Soil Science for Vegetable Producers

By Dr. David Nagel, Extension Horticulturist, Department of Plant and Soil Sciences.
Publication 1977 (rev-250-9-00) Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 14, 1914.
The original version may be found at: http://msucares.com/pubs/publications/p1977.pdf
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Text below has been edited by Robert Moore, Director of Organics Recyling Group, International LLC (ORG) as indicated by [brackets] and/or bold and italic emphasis for clarity - as this was published almost ten years ago.
Reader should also be aware that this document reads like a technical paper - not surprising, since it was written by a soil scientist - with focus toward commercial agriculture, not home gardening...
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Soil is a natural body [in its undisturbed state] synthesized from a mixture of broken and weathered minerals and decaying organic matter that provides support and sustenance for vegetable plants. Soil consists of four major components: mineral matter, organic matter, air, and water. The relationships between these four constituents and the types of mineral and organic material determine the suitability of soil for producing vegetables.
The properties of soil are divided into two major groups: physical properties and chemical properties.

PHYSICAL PROPERTIES

Soil’s physical properties are governed by three major factors: the type and size of soil particles, the structure of aggregates of those particles, and the type and amount of organic matter [along with resultant microbe population diversity and density] in the soil. The most dominant factor in a soil’s physical properties is the size of the individual mineral particles. [only according to proponents of soil science - this author believes that the percent of organic matter is more important than soil particle size since it is the determining factor for CEC, thereby water, nutrient and air retention]. The differently sized particles are grouped in separates according to USDA standards in the following table.

Information below is provided in the following order:
Separate name -- Size diameter in mm -- Particles per gram of soil -- Sq cm per gram (Surface Area)

Gravel -- 2 mm -- <90 p/g -- <10 sq.cm/g
Coarse Sand --1 to 2 mm -- 90 p/g -- 11 sq.cm/g
Medium Sand -- 0.5 to 1 mm -- 720 p/g -- 23 sq.cm/g
Medium Fine Sand -- 0.25-0.5 mm -- 5,700 p/g -- 45 sq.cm./g
Fine Sand -------0.10-0.25 mm - 46,000 p/g -- 91 sq.cm/g
Very Fine Sand -- 0.05-0.10 mm -- 722,000 p/g -- 227 sq.cm/g
Silt -- 0.002-0.05 mm -- 5,776,000 p/g -- 454 sq.cm/g
Clay -- <.002 mm -- 90,260,000,000 p/g --------- 8,000,000 sq.cm/g

Clay

As shown in the table, clay particles are very small. This small size allows clay particles to become tightly packed together. Additionally, clay particles are normally flat and will stack like plates. The small size and stacking tendency make soils dominated by clay particles relatively impervious to water [which means almost no percolation]. The holes or voids between the clay particles are quite small [which means that clay soil will not contain much air, or respiration, which is a major limiting factor of aerobic microbes], and the large surface area binds water to the particles [meaning, when clay gets wet, it stays wet for a long time].

There are three major groups of clays. Kaolinitic clays are six-sided flat crystals [more properly termed 'particles' as before] formed in highly weathered soils. Smectites, or montmorillinitic clays, are more complex, flat crystals [particles] that form in younger soils. Illitic clays are found only in young soils. Although these three clays may have particles the same size [relative thickness], they have different properties. Smectite clays are made of two layers or sheets of crystal units loosely held together. When these clays get wet, the sheets get farther apart [swell in thickness] . When these clays dry, the sheets get closer together [shrink in thickness]. The ability of clays to undergo this transformation is called their shrink-swell capacity [also termed "Potential Vertical Rise" (PVR) of the ground due to the combined internal swelling force, which can break concrete structures].

The soil’s ability to pull the plant roots apart during dry spells [due to the force of swelling/shrinkage] makes growing vegetables on a soil with a high shrink-swell capacity difficult [but amending soil within the plant root zone will significantly reduce the problem, to the degree of organic matter present in the dirt]. Illitic clays also are made of two sheets, but potassium ions between the sheets do not allow the clay to swell [as much]. Kaolinite has only one layer and is not subject to the shrink-swell phenomenon [although individual clay particles do swell when wet, and shrink when dry].

Silt

Silt particles are between clay and sand particles in size, but their properties are more like small sand particles [with regard to reaction with moisture].

Silt particles are irregular in shape and are seldom flat or smooth. Many are eolian or wind-deposited and tend to have rounded edges. Since most particles are the same size and shape, there is little arrangement or structure in silt soils. Wet silt tends to have a film of water around each particle [which is a noted condition with relation to certain bacteria that inhabit water films], and this water acts as a lubricant, allowing each particle to slide past another.

For this reason, predominantly silt soils have little load-bearing strength when wet [such as 'quicksand' has very little load-bearing capacity].

Silt soils also tend to form a fragipan, a layer of dense, compacted soil 12 to 24 inches below the surface. This dense layer restricts water and air movement and prevents root penetration, even of [strong] trees. Reflecting its name, the fragipan shatters when horizontal pressure is applied. Inexplicably, it is found in sites with no vehicular traffic [which would tend to 'shatter' the fragipan layer from resultant vibration, and to that extent is subject to excessive wind and water erosion].

Sand

Sand is the largest of the soil particles. In the Southeast [U.S.], most sand is made of quartz. The individual particles or grains can be irregularly shaped and jagged, or they can be smooth and almost round. Sand is very important to vegetable growers, since the sand grains are separated by relatively large voids, which allow movement of air and water [but can be a detrimental factor with regard to the lack of sand to retain water]. Sand grains also provide a soil that is resistant to compaction because of their large size and toughness [air voids are maintained regardless of compaction attempts].

Soil Structure

Individual soil particles tend to flocculate [clump together] into aggregates [groups of particles] because of chemical and physical attractions. [For example] Cations such as calcium, magnesium, and aluminum with more than one positive charge act as an electrostatic “bridge” to bind two negatively charged clay particles together.

Limestone is added to acid[ic] soils to replace [actually, to combine] singly charged hydrogen ions on the clay particles. This has a beneficial effect on the soil pH, and the calcium [primarily via application of agricultural gypsum, which] improves soil structure by binding together the clay particles [into aggregates].

Improved structure forms larger voids which increase air and water movement through the soil [but still, mostly in conjuction with organic matter - such as mature compost - to improve overall tilth].

Soil_Textural_Triangle.jpg?

Soil Names

Soil names are based on the amount of sand, silt, and clay [which is the ONLY requirement for soil taxonomy] found in the surface horizon and on the physical and chemical properties of the subsoil [no - just on the taxomomy - not including the chemical properties, which are primarily affected by the percentage of organic matter]. The textural triangle gives the terms used to describe various mixtures.The best soils for vegetable production are found in the left corner of the triangle — those with at least 40 percent sand and less than 20 percent clay. Vegetables can be grown in any soil, but better management techniques are required on a less-than-ideal soil
[OK - Dr. Nagel is saying that 40%+ sand, with less than 20% clay is ideal But, that leaves a LOT of room for the 40% in between, supposedly filled ONLY by a higher percentage of more sand and silt, since the taxonomy ONLY considers those three as valid. This author would like to remind readers that WHATEVER the ratio of sand-to-silt-to-clay - what you have, is what you have - with the IDEAL combination including less than 20% clay. So where does that leave folks who garden on land that is more than 20% clay? While not ideal, the answer is NOT just more sand OR silt (although some might be needed) added to clay, but rather the amount of ORGANIC MATERIAL to make the difference to making whatever you do have, better FERTILE garden DIRT].

Soil Horizons

Soils are not uniform throughout their depth.
In general, the surface layer or horizon will be higher in organic matter and lower in clay than the layers beneath it [because of natural 'settling']. The fragipan is an example of a special horizon [which is NOT often found]. The top layer is called the A horizon [where most of the organic material is found, except in the case of gardens that have been double-dug]; the second layer, which is normally higher in clay, is called the B horizon [in which organic matter is not high, BECAUSE most of it has decomposed by the time it 'settles' into that zone, but it is the horizion at which most ACTUAL humus is found]; and the C horizon is found above unweathered material [i.e., bedrock - at which level almost NO organic matter is usually found in ANY type of soil].

Soil series are separated by the properties of their subsurface [below-ground] horizons. Soil types are soil series separated by the texture of the surface horizon ['in other words, as soil minerals become increasingly larger, soil horizons are distinguished, base on an arbitrary 'standard' depending on the type of minerals present]. This naming system indicates that a spot on a soils map marked “Arredondo sandy loam” [for example] would be an excellent site for a vegetable field because of its deep, well-drained characteristic. [well, this notation indicates that a soil that is a bit higher in volume of sand, than for silt and clay, is best - but at best, is a "generalization" that organic gardeners should IGNORE, since this taxonomic-based recommendation totally ignores the amount and quality of organic material - and incorporated microbe density and diversity - needed in DIRT to grow vegetables in a home garden. Soil is NOT the 'whole story' about the best medium in which to grow vegetables, REGARDLESS of the environmental climate, of ANY particular garden planting area'].

Soil maps exist for most counties in the United States and are available from the Farm Service Administration [agree with Dr. Nagel that the Farm Service Administration has better online access to soil maps than the USDA/ARS/NRCS sites do].

Organic Matter

Organic matter is comprised of the [MATURE DECOMPOSED] remains of once-living tissue such as animals or plant roots, stems, or leaves. Although organic matter normally accounts for less than 1 percent of the soil’s volume in most states, it is important [yeah, because it is the ONLY viable material in ANY soil that significantly contributes to major growth of ANY plant]. It enables individual soil particles to bind together into aggregates, it contributes to cation exchange capacity, and it provides water-holding [AND air] ability ['capacity' is the correct term]. Humus is organic matter that is decomposed past recognition [WAY beyond visual recognition - humus is actually the FULLY decomposed form of organic matter, that microbes can no longer decompose ANY further - as close to pure carbon as an organic substance can get]. Although humus is important to soil, it is relatively short-lived and must be replaced constantly by the action of microorganisms on un-decomposed material [WRONG - just goes to show that a PhD may not be a specialist in certain areas. Humus is THE most PERMANENT organic substance found in soil which has been proven to last THOUSANDS of years. Dr. Nagel is making the very common mistake, that the word "humus" applies to NOT-fully decomposed organic MATTER (compost), between which, there is a MAJOR distinction].

Water-Holding Capacity

Water-holding capacity is the ability of a soil to retain water against the pull of gravity [well, that's not the best way to put it. A major condition is how well an organic material HOLDS water within its structure, which is also a function of cation exchange]. Smaller particles have a higher water-holding capacity because of the large surface area per unit of volume [if the reader did not comprehend that sentence, just 'blow it off'. Water is retained by surface tension in addition to sub-surface conditions]. Surface tension is displayed in the beads of water that form on the outside of a cold glass [condensation]. Until the beads gain enough weight to overcome the force holding them to the glass [or the water molecules are affected by a surfactant] , they seemingly will defy gravity [very poor definition of surface tension]. Surface tension causes an interesting phenomenon in soils. Water will not move out of a small diameter void into a large diameter void until the force of gravity overcomes the surface tension [so what happened to osmosis?]. Therefore, soil with a mixture of large and small particles may have a higher water holding capacity than a soil made up only of small particles [to: organic gardeners - just focus on amending your dirt with the highest quality organic matter possible - and Mother Nature will 'take care' of all those poorly-defined 'technical' issues].

CHEMICAL PROPERTIES

Cation Exchange Capacity

Cation exchange capacity, or CEC, is the ability of a soil to retain positively charged ions. Since potassium, ammonium, calcium, magnesium, zinc, iron, copper, and manganese [usually] exist in the soil solution [unless the soil is mostly sand, which contains VERY LITTLE] as cations, the soil’s ability to hold onto these [positively-charged ions] is important for the health of a plant. CEC is determined almost entirely by the amount and type of clay and organic matter in the soil. Kaolinitic clays have a CEC of 5, while illitic clays may be as high as 20, with smectites as high as 80. Humus has a CEC of 200 [hooray for compost and humus!!]. The ability of these colloids to hold onto cations results from the negative charges that arise from broken edges on clays and organic acids [such as humic and fulvic acids] in organic matter. Cations are bound to soil particles and move very little with water [unless you know the volume of clay in the dirt in which you garden, the focus should be on adding as much mature organic matterr as possible to the DIRT that you raise vegetable plants in. Mature HOME-MADE COMPOST is the very BEST form of mature, microbially-active organic matter that a gardener can hope to obtain].

Soil pH

Soil pH is a measurement of the amount of acidity in the soil solution or the water part of the soil. Technically, it is the negative logarithm of the hydrogen ion concentration [pH means: "percent Hydrogen"]. Vegetable growers should be aware that there is 10 times more acidity at pH 4.5 than at pH 5.5. [and 10 times as much alkakinity in 8.5 than 7.5] To neutralize acidity will require 10 times as much limestone in pH 4.5 soil as in pH 5.5 soil [and ten times as much elemental sulfur to neutralize 8.5 pH soil than 7.5 pH soil]. On the scale, pH 7 [7.0] soil is neutral. Numbers larger than 7 indicate basic or alkaline soil; numbers less than 7 indicate acidic soil.

Soil pH has a dramatic influence on nutrient availability. The following chart [depicted on the website] shows nutrients are most available at pH levels between 6.5 and 7.5 [expand that to 5.5 and 8.5 - beyond which most vegetable plants can uptake nutrients just fine]. Commonly grown vegetables in the Southeast [U.S.] prefer slightly acid soils [most vegetables prefer dirt that is higher in organic matter than most soils provide around the entire world. What every gardener should realize, is that organic matter contains microbes that decompose that matter, who work VERY hard to bring ALL dirt into a neutral pH] . The proper pH range benefits both the vegetable crop and the soil microorganisms [Hey - the pH does not just 'benefit' soil microbes - but rather soil pH is adjusted BY soil microbes who actually do the work to bring pH into balance. I'm sorry Dr. Nagel, but it is quite clear to this author, that you may be a great soil scientist, but are NOT an an experienced organic gardener who understands the inner-workings of microbes to decompose organic material to create organic matter from it, to benefit vegetable plants].

If soil is outside the pH 5.0 - 8.0 range, soil-dwelling bacteria and fungi cannot survive [that is simply a NOT TRUE statement] and fulfill their roles in decaying fresh organic matter [wait just a minute - what in the world is "fresh" organic matter? Hello - there is NO such thing - organic matter results from the decay (decomposition) of fresh (raw) organic MATERIAL by bacteria, fungi, actinomycetes, protozoa, algae and larger 'decomposers' and such microbes CAN exist - and function - in highly-acetic AND highly alkaline environments. Scientific documents should at least be technically correct...].

Apply limestone whenever soil is too acidic. Liming [can also help] prevents manganese toxicity. Normal levels of manganese can become toxic to plants if the soil pH is below 5.2. Finely ground limestone may react with the soil in 1 month, but it sometimes takes 3 months for full benefit. Dolomitic limestone can be used if low soil magnesium levels have been identified [WHOA !! What happened to the benefit of amending with organic matter? Here we have evidence that Dr. Nagel is not an organically-oriented scientist - because finished compost (organic matter) can and will accomplish what neither limestone nor sulfur can do, by microbial activity, without 'loading up' the planting medium with substances that are just as likely to 'throw off' pH too far the other way, as help it. Exactly how much of what kind of limestone or sulfur should be added to what volume of planting media? With finished compost, there is no need to be greatly concerned about those issues, because Mother Nature knows what 'she' is doing. If the reader does not believe that, then this author thinks that the reader should not consider themselves an 'organic gardener'].

Soil Testing

Soil for vegetable fields should be sampled and sent to a laboratory for testing at least every other year [Whenever a gardener substantially amends the vegetable planting dirt, the soil should be considered a candidate for testing, because it has changed. On the other hand, laboratory testing - while important to establish an initial 'baseline', is expensive to perform repeatedly. Testing of garden dirt can be accomplished by several types of self-tests, that can provide very reliable results with practice].

Testing gives the vegetable producer information on the available nutrient level in the soil [among other important aspects]. This information is necessary for proper liming and fertilization schedules [it is apparent to this author that Dr. Nagel is referring to a large extent of agricultural land, not the 'average' homeowner's garden plot. Seems that is the major distinction between the word "Producers" and the word "Gardeners". As the reader can probably discern, www.thesoilguy website is geared toward home-style organic gardening, and NOT synthetic gardening with heavy equipment. Unfortunately, there's not a lot of government-sponsored scientific research devoted to home-style gardening, and it is especially easy for the home organic gardener to become confused about conflicting recommendations from soil scientists - some of whom have never even gardened organically, much less operated an agricultural farming enterprise - and neither have most Agricultural Extension Service Agents (in this author's opinion)].

[So, this author's opinion is: organic gardeners - beware of what you read, that is written by soil scientists, with a focus on 'agricultural' farming practices {as compared with home-style organic gardening - even up to several acres of vegetable production).
Farming and organic gardening practices are NOT usually the same, and most commercial agricultural practices are almost entirely focused on synthetic chemical use - with the noted exception of farms which advertise as producers of organically-grown produce. Organic farming is a very laudible practice in this author's opinon, and I have great respect for people who go to the time and trouble to garden as nature intended.]

2010 [editorial comments] by Robert Moore, aka The SoilGuy