Guidelines for Organic Fertility Management

Soil fertility can be viewed as a function of the biological, physical and chemical characteristics of soil. An organic fertility program should consider all of these interrelated factors to optimize and sustain crop production.

Soil testing is useful for monitoring soil fertility and soil organic matter content, which influences the physical and biological quality of soil. Soil testing estimates the level of nutrients in the soil that are available to plants. This helps determine the quantity and type of soil amendments needed for good crop yields. Soil fertility amendments used in organic cropping systems are typically complex, whole nutrient source (e.g., compost, manure, seed meals and rock powders) making it challenging to maintain nutrient balance over time. As a result, excessive levels of certain nutrients (especially phosphorus) may build-up. This is why soil test reports should not be discarded after using for fertility recommendation, but rather the reports should be saved and filed by field and the nutrient amounts tracked over time. If a nutrient is rapidly accumulating, then adjustments should be made in the fertility program choosing amendments offering only those nutrients needed.  Monitoring nutrient trends over time enables growers to adapt their nutrient management strategies to optimize yield and minimize environmental impact.

In general, the goal should be to maintain nutrient elements within the optimum range as reported on the soil test. When nutrient levels are within this range, the needs of most crops will be met. If levels are below optimum (very low or low), most crops will benefit by increasing levels to optimum. However, if levels are above optimum levels, there will be no additional benefit, and excess levels may reduce crop yield or quality, attract pests, and may cause environmental harm. This frequently occurs on organically managed fields where soil testing was not used to monitor fertility levels and large amounts of manure or compost have been applied over the years. When a nutrient is above optimum it should not be included in any amendments until the excess is taken up by crops.

Routine soil testing can be performed at any time, but if testing for nitrogen is done it should be done in the late spring or early summer immediately before the greatest demand for this nutrient.   See pre-sidedress soil nitrate test (PSNT) for sampling and handling information. 

Organic matter management is an essential part of organic agriculture. Generous additions of organic materials, such as animal or green manures, are needed to feed soil microbes. Organic matter also is essential because the by-products of decomposition of organic amendments bind soil particles to improve the soil structure that is key to good drainage and soil aeration.   In addition, organic matter is the storehouse of nutrients in the soil. Many nutrients, especially N, P, S, Cu, and Zn, are released when organic matter decomposes (mineralization).

Decaying soil organic matter releases nutrients unevenly during the growing season.  The rate of release is primarily dependent on the type of organic material.  For example, compost, which has been primarily decomposed in the compost pile, is slower to release nutrients than manures (see Table 9 and 9a).  The rate of release during the season is also dependent on soil moisture and temperature that impacts microbial activity.  Cool, wet or dry soils typically slow decomposition and mineralization.  In the late spring after the soil warms there is usually a flush of nutrients, and commonly the rate declines after that.   When the release of nutrients is low, fertilizing with soluble forms of nutrients may benefit crops. This is why some relatively available phosphorus and nitrogen should be banded, or placed near the roots of crops early in the growing season. For example, use bone meal and dried blood or a seed meal like peanut or soybean to provide some available P and N, respectively, or use a commercial organic fertilizer blend. See Tables 7 and 7a, for the nutrient content of several common fertility amendments approved for organic production.

Nitrogen Anywhere from 10 to 90% of the N contained in compost, manure, and plant and animal byproducts may become available to plants during the season following incorporation (Table 9). Similarly, there is a release of about 20 lb/acre or more (on average) of N for each 1% soil organic matter. These releases of N vary with drainage and other soil conditions, and may not be well timed to crop needs, especially for early, short season crops. Many annual crops need N most intensely about three to four weeks after transplanting, or just before the period of maximum growth. Therefore, sidedressing, or spreading a rapidly available source of N along the crop row so it will release nutrients at this time is most efficient. Examples of appropriate materials include feather meal, blood meal, seed meals, and dehydrated poultry litter. These materials are relatively expensive, so it is advisable to use them on high value crops. A PSNT collected at the right time will provide information that can be used to estimate the most appropriate rate.

Calcium is typically supplied in sufficient quantities by lime applied to manage soil acidity. When liming is not required and soil Ca tests below optimum, the best alternative source of Ca for organic producers is gypsum.

Magnesium is best applied as dolomitic lime, but when liming is not required, other Mg sources are Sul-Po-Mag or Epsom salts. Sul-Po-Mag is the better choice if potassium is also required. However, Epsom salts can be applied as a foliar spray to temporarily alleviate Mg deficiency. Dissolve 15 lb per 100 gal water and spray at weekly intervals.

Limestone is a widely used rock powder. It raises the soil pH and provides calcium (Ca) and varying amounts of magnesium (Mg). The appropriate rate of lime stone should be determined by soil testing and adjusted based on the Equivalent Neutralizing Value of the material. The selection of dolomitic or calcitic lime should be based on soil test levels of Ca and Mg. When Mg tests below optimum, dolomitic, or high-Mg limestone, should be used for liming. If Mg is optimum, a calcitic, or low-Mg lime may be used.

Phosphorus is low in many New England soils, and can limit crop growth, especially early in the season. Soils testing below optimum in available phosphate (P2O5) usually require substantial applications of phosphate. Compost and manures are an excellent source of readily available P2O5. Maintain a pH of 6 to 7 with limestone to maximize P2O5 availability. Compost and manures tend to contain less P2O5 than N or K2O, but repeated applications of moderate rates will raise P levels substantially. Repeated use of these materials may result in excessive soil levels. Nutrient levels should be monitored with regular soil tests. Some growers use large amounts of rock phosphate to try to build fertility. Over time this can lead to levels much above optimum. To allow P levels to come down, no amendments containing P should be applied including compost.

Potassium is best applied at or near planting time because it is soluble and easily leached.  Sul-Po-Mag is the K fertilizer of choice when Mg is also needed. Potassium sulfate from natural sources is a better choice when K is needed but Mg is not.  Potassium is very slowly available (several years) from granite dust and greensand, which may be applied at 3 to 5 tons to the acre to build up K reserves. Wood ashes contain soluble K, but must be used with caution because they will raise the pH rather rapidly and can be caustic. The liming effect of ashes is roughly half that of limestone.

Micronutrients are generally sufficiently supplied to plants by regular additions of organic amendments. Wood ash is another excellent source of micronutrients. Some seaweed extracts may also supply micronutrients. In soils low in boron (B), remedial applications are widely recommended for crops that readily suffer from B deficiency, such as crucifers. In this case, 1 to 2 lb/acre of B should be applied to the soil. It is difficult to apply such a small amount uniformly, but boron can be ordered as part of a custom fertilizer blend. Most boron products are soluble and can sprayed evenly over the soil. Several forms of B are OMRI listed, including Solubor, Fertibor and Biomin Boron. It is advisable to monitor B levels with soil tests and tissue tests (for perennial fruits). Excess levels of B are toxic to plants, and some crops, such as beans and peas, are quite sensitive to high boron levels (see Table 4).

Table 9: Typical Nutrient Values for Common Fertilizers Approved for Organic Production.

 

Total N (%)1

C/N ratio

Fraction of organic N made available first season2

P2O5 (%)

K2O (%)

Relative Availability3

Plant residues

 

 

 

 

 

 

Alfalfa meal

2-3

15-20

0.25-0.4

0.5

2.5

slow/med

Cottonseed meal

6

5

0.6-0.8

2

2

med/fast

Soybean meal

7

5

0.6-0.8

2

2

med/fast

Peanut meal

8

11

0.6-0.8

1

 

slow/med

Animal products

 

 

 

 

 

 

Dried blood

12

3

0.6-0.75

1

0.5

fast

Bone meal (steamed)

3

4

0.25-0.35

15

0

med

Bone char

0

-

-

15

0

med

Feather meal

13

4

0.6-0.8

0

0

med/fast

Fish emulsion

4

3

0.7- 0.9

2

0

fast

Fish meal

9-10

4

0.6-0.8

7

0

med/fast

Manure

 

 

       

Dairy, with bedding

1

18

0.3-0.5

0.5

1

med

Horse, with bedding

0.5

25

0.2-0.4

0.2

0.5

med

Broiler litter

3-4

15

0.4-0.6

3

3

med/fast

Layer manure

1-2

10

0.4-0.6

3

1.5

med/fast

Bat guano

6

2

0.6-0.8

9

2

fast

Compost (mature)

 

 

 

 

 

 

Manure

1.5-2

15-25

0.1-0.15

2

1

very slow

Yard waste

0.5-1

20-25

0.1-0.2

1

1

slow
             

Table 9a: Typical nutrient values for common mineral materials approved for organic production.

 

Total N (%)

P2O5 (%)

K2O (%)

Ca (%)

Mg (%)

Relative Availability3

Potassium sulfate

0

0

50

0

0

fast

Sol-Po-Mag

0

0

21

0

11

fast

Epsom salts

0

0

0

0

10

fast

Wood ash

0

1

10

25

2

med/fast

Gypsum

0

0

0

19-23

0

med

Dolomitic lime

0

0

0

20-30

10-12

med

Calcitic lime

0

0

0

40

<5

med

Colloidal rock phosphate

0

254

0

20

0

slow

Rock phosphate

0

20-324

0

25

0

very slow

Granite dust

0

0

3-54

2

1

very slow

Greensand

0

14

4-94

0

0

very slow

1 Nutrient concentration of organic materials is inherently variable. Estimated values are provided for reference only. It is best to have materials tested in order to determine appropriate application rates. 

2 Compost, bat guano, poultry litter, and animal manures also contain varying quantities of NH4, which is immediately plant available; however, NH4 is subject to volatilization losses if material is not immediately incorporated.

3 Relative nutrient availability of lime and rock powders varies with origin of material, soil pH, and for rock powders, depends largely on fineness of grind.

   4 These values represent total K2O and P2O5, available K2O and P2O5 from these materials will be much lower.