Fundamentals of Soil Health and Fertility

Soil health (or soil quality) has been defined as the capacity of a soil to sustain plant and animal productivity, maintain or enhance water and air quality, and support human health and habitation over a human time scale (thousands of years). Similar but distinct from soil health, soil fertility can be viewed as a function of the biological, physical and chemical characteristics of soil that supply plant nutrient requirements. 

Holistically, a healthy and fertile soil must have: good structure and drainage, sufficient depth for root growth, sufficient (but not excessive) nutrient availability, small weed, insect pest, and plant pathogen populations, large populations of beneficial organisms (including microbes), no toxins, and resilience to adverse conditions. 

Soil Health

A number of individual soil tests may be used to assess aspects of soil health, including those obtained with routine soil analysis. However, a comprehensive evaluation includes a suite of complementary tests to measure chemical, physical, and biological soil properties. Researchers at Cornell University evaluated a large number of these measurements for use in Northeastern cropping systems and now offer the most meaningful measurements as part of the Cornell University Soil Health Test. This suite of soil health indicators is designed to provide growers with helpful information about problems that may limit crop productivity and/or soil performance. Recommendations are provided with the results to help growers address problems that may be identified. More information about the Cornell University Soil Health Test (including sampling protocol, current prices, and submission forms) can be found at http://soilhealth.cals.cornell.edu/. Other soil testing laboratories in New England now offer soil health tests as well; check with your local lab.

Soil Organic Matter

Soil organic matter (SOM) is critical for soil health because of the beneficial chemical, physical and biological properties it imparts. SOM increases soil nutrient availability, being a source of nutrients itself as well as by increasing cation exchange capacity. SOM also benefits the physical properties of soil by contributing to soil structure and increasing water holding capacity. Finally, SOM supports soil biology as a substrate for the growth of soil microbial communities. In turn, these soil microbes mineralize SOM into plant-available nutrients, develop symbiotic mycorrhizal relationships between soil fungi and plant roots, and produce sticky substances responsible for binding soil aggregates.

SOM is composed of materials containing carbon that came from living organisms, including plant and animal residues, organic-based amendments, and soil bacteria and fungi, all in various stages of decomposition. SOM is often divided into two categories. The more stable component is humus, which is primarily comprised of long-dead material that is highly decomposed and beneficial for soil structure and carbon storage. Active SOM, on the other hand, is relatively undecomposed and accessible to soil microbes for mineralization. It is important to maintain both types of SOM (see Building Soil Organic Matter). In any case, it is important to maintain SOM because it is typically the most important component of soil for nutrient supply, water holding capacity, cation exchange capacity, and soil structure. 

Native total SOM content in soils used for vegetable production in New England is almost always less than 10% and typically in the 2-6% range. SOM accumulation is generally limited by several abiotic factors, including temperature, moisture, and soil texture. Well-drained, coarsely textured soils tend to have lower levels of SOM, due in part to the rapid microbial decomposition rates favored by these soil conditions. In contrast, loamy soils often have 3-6% SOM. 

SOM supplies nutrients through the process of mineralization, which is the microbial decomposition of organic compounds into carbon dioxide and their mineral constituents. Soil microbes are most active in warm soils (over 70°F) that are moist, well-aerated, and have a pH between 6 and 7 (also ideal conditions for most vegetable crops). Mineralization of nutrients will proceed rapidly under these conditions, provided there is an adequate supply of SOM and an abundant soil microbial community.

SOM directly influences water holding capacity through its capacity to absorb large amounts of water. It also indirectly boosts water holding capacity by improving soil structure, creating more pore space for water storage and larger pores for air. Soil structure is enhanced by SOM because, as it decomposes, sticky compounds like gums, carbohydrates, and resins are produced by microorganisms. These gums bind soil particles together into secondary aggregates. This in turn bolsters cation exchange capacity, which is influenced by both clay and SOM content (both supply negatively charged sites that hold cations). In most New England soils, the stable humus portion of SOM accounts for the vast majority of the cation exchange capacity, as these soils are typically low in clay content. See also Cation Exchange Capacity and Base Saturation

Building Soil Organic Matter

Soil organic matter (SOM) is in a constant state of flux, with additions and losses simultaneously occurring. To maintain SOM levels, one must ensure that losses do not exceed additions. 

SOM can be lost from soil through both wind and water erosion. However, the primary mechanism for SOM loss is microbial decomposition. Soil microbes use SOM as a source of energy and nutrition, converting SOM into carbon dioxide and its constituent mineral elements. The rate of SOM decomposition is controlled by a number of factors including soil temperature, moisture, aeration (oxygen), and the quality or characteristics of the SOM. In agricultural soils, these factors are all greatly influenced by soil management. For example, aggressive tillage and cultivation increases aeration and break apart soil aggregates that protect SOM from microbial decomposition. Reducing tillage and cultivation is an effective management strategy to maintain, or even increase, SOM content  (see Reduced Tillage).

There are a number of ways to increase or maintain SOM. Increasing the quantity of plant residues returned to the soil is one of the most sustainable methods strategies for maintaining SOM. Most vegetables leave little residue in the field, and SOM will usually decrease if these are the only residues provided to the system.  Although cover crops can add enough biomass to maintain SOM, it can be difficult to increase SOM with some cover crop species or mixes. Including sod-forming crops in the rotation, however, can increase SOM. A more rapid and direct method of increasing SOM is by adding organic amendments, like organic mulches and compost. While the application of organic amendments can rapidly increase SOM, extractable soil phosphorous (P) concentrations must be monitored to avoid excessive applications (see Fertilizers and Soil Amendments).

Cation Exchange Capacity and Base Saturation

Cation exchange capacity (CEC) is a measure of the soil’s ability to retain and supply nutrients, specifically the positively charged nutrients called cations. These include calcium (Ca++), magnesium (Mg++), potassium (K+), ammonium (NH4+), and many of the micronutrients. Cations are attracted to negatively charged surfaces of clay and organic particles called colloids. CEC is reported as milli-equivalents per 100 grams of soil (meq/100g) or as centimoles of charge per kilogram (cmole+/kg). CEC can range from below 5 meq/100g in sandy soils low in organic matter to over 20 meq/100g in finer textured soils and those high in organic matter. Low CEC soils are more susceptible to cation nutrient loss through leaching, and may not be able to hold enough nutrient cations for a whole season of crop production.

The cations Ca++, Mg++, K+, hydrogen (H+) and aluminum (Al+++) account for the vast majority of cations adsorbed on the soil colloids in New England soils. It is important to note that H+ and Al+++ are not plant nutrients. Both H+ and Al+++ are considered acidic cations because they tend to lower soil pH while Ca++, Mg++, and K+ are considered basic cations and have little to no influence on soil pH. If all the cations are basic and none are acidic, there would be a 100% base saturation and the soil pH would be close to 7 or neutral. In acid soils there are acidic cations adsorbed on the soil colloids (called exchangeable acidity) and the percent base saturation is less than 100. A soil with a pH between 6.5 and 6.8 will typically have a base saturation of 80%-90%.

Soil Acidity, pH, and Liming

One of the most important aspects of nutrient management is maintaining proper soil pH, a measure of soil acidity. A pH of 7.0 is neutral, less than 7.0 is acidic, and greater than 7.0 is alkaline. Most New England soils are naturally acidic and need to be limed periodically to keep the pH in the range of 6.5 to 6.8 for most vegetable crops. Scab-susceptible potato varieties are an exception, but some lime may still be needed to maintain the recommended pH of 5.0-5.2. When the soil is acidic, the plant availability of nitrogen (N), phosphorus (P), and potassium (K) is reduced and there are usually low amounts of calcium (Ca) and magnesium (Mg) in the soil. In contrast, most micronutrients are more soluble and are therefore more available to plants. Under very acidic conditions aluminum (Al), iron (Fe), and manganese (Mn) may be so soluble they can reach toxic levels. Soil acidity also influences soil microbes, which decompose organic matter and recycle crop nutrients. For example, when soil pH is low (below 6.0), bacterial activity is reduced and fungal activity increases. Acidic soil conditions also reduce the effectiveness of some pesticides. These conditions also limit the ability of cover crops like legumes to fix nitrogen. 

The most effective way to manage soil acidity is to apply agricultural limestone. The quantity of lime required is determined by the target pH (based on crops to be grown) and the soil's buffering capacity, measured in a soil test. Buffering capacity refers to the soil’s tendency to resist change in pH. Soil pH is only a measure of active acidity, the concentration of hydrogen ions (H+) in soil solution. When lime is added to a soil, active acidity is neutralized by chemical reactions that remove hydrogen ions from the soil solution. However, there are also acidic cations (H+ and Al+) adsorbed on soil colloids (see CEC, previous section) that can be released into the soil solution to replace those neutralized by the lime. This is called reserve acidity. Clays and soils high in organic matter have the potential for large amounts of reserve acidity. These soils are said to be well-buffered. To effectively raise the soil pH, both active and reserve acidity must be neutralized. Soil test labs determine buffering capacity and lime requirement by measuring or estimating the reserve acidity. 

The neutralizing power of lime is determined by its calcium carbonate equivalence. Suppliers can tell you the calcium carbonate equivalence of the lime you are purchasing. Recommendations are based on an assumed calcium carbonate equivalence of 100. If your lime is lower than 100, you will need to apply more than the recommended amount, and if it is higher, you will need less. To determine the amount of lime to apply, divide the recommended amount by the percent calcium carbonate equivalence of your lime and multiply by 100. Wood ash is another amendment that may be used to manage soil acidity. The calcium carbonate equivalence of wood ash is typically between 30-50%, but it can vary widely. If purchasing wood ash from a supplier, they will provide a recent analysis. Otherwise, the wood ash should be submitted to a lab offering lime analysis to determine the calcium carbonate equivalence.

The speed with which lime reacts in the soil is dependent on particle size and distribution in the soil. To determine fineness, lime particles are passed through sieves of various mesh sizes. A U.S. Standard 10-mesh sieve has 100 openings per square inch while a 100-mesh sieve has 10,000 openings per square inch. Lime particles that pass through a 100-mesh sieve are very fine and will dissolve and react rapidly (within a few weeks). Coarser material in the 20-30-mesh range will react over a longer period, one to two years or more. Agricultural ground limestone contains both coarse and fine particles. About half of a typical ground limestone consists of particles fine enough to react within a few months, but to be certain you should obtain a physical analysis from your supplier. Super fine or pulverized lime is sometimes used for a “quick fix” because all of the particles are fine enough to react rapidly.

Lime will react most rapidly if it is thoroughly incorporated to achieve intimate contact with soil particles. This is best accomplished when lime is applied to a fairly dry soil and disked in (preferably twice). When spread on a damp soil, lime tends to cake up and doesn’t mix well. A moldboard plow has little mixing action, therefore, disking is preferred. If growing vegetables in no-till fields, it is common to apply needed lime and work it in before limiting tillage in the field, then to apply a smaller amount of lime more frequently to the surface to maintain adequate pH. Lime moves slowly through the soil profile without incorportation. If lime is applied and not incorporated the material likley won’t be spread throughout the entire rootzone, reducing the neutralizing ability.  

Besides neutralizing acidity and raising soil pH, lime is also an important source of Ca and Mg for crop nutrition. It is important to select liming materials based on Ca and Mg soil content with the aim of achieving sufficient levels of each for crop nutrition. If the Mg level is low, a dolomitic lime (high magnesium lime) should be used; if Ca is below optimum, a calcitic (low-Mg lime) should be used.
 

Nutrient Recommendations

Normally it is not necessary to supply all the nutrient needs of a crop from fertilizer alone, because the soil already contains some quantity of nutrient elements available for crop growth. It is necessary to test the soil to determine its nutrient status, because only then can you determine the additional amounts of appropriate fertilizer materials to apply. Frequently, growers apply more fertilizer than the crop can use and the excess, especially nitrogen, is leached from the root zone into groundwater. 

The "Plant Nutrient Recommendations" tables in each crop section can be used to determine nutrient needs based on soil test results. Read the section on Soil Testing to better understand nutrient recommendations.

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 (low or medium), most crops would benefit by adding the appropriate nutrient(s) to increase levels to optimum. However, if levels are at above optimum (very high) levels, there will be no additional benefit and excess levels may reduce crop yield or quality and may cause environmental harm. This happens in fields where soil testing was not used to monitor fertility levels or when nutrients are applied even when soil levels are sufficient. When a nutrient is above optimum levels it should not be included in any amendments until the excess is taken up by crops. In this case, it is wise to temporarily stop applying compost or manures until nutrient levels are in the desired range because the addition of these amendments can add high levels of nutrients, especially phosphorus. This is a practical way to manage nutrient levels if small to moderate amounts of mixed crops are to be grown.

If a significant acreage of a particular crop is to be grown, fertilizers should generally be tailored to the specific needs of that crop, based on the amounts of nutrients that the crop is expected to remove during the growing season (see Table 4). If the soil tests indicate that a nutrient is optimum/high it is likely that the soil will supply enough to meet the crop's needs. However, many growers will apply enough of the nutrient to replace what is removed by the crop. If the test level is above optimum/very high, additional applications should normally be avoided unless the crop has an unusually high demand for a specific nutrient. Occasionally, nutrient applications may exceed the soil test recommendation or the expected average removed by the crop (Table 4) because a particular cultivar is considered a heavy feeder, such as long season Russet potatoes. Or, for example, a large crop of tomatoes can be expected to remove a large amount of potassium and it may be justified to apply some of this nutrient even if the soil test indicates a level somewhat above optimum. The nutrient recommendation tables for each crop have been developed on this basis of expected crop removal. This can also be a practical way to determine nutrient needs of high value crops, even when they are grown on a small scale. It is important to keep in mind that many factors can limit crop yield potential, and that simply adding more nutrients will not address any issues beyond nutrient deficiency.
 

Removal of Nutrients from the Soil

Table 4 lists amounts of certain nutrient elements that are removed by vegetable crops. It includes both the amounts removed by harvest and those that remain in crop residue and are returned to the soil. These figures for crop removal should be considered approximate. They can be used as a basis for adjusting your own application rates up or down on a trial basis. Keep in mind that nutrient removal varies with factors such as soil moisture, temperature, and pH. Plants can absorb large amounts of a nutrient if it is in abundance; but this may not increase yield. Excess levels of some nutrients can reduce the yield and/or quality of some crops.

Nutrients that are relatively immobile in the soil, such as phosphorus, calcium, magnesium and the micronutrients, are not all extracted by plants because the roots do not come in contact with all of the nutrients. This is especially true of certain vegetables that have small or sparse root systems. Most of the phosphorous applied to the soil becomes fixed in a form unavailable to plants. Thus, it is necessary to provide some excess amounts of nutrients when soil test levels are below the optimum range, and this is why rates sufficient to replace the amount removed by crops are sometimes recommended for soils that test in the optimum range.

Table 4: Approximate Nutrient Removal by Selected Vegetable Crops. 

Vegetable

 

Yield per acre1

Nutrient removal, lbs/acre

N

P2O5

K2O

Ca

Mg

Snap beans

Total

250 bu

30

20

35

7

3

Broccoli

heads

5 tons

20

2

45

 

 

 

other

 

145

8

165

 

 

 

Total

 

 

 

 

 

 

Cabbage

Total

20 tons

125

30

130

28

10

Carrots

roots

25 tons

80

20

200

 

 

 

tops

 

65

5

145

 

 

 

Total

 

145

25

345

 

 

Cauliflower

 

6 tons

45

18

43

4

3

Celery

tops

50 tons

170

 

380

 

 

 

roots

 

25

 

55

 

 

 

Total

 

195

80

435

110

27

Cucumbers

 

24 tons

100-200

33-72

100-400

 

 

Eggplant

 

16 tons

207

46

34

 

 

Kale

 

10 tons

125

30

110

50

10

Lettuce

 

15 tons

75

35

150

13

5

Muskmelons

fruit

11 tons

95

17

120

 

 

 

vines

 

60

8

30

 

 

 

Total

 

155

25

150

 

 

Onions

bulbs

20 tons

110

20

110

12

14

 

tops

 

35

5

45

 

 

 

Total

 

145

25

155

 

 

Peppers

fruits

12 tons

137

52

217

 

 

Potatoes, White

tubers

300 cwt

90

45

160

 

 

 

vines

 

60

20

60

 

 

 

Total

 

150

65

220

 

 

Potatoes, Sweet

roots

15 tons

75

55

160

 

 

 

vines

 

35

5

280

 

 

 

Total

 

110

60

440

10

15

Spinach

 

10 tons

100

25

100

24

10

Squash, Summer

 

10 tons

32

12

56

 

 

Squash, Winter

 

6 tons

12

10

58

 

 

Sweet Corn

ears

250 cr.

55

8

30

 

 

 

stalks

 

100

12

75

 

 

 

Total

 

155

20

105

 

 

Tomatoes

  fruit

30 tons

110

48

180

15

15

 

  vines

 

90

30

100

24

21

 

Total

 

200

78

280

39

36

1 These are assumed yields. Actual yields may vary depending on weather and cultural practices. Adjust nutrient removal rates accordingly. To convert to volume or count yield units see Table 15: Approximate Yields (page 37).

 

Nutrient Management Regulations

Several states in New England have nutrient management regulations that impact vegetable production practices. The EPA began regulating municipal waste water treatment facilities to manage water pollution, and more recently, regulations have gone in place to regulate non-point source pollution such as that coming from fertilizer or manure applications on agricultural fields. Below is a list of states with regulations and recommendations on how to comply.

Maine: Regulations for crop farms require the development of a nutrient management plan (NMP) if a farm is importing more than 100 tons of manure or regulated residuals annually. The nutrient management plan must address storage and utilization of manure and off-farm nutrients on land to which the regulated residuals or manure are applied. The NMP must include or provide for minimum distances between manure storage, stacking and spreading areas and property lines and surface water based on site-specific factors determined to be effective for controlling runoff and for preventing contamination of surface water. The NMP must include soil test reports for each field where manure or other crop nutrients will be applied, and soils must be tested for each field at least every 5 years. For each field, the NMP must show the calculation of nutrients required to grow the specific crop. The producer may write his/her own plan, but it must be approved by a Maine certified nutrient management planner licensed through the Nutrient Management Office. To find a licensed nutrient management planner, contact:

Mark F. Hedrich, Nutrient Management Program Manager. Maine Department of Agriculture, Conservation and Forestry, Division of Animal & Plant Health, 28 State House Station, Augusta, Maine 04333; 207-287-7608; mark.hedrich@maine.gov

New Hampshire: Best Management Practices (BMPs) for Agriculture are prepared by the Agricultural Best Management Practices Task Force and the USDA Natural Resources Conservation Service (NRCS) in Durham, NH and provides a guide for growers for handling manure, agricultural compost and chemical fertilizer. There is not a law that explicitly requires growers to follow them; however, it is in the producers’ best interest to follow these BMPs as they provide protection from nuisance allegations. State law under RSA 431:35 requires the New Hampshire Department of Agriculture, Markets & Food to respond to complaints involving the mismanagement of manure, agricultural compost and chemical fertilizer. Copies of the BMP manual are available to producers as a tool, but the NH Department of Agriculture, Markets & Food does not have the authority to enforce the implementation. A copy of the manual can be found here: http://agriculture.nh.gov/divisions/regulatory-services/nutrient-management.htm

Vermont: All farms must comply with the Required Agricultural Practices (RAPs) effective since July, 2017. The RAPs are practices and management strategies to which all types of farms must be managed to reduce the impact of agricultural activities to water quality. These standards are intended to improve the quality of Vermont’s waters by reducing and eliminating cropland erosion, sediment losses, and nutrient losses through improved farm management techniques, technical and compliance assistance, and where appropriate, enforcement. The RAPs establish nutrient, manure, and waste storage standards, make recommendations for soil health and establish requirements for vegetated buffer zones and livestock exclusion from surface water. The RAPs also establish standards for nutrient management planning and soil conservation. Full text of the RAPs regulations may be found on the Vermont Agency of Agriculture, Food and Markets here: https://agriculture.vermont.gov/rap. Questions regarding these regulations should be directed to the Vermont Agency of Agriculture, Food and Markets, Water Quality Division, AGR.WaterQuality@Vermont.gov, (802) 828-2431. A factsheet specific to the RAP applicability to vegetable producers can be found here: https://agriculture.vermont.gov/sites/agriculture/files/documents/VeggieFactsheet.pdf

Massachusetts: In 2012, the Massachusetts Legislature passed Chapter 262, An Act Relative to the Regulation of Plant Nutrients. The text of the enabling legislation can be found at: https://malegislature.gov/Laws/SessionLaws/Acts/2012/Chapter262. The Act requires the Department of Agricultural Resources (MDAR) to promulgate state-wide regulations to ensure that plant nutrients are applied in an effective manner to provide sufficient nutrients for plant growth while minimizing the impacts of the nutrients on water resources in order to protect human health and the environment. The Act also requires that these regulations are consistent with UMass Extension’s educational and outreach materials relative to nutrient management and fertilizer, which may be found here: https://ag.umass.edu/resources/agriculture-resources/umass-extension-nutrient-management. In response to the Act, MDAR developed regulations entitled “330 CMR 31.00: Plant Nutrient Application Requirements for Agricultural Land and Land Not Used for Agricultural Purposes”, found here: https://www.mass.gov/service-details/plant-nutrient-management. A detailed factsheet specific to nutrient regulations on agricultural land can found here https://www.mass.gov/doc/plant-nutrient-regulations-fact-sheet-for-agricultural-land/download. The regulation gives MDAR state-wide authority to regulate and enforce the registration and application of plant nutrients including, but not limited to, fertilizer, manure and micronutrients.  The Cape Cod Commission, Martha’s Vineyard Commission, and Nantucket Commission have the option to adopt their own ordinances in regard to nutrients and fertilizers, but they cannot be less restrictive than the state regulation.

Connecticut:  Currently there are no nutrient management regulations affecting vegetable farmers in CT. There is a Phosphorous Law (CT PA-1255) but it primarily affects wastewater treatment facilities and homeowners who may only apply P if a soil test shows a need and not during winter months.

Rhode Island: No regulations of nutrient applications are in place in RI, however, on a case by case basis (and rarely at that) regulation may occur as part of a response to an environmental violation, for example, a consent agreement resulting from water quality impairment.