Mastering the Art of Soil Nutrients: Blending Intuition and Science for Optimal Soil Health
I grew up in a family of strict recipe followers; every baking project followed a structured procedure. Flour was always sifted twice – once before measuring, once after – and we used a special knife to accurately level flour in the measuring cup. Every instruction was followed precisely.
As a rebellious teenager and young adult, I improvised everything in the kitchen. Recipes were for conservative people; I scorned measuring cups and substituted ingredients with aggressive and sometimes unpalatable creativity. Although I still love improvising in the kitchen, I have since matured into a respect for the precision of following a recipe and understanding the underlying chemistry. I have discovered my cooking is consistently better as a result.
Managing soil is not unlike navigating the culinary world; you can operate with a level of intuition that is liberating, or you can operate governed by a strictly scientific approach. Most successful vegetable growers strike somewhere in the middle: embracing a holistic approach to their soil health through cover cropping, composting, and minimizing tillage complemented with carefully calculated soil amending. While soil tests play an important role in nutrient management, they don’t replace the knowledge of a farmer who works the soil every day. Soil tests are the fancy four-layer cake recipe you refer to only after you intuitively understand the basics of making a simple batter.
With a little extra planning and attention, implementing a specific “recipe” for soil amendments based on a soil test can transform a healthy, thriving farm into a vastly more productive one. There are two key elements to consider which can lessen the intimidation of creating a soil amendment plan. 1.) Usually the soil test comes with nutrient recommendations for lime, nitrogen (N), phosphorus (P), and potassium (K) calculated for you; and 2.) The soil lab, your state’s extension service, or a local fertilizer dealer are usually happy to offer assistance in interpreting soil test results and may even suggest which materials to use.
Quick Tips on Interpreting Your Soil Test Results
Note that soil test results may vary by lab because different nutrient extractors are used, and there is no easy way to compare results (it’s like comparing metric to English measurements). Standard soil tests typically cost around $14.00 and most state universities provide this service.
Soil pH. A soil pH of 6.5 to 6.8 is ideal for most vegetable crops. Lime is used to raise the pH, while elemental sulfur can be used to reduce it. Lime recommendations are made on most standard soil tests based on the pH of the soil as well as the soil’s buffer capacity (heavy textured soils require more lime to change the pH than a light, sandy soil). Lime is affordable, easy to apply, and best if broadcast in the fall for the following season. You can use Ag lime (calcium carbonate), or if you need magnesium, use Dolomitic lime, which contains some magnesium carbonate along with calcium carbonate. Wood ash can also be used in place lime, although you need roughly two times more ash than lime for an equivalent shift in pH. Wood ash is also a source of phosphorus and potassium, close to a 0-1-3 fertilizer analysis. Visit this page for more information on wood ash.
Nitrogen recommendations on your soil test are merely a guide and NOT based on any measure of the nitrogen in your soil. This is because “available” nitrogen fluctuates over time based on temperature, moisture, and microbial activity. The key to effective nitrogen management is to understand what “credits” you already have in your soil. These credits come from soil organic matter built by previous applications of compost, manure, and cover crops. This fact sheet has an easy-to-use chart for calculating your nutrient credits. Once you have calculated your credits, subtract these from the nitrogen recommendation on your soil test to determine how much you should actually apply. For more on nitrogen management, click here.
Other nutrients. Aside from nitrogen, plants require additional nutrients from the soil including phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg). Your soil test should indicate how much additional phosphorus and potassium (in the form of phosphate and potash, respectively) your crops require, but for P and K, similar to calculating your nitrogen credits, you need to account for the levels you have already added to your soil with compost and manure applications. Ca and Mg levels are typically maintained by liming. Micronutrient and sulfur (S) deficiencies are rare in the Northeast, and optimum ranges are not defined. If your levels are low, the first step is to make sure your soil pH is in the optimum range. Additions of compost or a volcanic mineral fertilizer may help boost micronutrient levels. Boron (B) is the one micronutrient to pay special attention to, as it is often low on vegetable farms, particularly those with sandy soils. If B is low, 1 pound per acre of B is advisable for cole crops, spinach, and beets (Solubor is an easy-to-apply, OMRI-approved product).
After interpreting your soil test, you will need to calculate how much of your chosen fertilizer you need to supplement your existing nutrient levels. If you selected a crop code when you submitted your soil test, you should have crop-specific nutrient recommendations from the lab included on the test results (written either in pounds per acre or pounds per 1000 square feet, depending on your scale.) If you didn’t select a crop code, the New England Vegetable Management Guide is an excellent reference on crop nutrient needs for New England.
Calculating Bagged Amendments
All fertilizers are labeled by the percentages of nutrients they contain in the order N-P-K. For instance, the analysis of a bag of blood meal is usually around 12-1-0.5, which means it is 12% nitrogen, 1% phosphorus, and 0.5% potassium. Suppose you determined (after subtracting your nutrient credits from the crop recommendation) that you need to apply 50 pounds per acre of nitrogen, how much blood meal would you need to apply to meet the crop’s N needs?
RECOMMENDED NUTRIENT RATE ÷ % NUTRIENT IN FERTILIZER = AMOUNT OF FERTILIZER TO APPLY
50 LBS/ ACRE N ÷ 12% N = 416.7 LBS of blood meal per acre
Or suppose that you have a garden and that your soil test recommends 2 pounds of potassium per 1000 square feet. How much potassium sulfate (0-0-50) should you apply?
2 LBS/ 1000 SQ FT K ÷ 50% K20 = 4 LBS of potassium sulfate per 1000 square feet
Sometimes it is most economical to use a blended fertilizer to meet your crop nutrient needs. For instance, if your recommendation is to add 2.5 pounds of nitrogen, 2 pounds of phosphate, and 1.5 pounds of potassium per 1000 square feet, you could use North Country Organics Pro-Gro, which has a 5-3-4 analysis.
In this case, you can meet your basic nutrient needs with one blended fertilizer application. If the numbers don’t work out quite so tidily, you can always supplement with another single nutrient fertilizer. Below is a list of common organic fertilizers and how much of each is required to add one pound of actual nutrient. For a price breakdown by fertilizer, visit this site.
|Fertilizer||Typical analysis||Amount of fertilizer needed to add 1 lb of nutrient|
|nitrogen (N)||phosphate (P)||potash (K)|
|alfalfa meal||3-0.5-2.5||33 lb||200 lb||40 lb|
|blood meal||12-0-0||8.3 lb||--||--|
|bone meal||3-15-0||33 lb||6.7 lb||--|
|compost||1-1-1||100 lb||100 lb||100 lb|
|pelletized poultry manure||4-3-4||25 lb||33 lb||25 lb|
|potassium sulfate||0-0-52||--||--||2 lb|
|‘Pro-Gro’||5-3-4||20 lb||33 lb||25 lb|
|rock phosphate||0-30-0||--||3.3 lb||--|
|soybean meal||7-2-1||14 lb||50 lb||100 lb|
|sul-po-mag (+11% Mg)||0-0-22||--||--||4.5 lb|
|wood ash||0-1-3||--||100 lb||33 lb|
*Chart from UVM Fact Sheet
Putting it all Together
Creating a healthy soil fertility plan is not a matter of strictly following a recipe or plugging numbers into an equation, nor is it a matter of flippantly letting the soil manage itself. It is up to us, as stewards of the land, to gracefully balance intuition with science. A soil fertility plan should form the same way freshly baked scones do: by merging your preferred ingredients with a deliberately concocted recipe. Likewise, the focal point of your soil fertility plan should be your preferred materials – compost, cover crops, manure, or bagged amendments with careful accounting for all the nutrients contained therein. A simple spreadsheet or written log is an easy way to keep track, and can help you pay attention to production fluctuations and expenses from year to year. In the long run, this will help keep nutrients on your farm and out of the watershed, fine tune your crop production, and ultimately save you money.
For more information on soil fertility planning visit:
 Nutrient content and availability of compost varies, and only a small portion of N is available in the year of application.
For more information see: http://ag.umass.edu/fact-sheets/compost-use-soil-fertility
 Only about 10% of the total phosphate in rock phosphate is available in the year of application