How much fertilizer do I need?

This is one of the most common questions we get at Seven Springs.  The answer to this question is the same as the answer to most things in agriculture – it depends.  The amount of fertilizer to apply for a given crop or season will depend on…

The crop(s) you are growing - different crop families will have longer or shorter growing seasons and different nutrient requirements.  Production guides published by a regional land-grant university can tell you more about the general requirements for commonly grown crops in your region.    

Your goals for those crops – up to a point, there is an optimum amount of available fertility that can maximize the yield of a given crop.  Again, this will vary by crop family.

What you have to begin with – how much available fertility is already in your soil.  This will be the focus of this article.  

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The amount of available fertility in your soils will depend on the field history and management practices – previous crops, rotations, cover crops and tillage – as well as the soil type(s) and specific soil series.  To learn about the kind of soils you are working with you can use the Web Soil Survey tool.  

To know how much fertilizer to amend your soil with you will first need to know where you’re starting.  To determine this, you will need a soil test for each area you plan to plant to a given crop.  For home vegetable and flower gardens you can take a single composite test to get a general recommendation.  For commercial growing, you should take soil tests for each field.  If your soils are very consistent you can take fewer tests, if your soils are more variable you will want to take more tests to help you understand your different soil types.

Soil test kits are available from your local county extension office or from commercial labs such as Waypoint.  The test kit will give you instructions on how to take and prepare a sample.  Be sure to indicate the crop(s) you plan to grow and check the boxes requesting fertility recommendations and organic matter percentages.

After the lab has processed your samples, they will send you reports and recommendations for amending your soils.  Typically, these recommendations assume you will be using conventional fertilizers, like calcium nitrate or 10-10-10.  The important things to note are how much of each specific nutrient the lab recommends (typically expressed as lbs./acre or lbs./1000 sq ft), the soil pH and the organic matter percentage (OM%).  Soil pH and OM% will both affect the availability of nutrients to your crop(s).   

A soil test report for a field to be planted to corn might give you something like this:

Soil pH = 6.2 - Organic Matter % = 2.9 - Fertilizer, lb./A = N 140, P2O5 40, K2O 60

Converting soil test recommendations for conventional fertilizers into recommendations for the biologically-based fertilizers we offer is what we do every day and we will be happy to help you.  You can send us a completed fertilizer recommendation form, or if you’d like to do it yourself, we can share one of the tools we use. It’s a pretty straightforward process but there are a couple of key considerations.

The most important thing to know is that there is a difference between the plant-availability of conventional fertilizers recommended by soil labs and the biologically-based fertilizers used by organic growers, particularly as nitrogen is concerned.  The percentage of plant available nitrogen (or PAN) in conventional fertilizers like calcium nitrate (15.5-0-0) is typically 100%.  That means that if the stated analysis of calcium nitrate (expressed as % of N-P-K) is 15.5-0-0, 100 lbs. will contain 15.5 lbs. of readily available N.  One 50 lb. bag will contain 7.75 lbs. of available N.  The nitrogen in biologically-based fertilizers like composted poultry manure or feather meal must first be mineralized by soil life in order to become plant-available.  The illustration below details the nitrogen cycle.  

The portion of the stated analysis (or total N) of a fertilizer that ultimately becomes plant-available is referred to as the mineralization rate.  The mineralization rate will depend on the type of fertilizer, the activity and abundance of soil biology and the soil conditions (including soil structure, temperature and moisture content).  The mineralization rate for feather meal might be as high at 75%, whereas the mineralization rate for poultry manure typically falls between 25% and 55%.  This means that fertilizers like feather meal, with a 13-0-0 analysis will deliver nearly 10 lbs. of PAN per 100 lbs. applied (75% of 13 = 9.75), given strong soil biology and favorable conditions.  Composted poultry manures, like Harmony, will deliver between 1 and 3 lbs of PAN per 100 lbs. applied.  A good rule of thumb is to use 75% mineralization rates for fertilizers based on animal proteins like feather meal, blood meal and NatureSafe fertilizers, and 50% for manure-based fertilizers like Harmony and Symphony.    

For the corn example above, we would want to use 1400 lbs. of feather meal per acre (140 lbs. of N are recommended, 10 lbs. are available per 100 lbs. of feather meal, 140 x 10 = 1400).  You can follow the same steps to calculate the amount of P and K required.  The plant availability of P and K in the amendments we offer will be the same as the stated analysis in most cases.  To get the 40 lbs. of recommended P, with something like bone meal (2-14-0) take the recommended number of lbs. per acre, divide it by the percentage in the fertilizer analysis and you will have the amount of fertilizer you need to meet the recommended amendment.  For the corn example, we would take 40, divide it by 14% (or 0.14) which gives us 285.  So, we need 285 lbs. of bone meal per acre to get 40 lbs. of P.  For the 60 lbs. of K, if we want to use potassium sulfate (or SOP) granular (0-0-50), it looks like this: 60/0.50 = 120 lbs. of SOP for 60 lbs of K per acre.    

Of course. things get more complex when you have fertilizers with a mixed analysis.  This is where spreadsheets come in handy.

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