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I recently read a NY Times article about straw bale gardening. It says that you should add about 2 1/2 cups of 20% N fertilizer or three pounds of blood meal and feather meal per bale. Add water, wait for the temperature to decrease, and then plant.

I would really appreciate a more objective approach - but I can't find it in any of the guides (including the few instructions from ag extensions*) that I reviewed.

A typical two-string bale is 45lb with, say 10% moisture, so it has 40 lbs (18kg) of dry straw. composed of 45% C and 2% N: a carbon to nitrogen ratio of 24:1 (according to the USDA-NRCS).

What I am missing is the target range of C:N for the bale - for a mixture of vegetables (tomatoes, squash, beets, carrots) as seen in the article. Then, I could better estimate - and evaluate - what I am doing.

If I knew the target C:N ratio, and how to account for the uneven decomposition and N loss (leaching, denitrification) as well as the "tolerance" for over / under fertilization (i.e., the range of applications rates that would optimize food production), then I could take an arbitrary fertilizer with complete nutrition (NPK+) and have an estimate of how much to add based on the % N.

in summary, my questions are:

  1. What is the target C:N for a straw bale garden?
  2. How much fertilizer should I add, in units of mass (g or lb) N?
  3. What is the range of values in 1 and/or 2 that would optimize mixed vegetable food production?

* for example, while the OKState extension says "The nutrient supply for plants established in straw bales is also critical." but gives no information on fertilization rate, only that insufficient nutrient supply can be be diagnosed (yellow leaves, unhealthy plants, etc.).

  • The C:N ratio of straw is more like 70:1 -- double check the table in the NRCS link. – bstpierre Apr 5 '13 at 12:33
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The composting process works best when the C:N ratio is about 30:1. So you will want to add N to reach that ratio. If a 20kg bale is 45% C and 0.5% N (section 4), there's about 8.25kg C and 0.09 kg N or about 90:1 C:N. If you add 0.25 kg N you'll be around the 30:1 ratio.

Now, that 30:1 is ideal for composting, but there won't be anything left over for growing vegetables or to account for losses from the system. Keep in mind that vegetables need a fairly wide range of N: peas can pull N from the air, for example; spinach and other greens are N-hungry; carrots don't need much; and providing too much N to tomatoes will result in lots of vine and not much fruit. You also need to make sure you provide the other nutrients that plants need (P, K, Ca, S, Fe, etc) -- some of these may be present in your straw bales, some may not.

Also keep in mind that N is volatile: you're going to lose some over the growing season, and this is hard to quantify. I find it easiest to adjust on the fly -- observe how the plants are doing and if they need supplemental fertilizer mid-season, give it to them. (If my spinach starts to yellow, I know it's low, so I sprinkle a little blood meal around the beds.)

I think the right answer will be to add whatever amount of N you'd use when growing in soil for the particular crop in question. E.g., more for spinach, less for peas and carrots.

  • thanks. At first approximation - I would assume more N would be lost via leaching than volatilization. I believe volatilization is associated with an anaerobic process (dentirification) ... which leads to the follow-up: is it likely that the bales would get anaerobic (I was watering them today and the answer didn't seem obvious)? – Abe Apr 5 '13 at 15:34
  • Are you leaving them tight, or breaking them apart? I'm not sure, but it seems like they should be getting some air because straw is hollow, potentially carrying some air to the center. Though tightly packed and water saturated could make them anaerobic. – bstpierre Apr 5 '13 at 15:59
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The idea behind the straw bale technique is to take a predominantly carbon source, add nitrogen to parts of it to create compost, and to grow plants which are going to use the compost/humus while it is being created.

The nitrogen provides the energy for composting bacteria to convert the straw to humus thereby freeing up the nutrients in the straw so that it becomes accessible to the plant roots. The plant will then take up what it needs. So, in a normal garden, you add nutrients for the plant. Here you create them in situ.

Someone has calculated the carbon in a straw bale to be 8.5 kg or 18.7 lbs (http://earlywarn.blogspot.co.nz/2010/08/carbon-offset-value-of-straw-bale.html). The carbon nitrogen ratio for straw is 70:1 so the amount of nitrogen present is 18.7 / 70 which is 0.25 lbs. To make compost effectively, we need a ratio of carbon to nitrogen of over 30:1. Ratios of 30:1 and over minimize nitrogen loss which would otherwise be lost through volatilization. So, we need to add approximately a further 0.25 lbs of nitrogen to compost the straw bale.

Joel Karsten advises adding over the 12 day conditioning period 2.25 cups of a lawn fertilizer, and finishing on day 10 with a cup of 10:10:10 fertiliser. With 4 oz per cup, the total weight of fertilizer being added is 0.81 lbs. This seems to result in a ratio higher then 30:1 for the whole bale but this makes sense as he wants to compost the inside of the straw bale, and not the outside which is providing insulation.

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