Amending soil

We had some soil testing done last year before the garden was manured and cover-cropped, so things have certainly changed since then. (Back then we combined 20 random samples from all over the garden and sent a small portion of the mix to the soil testing service.)

At that time, MSU gave us recommendations for amendments in terms of 'synthetic fertilizers to add per 1000 sq.ft.'

I converted those numbers to 'quantities of organic amendments that might benefit an individual plot of 400 sq.ft.'

Gonzalo checked my math and edited for clarity, and added much helpful information about what different nutrients are good for, when to apply, how not to overdo it, etc.

Hope this is useful info for some - read all the way down, including the notes and caveats!

  • Using a moderate of compost or manure is your safest bet.
  • Before amending, new soil tests are definitely in order.

I'm far from an expert, so conversation and commentary from others is very welcome.


2009 MSU recommendations, converted to  the needs of a 20x20 plot:

N       1.2 lbs per 400 sq.ft.
P       2.0 lbs per 400 sq.ft.
K       1.8 lbs per 400 sq.ft.

Using organic sources this translates to the following:

8 lb blood meal, or 16 lb of cottonseed or soybean meal

Note: Very important for leafy greens, less so for fruit and root crops - so application will depend on what you are growing.  Also if using blood meal best to spread small applications throughout the season, avoiding using on
seedlings and peas and beans. Cottonseed meal releases nitrogen slowly - so first application should precede planting by several weeks.

16 lb of bone meal or 13 lb colloidal phosphate

Note: Important for root crops. Also contains calcium (good for fruit) and also may contain nitrogen. Slow release - apply a month before planting - mineral phosphates elevate ph (not good in our soil) and will last several seasons.

Greensand is the traditional organic source of potassium - but converting MSU's recommendations to using greensand (that has 3% K) gives us a very high number - about 60 lbs. Some sources recommend 20 lb per 20 X 20 plot. Still seems like a lot. Other sources of potassium, like wood ashes have the unfortunate side effect of elevating the pH, which we do not need, so use sparingly if at all.

NOTE 1: if using a moderate amount of compost or manure the numbers can be halved. Our soil is fairly alkaline - adding these makes more nutrients available, and lessens the need to fertilize.

NOTE 2: best to halve the numbers in any case. Although organic sources are not as aggressive and quick acting as inorganic, it is best not to overdo it.

NOTE 3: although the conversion from MSU's recommendations of inorganic NPK per 1000 sq feet to organic sources for 400 sq feet are accurate as these things go, their numbers derive from before we added manure and green manure
last year. It is very likely that we need considerably less NPK this year, but another soil test is in order to determine this.