Soil-Notes:

Cascade Analytical - Soil-Notes 1997

• What Does pH Mean Anyway?
• It's An Acidic Cycle
• The Complete Benefits Package
• Boron
• Who's Responsible?
• What's Goin' On Down There?
• Researcher's Corner

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What does pH mean anyway? (Soil Notes 1997)

We have all heard the term pH at one time or another, but what does it really mean? pH is the measure or concentration of free hydrogen ion (H⁺) activity in a solution. These free ions are what cause acidity. The higher the H⁺ ion activity the more acidic the soil is, indicated as a low pH.  Conversely lower H⁺ ion activity results in more basic solutions,  indicated as a high pH.  (It seems kind of backwards doesn’t it?)

What does this have to do with your soil? Well, the pH of a soil has a direct effect on orchard productivity. Soil pH primarily does this by affecting nutrient availability. At different pH values, nutrients may range from deficient to toxic making the management of soil pH an important cultural practice.

	Nitrogen
	Potassium
	Calcium
	Phosphates
	Iron
	Magnesium
	Sulfur
	Manganese
	Molybdenum
	Zinc
	Copper
	Boron

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It's An Acidic Cycle (Soil Notes 1997)

We know that soil pH is important to nutrient availability, but did you know that the type of nitrogen fertilizer that you apply can directly affect soil pH? This is all due to the natural breakdown of nitrogen in the soil by microorganisms through the process called nitrification.

There are three different forms of nitrogen that are commonly applied as commercial fertilizer: urea, ammonium, and nitrate. Regardless of the type of nitrogen fertilizer that is applied, the predominant form of nitrogen in the soil ends up being Nitrate (NO₃^-). This stems from the culprit pin-pointed earlier; nitrification. Through this process both urea and ammonium are broken-down to the nitrate form:

CO(NH₂)₂ + H₂O --> NH₄⁺ + C0₂

Urea + Water --> Ammonium + Carbon Dioxide

NH₄⁺ + 1½ O --> NO₂ + 2H⁺ + H₂O

Ammonium + Oxygen -->Nitrite + Hydrogen + Water

**Watch the "H" in these equations, these are H⁺ ions!

Notice the order of breakdown: urea will be broken-down to ammonium, ammonium is broken-down to nitrite. Nitrite is then turned to Nitrate. This breakdown of urea and ammonium to nitrate results in an acidification of the soil because of the release of hydrogen ions. (Remember, H⁺ ions are what cause acidity) Nitrate is the base form and therefore is not acid forming.

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The Complete Benefits Package! (Soil Notes 1997)

The beneficial affects of soil organic matter on soil and plant health are commonly known. Although, more specifically it is the partially decomposed material called humus that deserves most of the credit. Humus is what is left after the readily decomposed matter has been broken-down and accounts for anywhere from 50-85% of all soil organic matter.

Humus affects both the soil's chemical and physical properties. Physically, it improves soil texture by increasing pore space that allows the movement of air and water through the soil, additionally it decreases erosion and loosens compacted soil. Organic matter also greatly contributes to the water retention of the soil, holding up to 20 times its weight in water! Chemically, humus is composed of negatively charged particles that help prevent the leaching of positively charged particles (cations). It also buffers against changes in soil pH helping maintain uniformity for soil reactions. The large amount of microorganisms found in humus insure the slow release of organically bound nitrogen, phosphorus, and sulfur.

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Boron (Soil Notes 1997)

Boron is not one of the more well known nutrients required by plants for growth, yet it is essential in many of the plant processes (new growth, proper pollination and fruit set). It is not required by the plant in large quantities, in fact it can be toxic! Boron is considered a micronutrient and in Eastern Washington soil it is, on the average, found only in "micro" quantities! Boron occurs naturally in the soil, common in most igneous rocks. So what is it about our soil that seems to set us apart from normal boron levels?

There are several factors that are characteristic to our area that are trademarks for boron deficiency:

• The natural boron released from the soil is slow, therefore it may quickly leach out of sandy or coarse textured soils.
• Soil boron becomes less available to plants as the pH of the soil increases, especially above 6.3 to 6.5. This also may be seen as a temporary deficiency when liming acidic soils.
• Applied potassium may emphasize boron deficiency in low boron soils.

Most important, as with all things in soil, is the soil moisture. Boron deficiency is common in an arid climate with low soil moisture. These conditions decrease the movement of boron in the root zone and its release from organic matter. (Does this sound familiar?!)

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Who's Responsible! (Soil Notes 1997)

A healthy soil environment teaming with a lot of microbes is responsible for the productive growth of our trees.  What’s the name of the ones that turn the nitrogen into the form that the tree can use (nitrification)? The two types of bacteria involved in this process are named, Nitrobacter and Nitrosomonas. These are the major soil nitrifying chemoautotrophic bacteria which are essential for the production of nitrate, the main form of nitrogen absorbed by plants.

Nitrobacter, by definition, is a genus of obligate aerobic chemoautotrophic bacteria which oxidize ions to nitrate in the final stage of the nitrification process. What this means is that these bacteria have to have oxygen to survive (obligate aerobe) and they need only carbon dioxide as a carbon source (autotroph), but that they obtain their energy by oxidizing inorganic substances (Chemoautotroph). The oxidation of nitrite ions to nitrate by the bacteria yields energy for the organism!

NO₂^- + ½O₂ >>>>>>>>> NO₃^-

Nitrite Oxygen Nitrobacter Nitrate

Nitrosomonas, by definition, is a genus of obligate aerobic chemoautotrophic soil bacteria which oxidize ammonium ions to nitrite in the first stage of the nitrification process. The specific process mediated by these bacteria is:

NH₄⁺ + 1½O >>>>>>>> NO₂^- + 2H⁺ + H₂O

Ammonium Oxygen Nitrosomonas Nitrite Hydrogen Water

What can you do to help this process along? The rate of conversion of nitrite to nitrate is affected by soil temperature, pH, and the amount of molecular oxygen in the soil (soil aeration is important here!). The number of both organisms is stimulated by the addition of ammonium ions to the system (i.e. fertilizer). However, very high concentrations of ammonium ions, can inhibit the activity of Nitrobacter.  So fertilize judiciously?!

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What's Goin' on Down There? (Soil Notes 1997)

It’s called nitrification and it is necessary for the provision of the main form of nitrogen used by plants, nitrate. The process of nitrification is the formation, in soils, of nitrites and nitrates from ammonium ions through the activities of certain soil bacteria.

Nitrification is a two-step biological process in which the ammonium ion (NH4⁺) is first converted to nitrite(NO₂^-) then to (NO₃^-) by bacteria. The process is the same regardless of where the ammonium ion originates (soil organic matter, legumes, or fertilizers). Nitrification is most rapid in warm, moist and approximately neutral pH soils. The process halts entirely when the temperature drops to about 37 °F.

Although this process is all fine and good during the growing season, it becomes a disadvantage when applying fall ammonium-producing fertilizers. Nitrate produced by nitrification during this time can be leached from the soil before it can be utilized by the plant in the spring, but Mother Nature can be fooled. In an effort to reduce nitrification of fall-applied fertilizers, many consultants and commercial laboratories recommend delaying N application until the soil temperatures reach 45-50 °F. Nitrification inhibitors are now available to slow the rate of nitrification. These inhibit the growth or the activity of the nitrifying bacteria but are eventually decomposed and nitrification continues.

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Researcher's Corner (Soil Notes 1997)

Composts and Nitrogen Content

This discussion was excerpted from The Compost Connection (January 1997) by David Granastein on the important properties to consider when buying compost.  Here we will focus specifically on nitrogen content, but many factors influence the suitability of a compost for use.  The end use of the compost will influence what major nutrient content you seek in a compost product.  

Some growers are particularly interested in compost with a high nitrogen content.  Composts vary widely in N content, largely due to differences in N of the original materials.  Composts vary even more in the amount of N they release over time.  It is relatively easy to determine the N content of compost (total N, available N) with laboratory tests as presented in Table 1.  However, it is much harder to predict how the N will be released during the growing season and beyond.  In the table, you will note a wide variation in total N, ammonium N (NH4+), and nitrate N (NO3-) among the composts.  Total N usually represents the sum on organic N and inorganic N (ammonium plus nitrate).  This is one reason many growers prefer an inorganic fertilizer over an organic source - the former is more predictable.

Available N.  In agriculture, available N generally refers to the inorganic forms nitrate and ammonium which is considered readily "available" to plants.  The process of conversion of organic N to inorganic N (as ammonium) is called mineralization.  Microbes are responsible for this.  A nitrogen mineralization test can be run on compost to estimate when and how much N will be released.  On a wet yard or ton basis, one can calculate the amount of available N from a compost.  This is calculated as follows:

dry matter fraction = (100 - %moisture) / 100

dry bulk density = wet bulk density x dry matter fraction

%available N = (ppm ammonium-N + ppm nitrate-N) / 10,000

lb. available N/yd compost = (dry bulk density x %available N) / 100

Another set of microbes then converts the ammonium to nitrate in a two-step process called nitrification.  Under normal conditions, there will be little ammonium in soil or in mature compost because it quickly converts to nitrate.  Ammonium is not subject to leaching in the soil, but can be lost through volatilization (gaseous loss), and can be toxic to roots.  Nitrate is subject to leaching.

Release of organic N. A fraction of the organic N will be released over a growing season, depending on factors such as temperature, moisture, soil pH, and C:N ratios.  Generally from 10 to 50 % of the organic N is released for plant growth over the first growing season.  There is a rough relationship between the C:N ratio of the compost and the release rate of organic N to available N.  Generally, composts in the range of 15 to 25:1 will release N at an intermediate rate.  Composts with a C:N ratio of less than 15:1 would release N more quickly and would fall into the higher range of 10 to 50% release rate for most composts.  Composts with a C:N ration of 25:1 or higher would release little available N immediately.  If one were to use this compost as a N nutrient source, additional N would need to be added.

Compost maturity.  A mature compost is one where most of the biological activity has ceased and it is relatively stable.  Mature compost will generally have a dark color, and earthy smell, and will not have a detectable ammonia smell.  Mature compost is generally at or near air temperature, and not hot or steaming.  As a rule, these composts will generally contain more nitrate than ammonium.  Nitrate is formed through the process of nitrification which occurs later in the composting process.  Composts made from chicken manure may have a detectable ammonia smell because of the high N content from the uric acid in poultry manure or because the compost is not mature.  This compost still may be used, but more caution is needed where the material may contact plant roots.

If nutrient additions are the primary purpose for using a compost, then the balance of major nutrients must be considered and agronomic rates determined.  Guidelines for compost end use are being developed in several states, including Washington.

Table 1 - Compost Analysis

Manure - In the US, manure generally means the excreta of animals, dung and urine with straw or other materials used as the absorbent.  Cattle, poultry, sheep and goat manure are dried and sold as fertilizer.  In this form, the plant nutrients in feces are almost entirely present as insoluble compounds that must be decomposed before they are available to the plants.  Those in urine are immediately available in the form of urea.  Listed below is a table for the average composition of manure reported in percent.

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