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Using Natural Fertilizers in Ground Beds in Greenhouses: Converting Chemical Fertilizer Recommendations to Organic Fertilizers

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Granular organic fertilizers and compost are becoming increasingly popular for fertilizing cut flowers, vegetables, and other crops grown in the field or in ground beds in greenhouses and high tunnels. Many growers report excellent results with these materials. Generally, organic fertilizers release nutrients slowly providing more timely nutrient release for the plants and potentially less nutrient leaching than chemical fertilizers.

When using natural fertilizers for plant nutrition, careful management of the structure and chemistry of the soil is essential. Soil pH between 6.2 and 6.8 is generally considered optimum for most crops growing in soil. A soil test will indicate whether or not the acidity should be changed. Fine ground limestone is used to raise the pH of acid soils; sulfur is used to lower the pH of alkaline soils.

It is equally important to maintain a high level of organic matter in the soil each year to maintain good soil structure - preferably at least 5% (at least 1 lb. per 1 sq. ft.). A soil organic matter test can estimate the required level to add. In greenhouses, there is a continual loss of organic matter due to cultivation, warm soil temperatures, frequent watering, and crop harvests. Animal manure-based composts should be avoided due to high nitrogen content which can lead to ammonium toxicity and high soluble salts.

An annual routine field or garden soil will provide information on what nutrients are adequate or limiting and provide a fertilizer recommendation. Visit the UMass Extension Soil and Plant Nutrient Testing Laboratory for information on soil sampling and testing . Once a soil test and fertilizer recommendation is received, the following steps and charts can be used to calculate the amounts of organic fertilizers to use for greenhouse crops grown in soil in ground beds.

If the soil test results recommend a commercial fertilizer such as 5-10-5 expressed as pounds per 100 or per 1000 sq. ft, then the first step is to convert the soil test recommendation to the number of pounds of nitrogen (N) to apply. Even though many fertilizers supply phosphorus and potassium also, N is the element most likely to be the deficient.

Step 1. Test soil and obtain fertilizer recommendation. Example: apply 30 lbs. 5-10-5 per 1000 sq. feet. The fertilizer analysis translates to 5% N (or 0.05) (The fertilizer also contains 10% phosphorus expressed as P2O5 and 5% potassium expressed as K2O).

To convert the soil test recommendation to the quantity of organic fertilizer to use, the number of pounds of N is needed (Step 2).

Step 2.  Determine the number of pounds of N that is needed by multiplying the %N by the recommended rate. Example: 0.05 x 30 lbs. (recommended rate) = 1.5 lbs. nitrogen per 1000 sq. feet. (In other words 5% of 30 lbs. of 5-10-5 is 1.5 lbs. of N).

Step 3.  Using Chart 1, choose the organic fertilizer you want to use instead of chemical 5-10-5 to supply the required amount of N. For example, you decide on dried blood which has an analysis of 12-0-0.

Step 4.  Use Chart 2 to determine the lbs. of the organic fertilizer to apply. Look at the lbs. of nutrients desired (determined in Step 2) on the top of the chart. Follow the column down until you find the percent analysis of the natural fertilizer. Example: 1.5 lbs. N is recommended Find 1.5 at top of chart (pounds of nutrient desired) and then find 12% (percent N in dried blood) in the left column. The chart indicates 12.5 lbs of dried blood would provide 1.5 lbs of N.  So, 12.5 lbs of dried blood would be applied to 1,000 sq. ft. based on the recommendation.

Chart 1. Common Organic Fertilizers
Fertilizer Analysis
N-P2O5-K2O
Nutrient Availability Comments
Dried Blood 12-0-0 High  
Bone Meal 2-20-0.2 Moderate May attract pests
Rock Phosphate 0-20-0 Low Must be ground to fine mesh (200) powder
Fish Emulsion 4-1-1 Moderate May attract pests
Fish Meal 10-4-0 Moderate May attract pests
Leaf Mold 1-1-1 Moderate  
Seaweed 1.5-0.7-5 Moderate  
Cottonseed Meal 7-2.5-2 High May contain pesticide residue
Wood Ashes 0-2-5 High Hardwood ash preferred
Fresh grden compost 1-1-1 Moderate Quality depends on ingredients

Composted manure
(not dehydrated)
Cow

0.5-0.25-0.5 Moderate  
Composted manure
Horse
0.7-0.5-0.6 Moderate  
Composted manure
Rabbits
4-3-1 Moderate  
Composted manure
Hen
1.8-1-0.5 Moderate  
Composted manure
Hog
0.3-0.3-0.45 Moderate  
Composted manure
Sheep
1-0.35-0.5 Moderate  
Sludge 4-2.5-1 Moderate Contains toxic metals
Granite Dust 0-1-6 Low
(nearly insoluble)
 

Fertilizers in Chart 1 rated "moderate" for availability should be applied at double the amount needed; those rated "low" should be applied at 4 times the amount needed. Do not use cat, dog, or human waste as a manure to prevent the spread of disease. Also, nowadays many feel that it is safer to use composted rather than fresh farm animal manure.

Note: One full bushel of compost, manure or seaweed will weigh about 40 lbs.

Chart 2. Amount of Fertilizer Needed Based on % Nutrient in Fertilizer and
Number of Pounds (lbs) of Nutrient Desired
  Pounds of Nutrient Recommended or Desired
%Nutrient
in Fertilizer
0.5 1 1.5 2 2.5 3 4 5 6 8 10
0.2 250 500 750 1000 1250 1500 2000 2500 3000 4000 5000
0.25 200 400 700 800 1000 1200 1600 2000 2400 3200 4000
0.4 125 250 375 500 625 750 1000 1250 1500 2000 2500
0.5 100 200 300 400 500 600 800 1000 1200 1600 2000
1.0 50 100 150 200 250 300 400 500 600 800 1000
1.5 33 66 100 133 166 200 250 330 400 500 660
2.0 25 50 75 100 125 150 200 250 300 400 500
2.5 20 40 60 80 100 120 160 200 240 320 400
3.0 17 33 50 66 83 100 133 165 200 266 330
4.0 12 25 37 50 73 75 100 125 150 200 300
5.0 10 20 30 40 50 60 80 100 120 160 200
6.0 8 16 25 33 40 50 66 83 100 132 166
7.0 7 14 20 28 35 40 55 70 80 110 140
10.0 5 10 15 20 25 30 40 50 60 80 100
12.0 4 8 13 17 20 25 33 42 50 66 82
15.0 3 7 10 13 17 20 25 33 40 50 66
20.0 2 5 8 10 13 15 20 25 30 40 50

Most often, fertilizer recommendations are based on the amount of N to apply. However, the information in Chart 2 and the earlier four step calculations can be used for any element. For example, a soil test might recommend an application of P, but not N and K. In this case the choice of rock phosphate would be best and so the procedure outlined here and Chart 2 would be used for P rather than N.

More information: Organic Growing Media and Fertilizers for Greenhouses

Adapted from:
Morehouse C. and A. Barker.  Natural Fertilizers in the Home Vegetable Garden. Cooperative Extension Service, University of Massachusetts. 1980.

Tina Smith, University of Massachusetts Extension
Douglas Cox, Plant Soil and Insect Sciences, University of Massachusetts Extension
Greenhouse Crops and Floriculture Program

 

Topics: 
Commercial Horticulture
Commercial Horticulture topics: 
Plant Nutrition