to do some more research before deciding how I feel about borax, copper sulfate, and zinc sulfate.  On a small scale, choosing dynamic accumulators that concentrate the same nutrients and using them as part of your compost pile could be a more harmless solution, and some people use rock dusts to boost micronutrients (although Solomon thinks they don't provide enough minerals to make the dusts worth your while).  I'm curious to hear from anyone who has done more research than I have about these amendments.

Thanks for hanging in there through all this math.  Hopefully the analyses week after next will have fewer numbers and will help broaden these ideas out so you can see the big picture.  (Next week we're taking a break to talk about cover crops.)

Trailersteading shows how to homestead on the cheap.



This post is part of our The Intelligent Gardener lunchtime series.  Read all of the entries:




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Just wanted to say thanks for going over this. As someone who is working to get more serious at a garden (been a major brown thumb all my life) this is really helpful. I am just letting it wash over me at this point but will be referring back to it when I am able to actually apply it.
Comment by Christopher Scoggin Fri Jan 18 13:32:12 2013