Preparing new garden beds: real-world math problems

We have about half a dozen buckets of wood ash

Now that winter's almost over, Bea's been reviewing some old gardening notes about which are the best organic sources of nutrients to use as soil amendments, and she came across a note that hardwood ash is high in potassium.  That's great news, because Chris has made quite a stash of 5-gallon buckets of stove ashes!  The caveat here is that our starting pH is so low (5.4) that our soil test analysis already recommended adding 13 lbs of limestone per 100 square feet -- so since wood ash is alkaline, we can cut back a bit of the limestone and use the ash as a potassium source without worrying that it will adversely affect our pH.  However, if your pH is where you want it or slightly high, use wood ash sparingly or not at all.

Chris raking in the soil amendments

By now, we have half the beds dug up in the hoophouse and last week (before this most recent round of snow) we were out there prepping our soil and planting our first spring crops!  Since it was helpful to us to review the math that goes into soil amending, we'll go over it here in case it helps anyone else who's ever wondered what those 3 dashed numbers on the bags of fertilizers were for!






1.  Get a soil test!

DON'T bake your soil to make it dry faster,
but old cookie sheets make great drying racks
for soil samples!

This is critical to having a successful garden, but lots of new gardeners skip it, thinking it's difficult or expensive.  You can test your own soil with a basic kit from a hardware store, or send it off to a lab for around $10 to get way more finely-tuned and detailed results.  Historically, we've sent ours to UMass because they were the cheapest, but now that Bea's a Master Gardener through Virginia Tech, we should really encourage use of their soil-testing services!

In true Fixettwell fashion, Bea brought a trowel and handful of ziplock bags to our 2nd appointment to view this farmhouse, stalled Chris from leaving until after our realtor pulled out of the driveway, and then started digging up the lawn in various places to take soil samples!  Oh, and it was pouring down rain.  Luckily, we didn't meet any of the neighbors that day; we would have been such a sight!  The sodden soil she collected took a long while to dry out, so it turned out we had already made an offer on the house by the time we got our results back.  Luckily, they showed no lead or other major issues of concern.  Just good ol' acidic Virginia red clay, as expected.

2.  Interpret your results

If you send your soil samples off to a lab, usually you've specified a crop code of what you intend to grow in each area that was tested so the lab can give you custom-tailored results about what nutrients to add to achieve ideal growing conditions for your selected crop.

In our case, the lab has drawn us a helpful graph to easily visualize what we need and what's already present in sufficient concentration in our soil.

Furthermore, we specified that we want to grow Mixed Home Vegetables so they've adjusted our recommendations accordingly, and tailored our recommendation for “per 100 square feet" as opposed to “per acre".  Be sure to read your results carefully so you're amending with the proper proportions!

Note that the lab hasn't told us specifically which products we should add to achieve desired results, they've told us which nutrients are deficient and what we need to do to adjust our pH.  This is where the decisions and the math come in!

3.  Decide which soil amendments to use
Since we're gardening organically despite not having to worry about certifications, we're limiting our choices to products approved for use in organic farming.  Some common organic sources of major and minor nutrients include:

• pH adjustment – dolomitic limestone – raises pH, also contains calcium and magnesium
• Nitrogen – finished compost, composted manure, alfalfa meal, dried blood
• Phosphorus – bone meal, rock phosphate, soft rock (colloidal rock phosphate)
• Potassium – wood ashes, greensand, azomite
• Micronutrients – individual nutrients, kelp meal, azomite

Look at that smile!  Truckaroni is happy
to be running and put to work again!

Last month, Bea looked in our shed and took stock of which amendments we had left over from previous years gardening.  She then took the truck up to Southern States to buy what we still needed, including 400 lbs of limestone.  This turned out to be a total disaster because you go in and tell the clerk you want limestone and they ring you up, then you take the receipt out to the warehouse where they load you up.  Well, it was raining and Bea decided to act like a Southern girl and stay warm & dry in the truck while the guys loaded it, so she got home with 400 lbs of what turned out to be a type of chemically-treated limestone that's apparently meant for water filtration, definitely NOT for organic gardening!  Luckily she realized this before unloading the truck.  Still, it sat there for a couple of weeks with us being too embarrassed to go exchange such a large order.  Then we got over ourselves and went back to beg forgiveness and get an exchange, so the poor warehouse guys had to unload 400 lbs of one type of lime and reload 400 lbs of the correct type.  The warehouse guy only had one bag explode on him during the process.  Somehow Chris also managed to lock the keys in the truck (a first!) so we had to sit around the parking lot for 2 hours waiting on AAA to come bail us out.  Certainly not our best day in history, but we survived.  Luckily there are 3 Southern States within the same radius of us so we don't have to show our faces there again!

Anyway, based on our analysis and on what we have available, we decided to add limestone, bone meal, and wood ash, plus of course plenty of compost to boost nitrogen and organic matter.

4.  Do the math!

Any fertilizer (organic or conventional) will have an “analysis” shown somewhere on the bag (these are the 3 numbers with dashes between them).  The numbers stand for the percentages of N-P-K (Nitrogen, Phosphorus, Potassium) contained in the fertilizer.  For example, our 10 lb bag of bone meal has an analysis of 4-12-0, which means that 4% of the 10 lbs or 0.4 lb of pure N, 12% of 10 lbs, or 1.2 lbs of pure Phosphorus, and 0% of 10 lbs, or 0 lbs of pure potassium are contained in the bag.

Since our soil test analysis recommends adding ¼ lb of phosphorus per 100 square feet, we need 0.25 lbs of pure phosphorus from our source – in this case the bone meal. We just calculated that in the full 10 lb bag we have 1.2 lbs of pure phosphorus, so simple division shows us that in each pound of bone meal we have 0.12 lbs of pure phosphorus. We then need to apply about two pounds of bone meal to get the recommended ¼ lb of pure phosphorus (0.25/0.12 = 2.0833333 lbs, but we're rounding for simplicity).

Since we don't have a scale set up in the hoophouse, we convert this weight to volume.  Rather than come up with a ratio of weight-to-volume, we just used our handy kitchen scale to weigh a clean yogurt-tub (quart) full of each amendment.  In the case of bone meal, the yogurt tub weighed 2 lbs -- so we need to apply 1 yogurt-tub-full of bone meal to each 100 square feet of garden area in the hoophouse!

We did similar math for our wood ash, although since it's in 5 lb buckets and obviously wasn't labeled with any analysis, we googled the analysis (0-1-4) and just weighed the yogurt-tub full to figure out how much we'd need to get our recommended application rate of 0.1 lbs of potassium per 100 square feet.

5.  Prepare your beds

Remove sod

Add compost

Dig the beds deeply

Soil amending should be the last thing you do right before you plant so your plants/seeds get the maximum benefit.  In our case, we had to remove sod, add piles of compost, dig that deep down into the beds, and then rake everything smooth (removing quite a lot of small rocks and breaking up chunks of clay in the process).  We treat compost as a necessity to be added every year, and then add other amendments near the surface as recommended on the soil test.

6.  Spread your amendments

When applying amendments, you want to sprinkle the recommended amount of fertilizer as evenly as possible over the full 100 square feet.  Then take a rake and lightly sift the amendments into the top 3 inches of the garden bed. 

7.  Plant your seeds!
Once your beds are prepped and ready, you're ready to plant!

Newly prepped and planted beds of root crops, onions, & spring greens

Then we just had to repeat the process on the other half of the hoophouse!