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Why Test your Soil?
Soil Testing
Prior to the 1900’s little was known about the plant
nutrients that naturally occur in soils. The Indians taught the first settlers
to drop a fish into the hole in which they would plant the corn that they grew
(We know now it provided the nitrogen all plants need to grow). Most farmers on
the east coast would farm the land until it could no longer grow a crop
successfully then they would move on further west to land that was still
fertile. In other parts of the world other techniques were used to keep the land
productive. In China and the Far East, human wastes were collected and put on
the farm land. Americans did not wish to use that method because they knew it
could carry parasites from one person to another. With the further development
of science it became known that the nutrients needed in the soil were three
major elements, nitrogen, phosphorus, and potassium. With this information and
with the development of the fertilizer industry, farmers at last had an easy
source of these three nutrients and as the Green Revolution came about the
knowledge of fertilization spread around the world. As short a time as 15 years
ago gardeners were exhorted at every point to fertilize, fertilize, fertilize.
This was good for the fertilizer industry but was it good for the soil? It was
easy to apply a balanced fertilizer that could supply all the major nutrients
that plants needed to grow exuberantly.
America had become the place that grew the wheat, corn and other grains that
kept other less prosperous parts of the world alive. As more and more fertilizer
was applied to the land, the waters that drained from that land became polluted
with nitrates and phosphates. Rivers and lakes began to die from the bloom of
algae in those enriched waters. Something had to be done. Although soil testing
had been going on for many years it was not widely practiced. The home gardener
rarely thought about having the soil tested. If the garden didn’t grow as he
wished it would he simply applied more fertilizer.
We in Albuquerque had bought into that idea along with the rest of the world.
In 1982 the Albuquerque Garden Center was given soil testing equipment by a
retired Extension Service agent. He gave it to the Garden Center with the
expectation that the Master Gardeners would be using it. The Garden Center was a
not-for-profit organization and the Master Gardeners were not. He could take a
tax deduction for the gift. Soil testing was immediately begun at the Garden
Center. Over the years records were kept of the soil test. Those doing the
testing noted that there were very few test that indicated a need for phosphorus
or potassium. In 1992 I began using the computer to prepare the reports. Those
of us doing the testing again noted the rarity of short phosphorus or potassium
supply in the soils we tested. I finally made up a report from the computer data
of the actual soil test readings. To my surprise, less that 1 percent of the
soils we tested were low on potassium and less than two percent were low in
phosphate. What does this mean to the Albuquerque gardener? We have always been
told to use a balanced fertilizer or at least one that has all three of the
three major nutrients. Now we must change our practices.
We should use only nitrogen fertilizers unless we have reason to believe that we
need phosphorus. Only the gardeners who repeatedly plant a crop and remove all
the crop and plant residues from the property, have any need for potassium and
almost equally rarely will the phosphorus be in short supply. If you have any
doubt about the nutrient levels of your soils a soil test will tell you whether
you need fertilizer and what kind. Along with testing for nitrogen, phosphorus
and potassium, we could test for pH ( the acidity or alkalinity), total soluble
salts and organic matter. Our native soils have very high pH and very low
organic matter and usually have low soluble salts. Soils that have had high
amounts of manure added may have high salts. Most plants do not do well in salty
soils and poor drainage in clay soils or in areas that are lower than the river
can aggravate the problem as the salt is not leached away be rainfall or by
heavy watering. We need to apply different criteria to fertilizing practices in
the Albuquerque area. Does the soil need that nutrient? Does adding the nutrient
even if needed change the effect of the other nutrients. Do we want to increase
the wasting of nitrogen and phosphorus by sending them down to the river in the
drainage water?
We have enough potassium in our soils to last us for many years. Why should we
pay for fertilizer that only adds more potassium to our soil and as it
accumulates causes stunted growth in some of our plants, particularly iris? If
you don’t know what is in your soil have it tested. The cost is very low
compared to the cost of adding potassium you don’t need or phosphorus you
probably don’t need.
In the years that have passed since the above article was written, more attention has been paid to the effects of the fertilizer leaching into the lakes and rivers and the eutrophication (deficiency in oxygen) that has taken place. Now we are seeing those same effects in the Gulf of Mexico. In a recent article in Science News it was noted that large bodies of waters in the Gulf have become totally dead. This is believed to be caused by the dumping of wastes from the rivers into the Gulf. Farmers know that losing nutrients to leaching is costing them money every time it happens. So they are careful about fertilizing, so much of that fertilizer going into the rivers and then to the Gulf is thought to be the result of the leaching of nitrogen from over fertilized lawns and gardens.
How to Calculate the amount of fertilizer you need.
Knowing how much fertilizer to apply to lawns or
crops and how and when to apply it is essential for the responsible gardener.
Soil tests can help in determining how much, although the
common practice of side dressing or broadcast fertilizing if practiced with some
restraint and observing the response will be effective for the experienced
gardener. The natural result of over fertilization is death of the plants that
have been over fertilized. This is usually sufficient to warn the novice that
too much is as bad (or maybe worse) than too little. For help in
calculating amounts go to Fertilizers.
Timing of Fertilizer Application
Soil type is one factor that dictates the frequency of
fertilizer application. Sandy soils require more frequent applications of
nitrogen and other nutrients than clay soils. Other factors affecting frequency
of application include the type of plants being grown and what is expected of
it, the frequency and amount of nitrogen or water applied and the type of
fertilizer and its release rate. The plants being grown influence the timing and
frequency of fertilizer application since certain plants feed more heavily on
some nutrients than others. For example, in the vegetable garden, root crops
require less nitrogen than leafy crops. Corn feeds heavily on nitrogen, while
most trees and shrubs are generally light feeders. Corn may require nitrogen
fertilization every 4 weeks, while most trees and shrubs perform nicely with a
single, well placed application every year. Turf grasses are heavy feeders of
nitrogen, requiring at least two applications per year for optimal growth and
appearance. Splitting the application into two smaller quantities therefore is
recommended.
A general rule of thumb is that nitrogen is for leafy top growth, phosphorus is
for root and fruit and potassium is for general health and durability.
Crop Fertilization Helps
Stabilize Carbon in the Soil
The primary role of crop
fertilization has been and will continue to be that of increasing crop yield and
quality. An additional benefit is
improved environmental protection. It has been shown that proper fertilization
results in fast growing, vigorous plants that rapidly close their above-ground
canopy to protect the soil from the effects of wind and water—and thus runoff
and erosion. Also, healthy crops develop massive root systems that help to hold
the soil in place. Another environmental benefit of fertilization is that it
contributes to the stabilization of carbon (C) in the soil. Carbon dioxide (CO2)
is one of the three primary global warming gases. When C is tied up or
stabilized in the soil, the release of CO2
into the atmosphere is lowered, thus
reducing the potential for global warming.
There is strong evidence that
there is a relatively stable sink of CO2
in North America. For example, the U.S.
exports more C in agricultural and wood products than it imports. Further, it
stores about 500 million tons of C annually in forest and non-forest soils.
Indications are that the agricultural sector is storing more C in soil organic
matter and crop residues than it once did, in part due to use of conservation
tillage and crop fertilization.
A proper nutrient management
system, one that considers existing soil fertility and the need for supplemental
fertilization, aids in the capture of atmospheric CO2,
improves photosynthesis, enhances the release of oxygen into the atmosphere, and
increases soil organic C. For example,
research conducted by the U.S. Department of Agriculture (USDA) has shown that
nitrogen (N) fertilization increases both soil organic C and the soil’s
productivity.
Extensive reports from
long-term research indicate that whenever N fertilization results in higher crop
yields, the accumulation of C in soil organic matter also increases.
Furthermore, there is evidence that N itself is chemically involved in the
stabilization of soil C. It is thought that N compounds are involved in the
formation of humus and, as a result, help to stabilize C in soil organic matter.
Other long-term studies have shown that soil organic C levels are highest when
conservation tillage is combined with rotations of high residue crops and
adequate fertilization to increase yields.
The
role of crop fertilization in protecting the environment is undeniable.
Helping to stabilize C in the soil is an
important example. The key to the total benefit of crop fertilization—for yield
and quality increases and environmental protection—is a sound fertilization
program.
Proper nutrient management should be an integral part of every farmer’s (and Gardeners) overall management program.
This page prepared from Potash and Phosphate Institute online. For a discussion of the role of potash to farmers and gardeners go to this site. Cleveland Potash
02/01/2009