From the Ground Up 9b

Fertilizer: How much is enough?

by Derek Burch

Let's assume that a properly done soil test has shown that the major elements are present in your soil in amounts that the testing group considers to be satisfactory.

In very broad terms a plot from which nothing is taken away would stay that way, but in fact that is not strictly true. We can't account for the addition of elements brought up from the lower reaches of the soil, or for what washes out with heavy rains, or any of the other things that would have to be measured if we wanted to be sure that it is so.

What would be a safe starting point for a fertilizer program - not too little to replace losses and not so much that the salts level in the soil builds or washes into the ground water to add to problems there? With all the reservations and caveats that need to go with it, most recommendations run in the region of 1-1.5 pounds of elemental nitrogen per 1000 square feet at each of two or three applications a year - say 500-750 gms for 100 square metres if I have my metric conversions right.

The amount of elemental nitrogen is given as a percentage in the fertilizer analysis (it is the first number when three major elements are given). To take an easy example, a 10-10-10 fertilizer would have 10 pounds of nitrogen in 100 pounds of fertilizer. (The other numbers represent compounds of phophorus and potassium respectively rather than the actual element, and are given this way from the tradition of analysis followed.) So, back to the nitrogen, 10 pounds of fertilizer would have the required one pound of nitrogen. A 6-6-6 would need about 16 pounds of fertilizer for the one pound of nitrogen, and so on.

The other two or the major elements given in a typical analysis are usually ignored in this calculation since they are fixed by the analysis of the fertilizer mix, but obviously you can control how much of each is being used by choosing the correct analysis. In practice, any N-P-K ratio from a 1-1-1 to 3-1-2 will work for most situations, although my personal choice is only rarely to go with a nitrogen level higher than that of potassium.

Oh, dear, generalities! This guideline hides so many more things than it illuminates! And does nothing to look at whether the fertilizer has slow-release characters, or the form of each material that is used in the mix. These can be very important considerations.

Using liquid feed is another matter. It has one strong advantage, which is that the analysis or the amount can be changed with each application and will have a very rapid effect. Applying at rather less than the recommended rate on the box is a good starting place.

How should you use this generalised recommendation? Obviously with caution if not downright scepticism. Make a table. if you like, with that figure as a starting point and add to it if you are doing heavy cropping, or do not compost your garden waste and return it. Subtract if you do compost, or if you bring in materials such as stable manure, or if you have a garden of mature trees that do not intend to grow more. Factor in your soil type - everything washes out of sandy soils, fertilizer elements stay around in clays and humus-rich situations.You can see the idea.

Next time I will try to get into fertilizer components, if anyone is still hanging on.

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