This is the second of a ten part series of posts chronicling core concepts from a class we are taking. Further details and links to walk through the series can be found on the class 1 post.
Soil Acidity (pH)
The primary focus of this class was a review of soil fertility concepts, predominantly focusing on pH. Soil acidity, pH, is one of the most common factors which will be listed on a laboratory soil test. Two metrics will be provided, soil pH and pH buffer. Soil pH is the present acidity, literally it stands for “concentration of hydrogen (H)”. pH uses a logarithmic scale, which means that a change of 1.0 on the scale indicates a 10x change in acidity. So a pH 5.0 soil is 10 times as acidic as a pH 6.0 soil and 100 times as a pH 7.0 soil. Other commonly used scales that are logarithmic (10x per increment based) include audio decibels (dB) and the Richter scale. The pH scale ranges from a low of 1.0 to a high of 14.0. Neutral acidity is 7.0, which represents distilled water. Stomach acid is quite acidic, with a pH of 3.0. Lye (used in making soap) is very “basic” (the opposite of “acidic”) with a pH of about 13.0.
Rain water is somewhat acidic, usually about pH 5.7. From this we can immediately guess that areas with high rainfall (Oregon west of the Cascades) will have acidic soils and areas with low rainfall (Oregon east of the Cascades) will have basic soils. Most western Oregon soils have a pH between 4.8 and 6.0. Unfortunately, most pasture plants prefer a soil pH of 6.0 to 7.0.
Next we looked at a chart which showed how common soil nutrients are affected by pH. In general, most nutrients (nitrogen, phosphorus, potassium, sulfer, calcium, magnesium, iron, manganese, …) become less available as the soil moves away from a neutral pH (7.0) in at least one direction. Phosphorus is especially effected by pH. Changing the soil from 5.0 to 6.0 will double the availability of phosphorus to plants. This doesn’t “add” any phosphorus the ground, but at a more neutral pH more of the existing phosphorus will enter solution where it can be used by plants. If you tell a soil scientist that you only have $100 to spend on fertilizer, he will tell you to spend it on lime to fix the pH first!
There are other important minerals affected by pH. Molybdenum availability decreases with pH. This is indirectly important for sheep. Sheep are easily affected by copper toxicity, but uptake of molybdenum in their diet ties up the copper in their gut, preventing harm. As the pH goes down, they are more at risk from copper. (Molybdenum is a key element in an enzyme used by nitrogen fixing bacteria, so legumes do worse as pH goes down also.) Minerals don’t just become less available as pH changes. Some become too available. Manganese, for example, becomes more plentiful with lower pH, so many plant problems from low pH are caused by manganese toxicity.
What followed was a rather involved chemistry explanation for soil acidity and how it is neutralize by lime. The short answer is that lime (calcium carbonate or CaCO3) raises the pH by means of a chemical reaction which produces water, carbon dioxide, and free calcium. Since calcium is a useful soil mineral, this is a simple and non-toxic way to fix pH problems, but how much do you apply?
As mentioned above, there are two pH-related metrics on a soil test. The second is “buffer index”, or “pH buffer”. This number is an indication of how resistant the soil will be to changes in pH. Sandy soils change pH easily. Clay soils respond slowly. Given a pH buffer value, you can consult a table and determine how many tons of lime are required per acre to raise the pH of the top 6″ of soil to a certain goal level. Note that pH changes are unfortunately not permanent, but should be considered a 4-6 year investment in pasture productivity. Rain will eventually return the soil to its original pH. Generally, no more than 2 tons of lime should be applied per year, and soil tests performed every 2-3 years to monitor the pH level. Lime is generally applied in the fall so that the reaction can work over the winter and the boosted pH be available for spring growth. Animals can still graze pastures after liming, as the mineral is often used as a calcium supplement in their feed.
In western Oregon, our goal should be to raise soil pH into the 6.0 range, but productive pastures can still be grown at lower levels. Many grasses, plaintain, and chicory will grow in soils as low as pH 4.9. Some legumes, such as red clover, can handle pH 5.5. Don’t plan on growing much alfalfa, which prefers a pH of 7.0 to 7.5.
Most people are familiar with the NPK numbers on the side of a bag of commercial fertilizer. For example, “triple 16” available here in Oregon is written 16-16-16-6. The 6 is for sulfur, and is sometimes omitted. These numbers do not represent the percentage of each of those elements available in the fertilizer by weight. This is a common misconception. The “N number” does represent “percentage of nitrogen”, and the “S number” does represent “percentage of sulfur”, but the remaining two are different. The “P number” is actual “percentage of phosphorus oxide”, which is P2O5. Phosphorus comprises only 44% of phosphorus oxide by weight. The rest is oxygen. Similar, the “K number” is “percentage of potash”, which is K2O. Potash is only 83% potassium.
Note that these definitions are by U.S. law, rooted in the history of how nutrients were once calculated, and no doubt kept in place by the fertilizer industry who wouldn’t want to rewrite 16-16-16-6 as 16-7-13-16-6. (Not very catchy is it?) Unfortunately, most topics in animal and plant nutrition will talk about actual weights of nitrogen, phosphorus, and potassium, while the fertilizers deal in these compounds containing the desired elements. To get around this ambiguity of expression, farmers will use the phrase “unit of”. A “unit of N” is a pound of nitrogen, and a “unit of P” is a pound of potassium.
Finally, a word of warning about fertilizers. Many chemical fertilizers can have an acidifying effect on the soil. This can drive down the pH, potentially lowering the availability of other minerals. Such fertilizers will be labeled with “equivalent acidity”. This value is the number of pounds of lime that should be added for every 100 pounds of fertilizer, to neutralize the acidifying affect. For example, adding 100 pounds of anhydrous ammonia (a highly concentrated nitrogen fertilizer used on factory farms) requires 148 pounds of lime to neutralize the acidity.
Mining the soil
Finally, we considered some numbers to reinforce the point, first presented last week, that selling hay off a field without restoring equivalent fertility was literally mining it’s nutritional value and reducing it’s future productivity. An average field might produce 2 tons of hay per acre. As the truck leaves your property with 2 tons of hay (3600 pounds of dry matter), it’s taking with it approximately:
- 72 pounds of nitrogen
- 8 pounds of phosphorus
- 36 pounds of potassium
- 8 pounds of sulfur
In general, the cost of replacing these minerals with fertilizers will exceed the profit from selling the hay. Famous pasture farming advocate Joel Salatin uses this to his advantage. His pasture-raised chickens are also fed a grain suppliment. Much of the nutritional value of this grain is then distributed onto the pasture in the form of chicken manure. This results in a net import of nutrients to his farm. Even if you aren’t importing feed, grazing animals are predominantly nutrient recyclers, not consumers. However, proper management is still needed to prevent animals from redistributing nutrients to areas of tree cover (where they rest and fertilize after grazing).
There was lots more I didn’t cover: grass characteristics, diversity of commercially available cultivars, lime sources, lime score, and use of selenium additives in Oregon. The reading for next week is taken from Greener Pastures on Your Side of the Fence: Preface, Chapter 1, and Chapter 3.