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 (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. 

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 breaks 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 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, such as 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 (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%.

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 between 6.5 and 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 range 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 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 legume cover crops to fix N. 

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 Cation Exchange Capacity and Base Saturation, 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, 1-2 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 incorporation. If lime is applied and not incorporated the material likely 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.
 

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.

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