A critical nutrient concentration is a concentration of nutrient below which yields decrease or deficiency symptoms appear. Thus many nutrients, yield decreases even before visible deficiency symptoms are observed. Therefore the critical nutrient level is the nutrient concentration at 90 or 95% of maximum yield. Critical levels occur at a specific plant part at a specified stage of maturity. Example in maize the ear leaf from tasselling to silking is most commonly used.
The following are the critical ranges for leaf nitrogen, phosphorus and
potassium concentration for maize, rice and beans
|
Crops |
Hybrid maize |
Improved rice |
Beans |
|
Nutrients |
At low At
medium At high |
At low At medium
At high |
At low At medium
At high |
|
Nitrogen
(N) |
2.76
3.75 >3.75 |
2.75 2.85 >2.9 |
3.5 5.5 >5.5 |
|
Phosphorus
(P) |
0.24 0.25 >0.50 |
0.2 0.24 >0.25 |
0.25 0.3 0.6 |
|
Potassium
(K) |
1.74 2.75 >2.75 |
1.2 1.6 >2 |
1.7 2.5
>2.5 |
Analysis
on dry weight basis of most recently matured leaves for Kjeldahl N, 2% HAc
extractable PO4 and K. Plants with a critical level of a nutrient produces
approximately 90% of maximum yield.
Qn.2. Plant analysis usually refers to
analysis of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca),
magnesium (Mg), sulfur (S), iron (Fe), manganese (Mn), copper (Cu), zinc (Zn),
and boron (B). Thus is the quantitative determination of
the elements in plant tissue.
It is unique from other crop
diagnostic tests in that it gives an overall picture of the nutrient levels
within the plant at the time the sample was taken. Its use is based on the
principle that the nutrient level present is a result of all factors affecting
the growth of the plant.
Plant analysis is useful in
confirming nutrient deficiencies, toxicities or imbalances, identifying hidden
hunger, evaluating plant
nutritional problems or to monitor effectiveness of a soil fertility program, determining the availability of
elements not tested for by other methods, and studying interactions among
nutrients.
Nutrient concentrations in the plant are not static
and may vary within parts of the plant, time of the year, and among forage
varieties and species. The factors that affecting plant nutrient concentration
Ø Physiological
maturity of the stand.
Ø Sampling
procedure and parts of the plant that are sampled.
Ø Sample
preparation and handling.
Ø Environmental
conditions
·
Parts
of the plant that are sampled and sampling procedures
A tissue samples are taken after initial flowering is
not accurate example in maize silks that are starting to turn brown, flowers in
soybean above the two or three lowest nodes. Sampling at the latest acceptable
stage (initial flowering) gives the best picture of the general nutritional
status of the plant because most of the nutrient uptake has occurred. Nutrient
deficiencies could still develop when samples are collected at earlier growth
stages. The sample should be taken to a stalk that represents the area being
sampled.
·
Sample
preparation and handling.
The samples are selected randomly from the suggested
number of plants throughout a field or desired sampling area to ensure accurate
tissue analysis. The collection of the selected plant parts and place in a
clean brown paper bag (No. 6 for grasses and small grains, No. 8 for legumes or
No. 12 for maize, grain sorghum, and tobacco). A truly representative sample
should be taken by sampling a large number of plants so that the sample
represents the field. Care should be taken to minimize soil contamination on
the sampled plant material.The samples are not washed with water as some
elements may be leached from the sample.
·
Physiological
maturity of the stand
The difficult aspect of plant analysis is that
nutrient levels within the tissue change as the plant or plant part ages.
Example maize leaves have a high concentration of nitrogen when they first
emerge, but the N concentration can decrease rapidly as the plant grows. This
happens because the plant has the ability to move nitrogen from older tissue to
younger tissue. Nutrient concentration tends to decrease as the plant grows
because nutrients are being diluted with greater amounts of plant tissue. Plant
tissue samples can be taken any time after emergence until the beginning of
flowering. At flowering, the plant changes from vegetative to reproductive
stages. Nutrients then move into the seed, fruit, or grain from other parts.
·
Environmental
conditions;
Nutrient concentration tends to decrease as the plant
grows because nutrients are being diluted with greater amounts of plant tissue.
To account for this variability, sufficiency levels have to be determined for
specific plant parts at critical times in the crop’s life cycle. The sample of
the same plant part in each area, and be sure that both areas have been treated
the same (same variety, same planting date, etc).
How to manage error in interpreting plant analysis
results
This can help in evaluating the fertilizer program up to the point of sampling and allows adjustments to be made. The tissue sampling method is critical for success. The procedure is unique to each crop. The plant must be at a specific stage of growth, and a specific tissue must be selected. Failure to follow the prescribed method for that crop will produce misleading results. Samples are then quickly rinsed in distilled water an immediately sent to the laboratory.
Once the analysis is done, the nutrient contents are compared with known minimum values for that crop (critical values) and nutrient deficiencies or excesses are identified. A more recent approach is to look at both the nutrient levels as well as the ratios of the nutrients, a process known as Diagnostic and Recommendation Integrated System (D.R.I.S.). This system shows a lot of promise, but is still being fine tuned.
There are certain things that plant analysis cannot do. It cannot predict nutrient needs before planting. In the case of short term crops, plant analysis does not identify problems early enough to solve them. It also does not identify acidity problems or other soil conditions which may affect uptake of the nutrients by the crop. It is therefore not a substitute for soil analysis.
When plant analysis is used as the sole method of determining nutrient needs, it can give misleading information even when sampling is done properly. A low level of nutrients in the tissue indicates that something is wrong, but does not necessarily indicate that fertilizers are needed. Nutrient uptake may have been inhibited by problems such as root pests and diseases, water logging, drought etc. An adequate level of a nutrient in the tissue does not always indicate that fertilizer application is unnecessary. Often, when one nutrient deficiency is solved and the plants begin to grow, the levels of other nutrients may fall below the adequacy level due to a process known as dilution. Plant analysis is a powerful tool, but the results must be interpreted intelligently.
Relationship between plant growth and available
nutrient supply typically follow a relationship similar to that depicted above.
The concept is when equal increments of a nutrient are applied to a crop; the
yield response becomes smaller with each increment.
Soil pH
Soil pH has an important role in the loss of N. Most
field crops like maize perform best at a soil pH between 6.0 to 7.0, this range
provide the best balance of available nutrients when the soil pH is below this
range some nutrients turn out to be less available example phosphorus and other
becomes toxic in highly acid soil <5.0 also maize production turn into
problems. Nitrogen, phosphorus (P) and potassium (K) are frequently the most
limiting nutrients for plant growth in numerous ecosystems.
Continuous cropping systems of maize requires important
amounts of N, P and K fertilizers and among these N fertilizer plays
significant role. Additional of lime and other materials can raise soil pH to
the ideal crop production and increase the levels of calcium ion.
Acid Neutral Alkaline
3 4
5 6 7
8 9 10
Range of soil pH
Low soil pH makes phosphorus, Nitrogen and Calcium
less available to growing plant Phosphorus is also less available in alkaline soils
and high levels of calcium may inhibit the uptake of potassium and magnesium.
Nitrogen involved in protein synthesis and plays an important
role in the protection of plants against nutrient stress and pests and
synthesis of vitamins and chlorophyll in the cell.The
combination ammonium with N (NH4-N) enhanced the P uptake. This
benefit typically requires that the N and P be applied in either a chemically
combined form or as a concentrated mixture, such as a banded fertilizer blend.
The exact mechanism for this reaction is not clearly understood.
Calcium is generally adequate for maize as long as crop lime needs (pH)
are being met. It combines with P
to make insoluble compounds that are unavailable to plants in the short term.
The general trend in the reaction is that as the soil Ca content and pH
increase more P will combine with Ca to form compounds with ever-decreasing
solubility.
Phosphorus reacts rapidly with soil constituents that reduce the
availability of this nutrient to plant roots. Phosphorus interacts positively
with nitrogen and potassium, while high soil phosphorus levels may also reduce
plant uptake of zinc and copper.
In soils of pH 6.0 to 8.0 nitrogen, phosphorus and calcium are
available to plants also phosphorus binds with calcium, forming insoluble
compounds that are largely unavailable to plant roots.
Qn.3.(b)
Nutrient imbalance
Is the changes of nutrient in the plant field,
usually appear only under extreme conditions. Nutrient problems often reduce
growth without any visual symptoms. One of the common symptoms of a poor
nutrition is low productivity and/or crop quality that cannot be explained. The problem
could be due to nutrient problems even though no deficiency symptoms are
visible and another related symptom is a gradual decline in yield over several
years.
Unusual pest and disease problems
are another symptom of imbalanced nutrition. If your field is especially susceptible
to pests and diseases or you are experiencing outbreaks of infections that
normally should not affect that crop, your plants may be under stress.
The productivity of maize largely depends on its nutrient requirement
and management particularly that of nitrogen, phosphorus and potassium.
The concept
of balanced nutrition is very simple and was in fact developed more than 150
years ago. The idea is that a crop requires an adequate supply of all nutrients for optimum growth. If
more than one is in short supply, growth is determined by the nutrient which is
in lowest supply.