How to translate a Non-Organic Soil Test for Organic Gardening

captaincompostal(z7 AL Bham)January 1, 2013

Do you ever feel alone when trying to discuss fertilizer needs or pH test results with non-organic gardeners? Well, it is difficult. How do you interpret soil needs organically, from charts that were designed for synthetic gardeners?

Here is one suggestion. Let's assume that a traditional soil test suggests that you need to add 50 lb of a 12-4-8 customized synthetic fertilizer to your soil to raise the classic NPK needs. You as an organic gardener don't want to deal with the synthetic fertilizers, but you want to build up your available soluble nutrients in your soil. Also since you are an organic gardener, you don't have to be so precise in your NPK calculations, because your soil amendments have plenty of OM in it and you are depending on the soil microherd to break down and supply the available soluble nutrients that your plants need from the existing soil and your new amendments. Also keep in mind that the normal P and K recordings usually are only the available soluble portions of P and K in the products, not the total portions of P and K. Not to mention all the trace elements in your natural soil amendments that the tradition gardeners forget about. What do you do?

CASE 1: Let's say you want to fulfill the basic N needs with blood meal (14-0-0), the P needs with bone meal (1-11-0), and the K needs with greensand (0-0-7). That means at least 14% of blood meal contains your N, approx. 11% of bone meal is your P, and approx. 7% of your greensand is K. Now to get close to 50lb of a 12-4-8 fertilizer, you need to first calculate how many pounds of NPK is in 50 lbs of 12-4-8.

That would be:
50x.12 = 6lb of N.
50x.04= 2lb of P.
50x.08= 4lb of K.

Now how much of blood meal, bone meal, and greensand does it take to equal the above amounts of NPK in lbs?

That would be for blood meal: 6lb /(.14 N) = 42.8 lb.
That would be for bone meal: 2lb /(.11 P) = 18.2 lb.
That would be for greensand meal: 4lb /(.07 K) = 57.1 lb.

Therefore 42.8 lb of blood meal for N, 18.2 lb of bone meal for P, and 57.1 lb of greensand for K, is approx. the same for NPK levels as 50 lb of 12-4-8 synthetic fertilizer.

CASE 2: How would you do the above translation with just homemade compost? Assuming most compost is between 1-1-1 and 4-4-4, let's go with an average 2-2-2 (that's 2%N, 2%P, 2%K). To get the same NPK levels as above for 50 lb of 12-4-8, here's a suggestion.

Since the 3 NPK numbers are the same for this compost example, let's calculate the largest number of the synthetic NPK, which is the 12%N portion of 12-4-8.

(6lb of 12-4-8 of N) / (.02 N in compost) = at least 300 lb of compost needed to satisfy the soil test recommendation of NPK levels of 50 lb of 12-4-8 synthetic fertilizer.

Of course these are just simple examples to help explain the vast differences in measurements between non-organic and organic gardeners. Unlike synthetic fertilizers that only stay in the soil for 1-3 months, compost keeps on working and breaking down for years, even decades!

The real truth is that contant year round composting, green manuring, and occasional natural foliar feeds like compost tea, should handle all your plant nutrient needs without being exact.

Happy Gardening!

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