Soil Basics Part IV: Putting it All Together

In the past three fact sheets, we have been discussing soil basics which included soil physical and chemical properties and the role of soil organic matter. This fact sheet will summarize a few key points and offer a perspective on using soil testing in managing soils and nutrients. You may wish to review previous articles.

Previously, we talked about soil texture (the size of soil particles), soil structure (the arrangement of soil particles) and pore space. We noted that light textured (sandy) soils are easily tilled, well drained and warm up early, but do not retain moisture and nutrients well. Sandy soils have a low cation exchange capacity (CEC), which is a measure of the ability of a soil to retain nutrients and make them available to plants. Heavy (clay) soils generally have opposite characteristics of sands, except that most clays found in New England are fairly low in CEC. Loamy soils are mixtures of sands, silts and clays and usually combine the good characteristics of heavy and light soils.

Be that as it may, we can't change soil texture. We have to work with what we have. But we can modify the soil's structure. Every time we till the soil we change its structure. Soil becomes packed when we walk or drive on it. Rain also packs the soil to some extent. Freezing and thawing and the penetration of plant roots help loosen soil structure. One of the most important things we can do for soil structure is to increase organic matter. Organic matter improves moisture holding capacities of sandy soils and makes clayey soils lighter, improving drainage and making root penetration and tillage easier. Most of the long term benefits of organic matter come from humus, which is the stable fraction that remains after most of the organic matter has broken down. Humus is the primary factor contributing to CEC in New England soils. You have to add large volumes of organic matter to get a small amount of humus. This may seem discouraging but the effort is worth it. As soil organic matter breaks down, energy is provided for microbes and nutrients become available for plant growth, thereby increasing biological activity and improving soil fertility.

Soil Testing

Soil testing should be a part of your soil management program. First of all, you should know the textural classification of your soils. The University of Massachusetts and many other soil testing labs can do a textural analysis. Remember that soil texture does not change, so you need to have this test done only once. However, soil texture commonly varies from one part of a field to another, so you may wish to test sections separately if they appear different in any way.

Routine soil tests are designed to evaluate the chemical status of the soil. Most labs also test for organic matter. Soil organic matter and nutrient status change over time, so soil should be tested annually. Most soil tests measure soil pH, provide a lime recommendation, and measure CEC and base saturation. These were discussed in the second fact sheet, Soil Basics: Part II, Chemical Properties of Soil, which you may wish to review. Soil tests also evaluate the status of phosphorous (P), potassium (K), magnesium (Mg) and calcium (Ca). Some testing laboratories report the level of nitrogen (N) and/or minor elements. Nutrient elements can exist in various chemical forms in the soil, some of which are available for plant uptake and some of which are not. Soil testing procedures are designed to measure the levels of nutrients which are available for plant uptake.

Phosphorus

Our goal should generally be to achieve and maintain P at a "high" soil test level. This status should provide for optimum growth of most crops. "Medium" or "low" levels of P may limit crop growth and quality. On the other hand we should avoid building this element to excess or "very high" levels. At excess levels, P can tie up other elements such as iron (Fe), and zinc (Zn) and inhibit their uptake by plants. Recommendations for adding P are based on the soil test levels. If the test level is "very high", we can not expect to have a positive response to additional P and none will be recommended. If the level is "high" there will probably be little or no response, but some P may be recommended as a maintenance application. In cold soils, root uptake of P is limited, and a small amount of soluble P placed near the seed or transplant is beneficial. A band of starter fertilizer, high in P, is frequently recommended when planting in cool soils early in the season even if the test level is high. Most plants use relatively little P compared to nitrogen (N) or potassium (K). However, growers commonly apply about as much P as N or K. This action is justified to build soil levels of P if soil test levels are low because most of the applied P becomes fixed in the soil and unavailable to plants. P does not move in the soil and should be mixed into the soil in the root zone to facilitate plant uptake. This should be done before planting or by banding at planting. P applied on the soil surface will be of no value until it is incorporated. P does not leach and poses no threat to ground water. However, pollution does occur when soil containing P is washed from fields into surface water.

Phosphorus (P) is referred to as P₂O₅ for the purposes of soil testing, fertilizer grades and recommendations. We don't apply P in this form, but it has become the standard over many years and fertilizers are calibrated as P₂0₅. Growers usually apply P as part of a blended fertilizer. Monoammonium and diammonium phosphates are common sources which also supply N. Superphosphate and triple superphosphate are also available and can be applied alone or as part of a blend. Rock phosphate, from which the other products are made, can be applied as a powder. It is very slow to become available and large quantities are required. Bone meal is an effective, but expensive source of P. Animal manures contain readily available P although the concentration is usually low. Several years of manuring can, however, increase P levels substantially. Continuous, heavy manure application can eventually lead to excessive soil P levels, but routine soil testing will alert you before this happens.

Potassium

Potassium (K) is expressed as K₂O similar to the way P is referred to as P₂O₅. The need for K varies considerably depending on the type of crop. It is important that the soil K plus the applied K are enough to meet crop needs. However, excessive levels should be avoided because K can interfere with the uptake of Ca and Mg (see "Base Saturation" in the second fact sheet Soil Basics: Part II, Chemical Properties of Soil). K is not usually subject to leaching except in soils having a low CEC. As with P, K is commonly applied along with other nutrients in a blended fertilizer. The most common source of K in fertilizers is potassium chloride, but it is also supplied as potassium sulfate, sul-po-mag. Wood ashes, manure and compost also supply K. Greensand contains K, but it is very slow to become available. K should be incorporated into the root zone of the soil before planting or applied as a band at planting. NOTE: Do not exceed a total of 5.5 lb of N plus K per 1,000 ft of row when banding.

Calcium and Magnesium

A good liming program is the most economical way to supply adequate amounts of Ca and Mg. It is important that liming materials be chosen based on the need for Ca and Mg as indicated by soil test levels and base saturation to avoid a deficiency. Refer to Soil Basics: Part II, Chemical Properties of Soil for discussions about base saturation and choosing liming materials.

In situations where soil pH is high, but Ca or Mg is low, we can use materials other than lime to correct deficiencies. Gypsum (calcium sulfate) is about 21% Ca and 16% sulfur (S) and has no effect on soil pH. Likewise, sulfate of potash-magnesia or "sul-po-mag" (11% Mg, 22% K, 23% S) can be used to supply Mg as well as K.

Minor Elements

Minor elements are required in very small amounts and deficiencies are not common. When deficiencies occur, they are usually induced by high soil pH, which limits their availability. When soil pH is at a proper level, minor element deficiencies are rare because they are required in very small amounts. Continuous cropping without replacing minor elements will eventually lead to deficiencies. This problem appears to be happening on some farms in New England. Building soil organic matter with compost or manure should assure an adequate level of minor elements. Although cover crops are useful in building soil organic matter, don't increase the levels of minor nutrient; they merely recycle them. If needed, minor elements can be added to fertilizer.

Of the minor elements, boron (B) is most likely to be needed to supplement soil levels. Cauliflower, broccoli, cabbage, turnip, rutabaga, radish, beets and alfalfa have relatively high B requirements, and may require application of additional B. Solubor or Borax can be used to supply B if needed. CAUTION: Some plants such as beans, peas, cucumbers, melons and Jerusalem artichoke are sensitive to high levels of boron and, in most cases, should not receive B applications. B leaches readily.

Minor elements are difficult to analyze with routine soil tests. Some testing laboratories do not test for minor elements. Plant tissue analyses are more reliable for determining whether or not plants are getting sufficient quantities. However, soil tests may alert you to potential problems before they occur.

Nitrogen

As mentioned above, soil tests measure pH and the levels of P, K, Ca, Mg. These elements are quite stable in the soil, and test results are useful in determining the amounts of additional P, K and lime that should be applied to provide for crop needs.

However, the amount of available N, varies considerably during the year. For this reason, standard soil tests of samples taken in the fall or spring are not reliable predictors of N availability during the growing season. Traditional recommendations call for the application of enough N to supply all or nearly all of the crop needs without considering the value of other sources. Substantial amounts of N can be supplied by residues from previous crops, cover crops, manure, compost and soil organic matter. By crediting these sources, N applications can often be reduced substantially while maintaining and sometimes improving crop yield and quality.

Cover crops

Cover crops are grown on land during fallow periods to protect soil from erosion, build organic matter, smother weeds and to capture unused N which otherwise might leach. Winter rye is a popular winter cover crop and is very effective in taking up residual N. Oats are sometimes used because they winterkill, but they are less efficient at recovering N. When these crops are plowed down they return nutrients to the soil. As discussed in Soil Basics Part III, the C/N ratio or C:N should be considered regarding N availability. If winter rye is allowed to grow late into May, it will typically have a high C:N and will depress N availability. This problem is unlikely if rye is plowed down in early Spring when it is short and fairly succulent and has a lower C:N.

Legumes fix N from the atmosphere and are becoming popular cover crops with vegetable growers. Legumes, most notably hairy vetch, are usually sown in late summer in combination with winter rye or oats. The legume can fix a substantial amount of N, a large portion of which is available to the following crop. A good stand of vetch and rye that is allowed to grow until late May and then plowed down can contribute 80 lb or more N per acre the same year. Nearly all the N in a legume is in the leaves and becomes available rapidly after plow down. This N can replace pre-plant broadcast N, and, like fertilizer N, can be easily leached if heavy rains occur before the crop can use it.

Manure

Animal manure can supply substantial amounts of nutrients for crop needs. The New England Vegetable Management Guide has information about the average nutrient content of various types of manures. Many factors affect the nutrient content (especially N) of manure, and it is best to have it analyzed to be certain. Up to half the N in dairy manure and 75% of the N in poultry manure is in the form of NH3, and is available for crops in the current year. However, NH₃ is volatile and subject to loss unless incorporated promptly. About 40% of the phosphate (P₂O₅) and 90% of the potash (K₂O) are available in the year of application. If the N in manure is protected from leaching and volatilization and incorporated right after application, a ton of dairy manure can supply about five lb of available N per acre and a ton of poultry manure from 15 to 25 lb per acre. It should be noted that there is increasing concern that fresh manure can be a source of food-borne illnesses such as E. coli.

Compost

Well-composted material is stable, and N is released slowly over a number of years. For this reason, N contributions from compost are usually small unless large amounts are added to the soil. To determine availability of nutrients from compost in the current year it should be analyzed for nutrient content. The UMass Soil Testing Lab can analyze compost samples.

Organic matter

On a dry weight basis, soil organic matter contains an average of about 3% N. This is about 600 lb per acre for each 1% organic matter in the soil. Highly decomposed organic matter (humus) has less N. As organic matter breaks down, a portion of this N is released and becomes available for crop uptake. The breakdown and release of N is called mineralization. This process is accomplished by microbes which consume organic matter as food. These microbes thrive in warm, moderately moist soils with good aeration and a pH above 6. Under such conditions we can expect mineralization to provide a significant portion of the crop needs. However, if any of these factors are less than favorable, mineralization will be limited. Research and several years experience on numerous farms indicates that for each 1% organic matter, we can expect 20 to 40 lb of N per acre per year to be mineralized when conditions are favorable. The amount available for crop growth is subject to several variables as mentioned above and it is wise to use the more conservative figure initially. Work to date has been with long season crops (corn, tomatoes, peppers, and squash) which are in the field when mineralization is favored by warm soils. Early, short-season crops are likely to receive only a small portion of the mineralized N.

Applying nitrogen

Fertilizer N is commonly applied as a pre-plant broadcast, a band at planting, liquid starter at transplanting, or as a top-dress during the growing season. Most growers use two or more of these methods. Heavy N applications at one time should be avoided due to the ease with which N can leach in the soil. When seeds are germinating and during early growth stages plant requirements for N are low. If large amounts of N are present during these periods the threat of leaching is high. Pre-plant application of N should be reduced or eliminated, if possible. It is more appropriate to apply a small amount of N (i.e. 20 lb/A) in a band at planting. Additional amounts of fertilizer N, if needed should be topdressed or sidedressed during the growing season when plants have an increased need. A pre-sidedress soil nitrate test (PSNT) is helpful in determining if additional N is needed. It is becoming increasingly popular to apply N as a liquid through trickle irrigation systems, especially where plastic mulch is used. The next fact sheet will address using the PSNT and methods of topdressing and sidedressing N.

Common sources of fertilizer N include urea, ammonium nitrate, monoammonium phosphate, diammonium phosphate, calcium nitrate and potassium nitrate. All of these form are readily available to the crop. Sulphur coated urea is a material which releases N more slowly over a period of several weeks.

Soil testing

Soil tests provide the best way to determine lime and fertilizer requirements. The uninformed "shotgun" approach to fertilizing crops is not practical or economical. You must know the nutrient status of the soil so you can make appropriate applications of lime, organic materials and other amendments. This information is important for cost effectiveness and to achieve optimum growth and plant quality and to safeguard water quality. It is important to achieve proper balance of nutrients and to avoid over application of nutrients, especially N, which can leach into ground water. Just as with commercial fertilizers, improper use of organic amendments can lead to excessive nutrient levels and potential environmental problems.

Late summer or fall is a good time to take soil samples for a few reasons: 1) fall is a good time to apply lime if needed; 2) there is more time to plan crop management for next year; and 3) by knowing what is left at the end of the crop year you can evaluate and refine your nutrient management program.

Soil samples should be representative of the field or area being managed. Unusual areas of topography and soil texture should be treated as different fields and be sampled separately as should individual plantings. Use a clean trowel, spade or coring device to take thin vertical slices, six to eight inches deep, from at least a dozen locations around the field or area. Mix these slices and air dry about one cup for shipping to the lab. Use zip-type sandwich bags and be sure to label them on the outside.