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Soil Analysis

Soil pH

1. What is Soil pH?

The pH is the negative logarithm of the hydrogen ion concentration.
It indicates whether the soil is acidic or alkaline and is an important parameter which affects pond condition.
One of the most important factors for maintaining pond productivity - controls most of the chemical reaction in the pond environment
Pond bottom soil pH can range from less than 4 to more than 9, but the best pH for shrimp pond soils is considered to be about neutral or slightly alkaline pH.

2. What are all the factors influenced by pH?

The Nutrient availability, mineralization rate, bacterial activities and phosphorus fixation are influenced by pH.

3. What is the ideal level of Soil pH?

The ideal pH is 7.5 to 8.5. It is best suited for brackish water environment as the availability of nutrients like nitrogen, phosphorus, potassium, sulfur, calcium and magnesium is highest under this range.
Most soil microorganisms, and especially soil bacteria, function best at pH 7 to 8.

4.Why Soil pH analysis is conducted?

Soil pH analysis is generally conducted to determine lime requirement. This is important for newly developed ponds where the occurrence of acid sulphate soils is common.

5. If pH is less than 7.0 means what to do?

The low pH of bottom sediment indicates unhygienic condition and needs regular check-up.
If the pH is too low - strongly acidic - reduce the availability of key nutrients in the water and lower the pond productivity
Pond soils with pH below 7.5 should be limed with the amount of liming material applied based on soil pH.

Soil pH	Agricultural limestone (kg/ha)
Above 7.0	0
6.9-6.5	500
6.4-6.0	1000
5.9-5.5	2000
5.4-5.0	3000

6. How to measure Soil pH?

There are many ways of measuring Soil pH but for aquaculture ponds, it should be measured in a 1:1 slurry of distilled water and dry pulverized soil using a pH metre and glass electrode.

Total Organic Carbon

1. What is Total organic carbon?

Total organic carbon (TOC) is the carbon (C) stored in soil organic matter (SOM). It is the main source of energy for soil microorganisms.
Organic carbon (OC) enters the soil through the decomposition of plant and animal residues, root exudates, living and dead microorganisms, and soil biota.
Acts as the source of energy for bacteria and other microbes that release nutrients through various biochemical processes
Soil fertility depend on the organic carbon contained in the soil.
pH ranging between 6.5 and 7.5, organic carbon content is varying between 1.5 to 2% and calcium carbonate content is more than 5% are best suited for shrimp culture.

2. Is Less Soil Organic Carbon affect the culture?

A direct effect of poor Soil Organic Carbon is reduced microbial biomass, activity, and nutrient mineralization due to a shortage of energy sources.
Some organic matter in bottom soils is good, because it favors benthic productivity. Ponds with less than 0.5% organic carbon may not have good benthic productivity.

3. If Organic carbon is high means what to do?

Soil with excessive organic matter, nitrogen fertilizers may be applied at 200 to 400 kg/ha to enhance microbial activity.

4. What are all the factors influence the Total organic Carbon?

Total organic carbon influences many soil characteristics including colour, nutrient holding capacity, nutrient turnover and stability, which in turn influence water relations, aeration and workability.

5. What is the importance of Total Organic carbon?

Soil organic carbon is fundamental to soil fertility
Soil organic carbon improves soil structure
Ecological soil function utilises carbon as the initial food source.

6. Explain about Soil Organic Carbon concentrations?

Concentrations of sediment organic carbon usually are below 4 percent in pond bottoms, and in new ponds, the soil can contain less than 0.25 percent organic carbon. The amount of organic carbon in pond bottoms can increase as a result of increasing carbon concentration in the sediment, increasing depth of sediment, or both
Soil Organic Carbon concentrations for shrimp culture as follows:

Organic Carbon (%)	Comment
>15	Organic Soil
3.1 to 15	Mineral Soil, High Organic Matter content, prone to anaerobic zones in bottom
1.0 to 3.0	Mineral Soil, moderate organic matter content, Optimum for fertilized ponds and acceptable for ponds with feeding and best range for aquaculture
< 1	Mineral Soil, low organic matter content will not support good growth of benthos

Available Phosphorus

1. Define Available Phosphorus.

Available phosphorous content was significantly higher in shrimp farm.
Total Phosphorus concentrations in bottom soil is greater in deep water than shallow water areas.
Phosphorous content is negatively correlated with pH and organic carbon content of farm sediment (soil).
Maximum availability of soil phosphorus usually occurs at about pH 7.
The soil available phosphorous increased with the increase in culture period.
The phosphorus in bottom soils is not highly soluble, and the equilibrium concentration between phosphorus in soil and phosphorus in water is low – often only a few micrograms per liter.

2. How to determine the available phosphorus in soil?

Phosphorus in soil occurs as orthophosphate in different forms and combinations. A small portion of total phosphorus is available to phytoplankton.

3. Why Phosphorus is necessary?

Phosphorus is tightly bound in soil, and fertilization generally is necessary to promote phytoplankton growth regardless of acid-extractable phosphorus concentration.

4. What is the importance of available phosphorus?

The importance of available phosphorus in soil for increasing productivity
The phosphorus in soil is in both inorganic and organic forms.
The organic form constitutes about 35-40% of the total phosphorus content of the soil.

5. Different levels of Available Soil Phosphorus and their comments

The available soil phosphorus (P2O5)
Below 3 mg/100gm (30ppm) as poor productivity
3-6mg/100gm (30-60ppm) as average
Above 6-12mg/100gm (60-120ppm) as high productivity and
Above 12mg/100gm (120ppm) as excess

Total Nitrogen

1. What is Total Nitrogen?

Total nitrogen forms an important component of soil fertility
Total Nitrogen is the sum of nitrate (NO3), nitrite (NO2), organic nitrogen and ammonia (all expressed as N).
Organic nitrogen in plankton and in aquatic animal feces may settle to the bottom to become soil organic nitrogen.
Nitrogen in soil organic matter may be mineralized to ammonia and recycled to the pond water.
The amount of nitrogen held in soil complex is higher at higher salinities and hence reduce nitrification.

2. What is the range of Total Nitrogen?

The range of available nitrogen is 50-75mg/100gm of soil relatively more favorable for pond productivity.
Respectively ponds with less than 250 ppm available soil nitrogen are considered to have low productivity while concentration is the range 250 to 500 ppm and above 500 ppm are considered to be medium and highly productive respectively.

3. What is the reason for determining total nitrogen?

The most compelling reason for determining total nitrogen concentration in ponds with feeding is for the calculation of carbon: nitrogen (C:N) ratio.
The ideal C:N ratio is probably 8.1 to 12:1.
onds with high concentrations of fresh organic matter have lower ratios and tend to develop anaerobic zones at the sediment-water interface.
Higher C:N ratios suggest that pond bottoms are accumulating stable organic matter that decomposes at a slow rate. This phenomenon usually is found in ponds with low natural productivity.

4. What is C: N Ratio?

The C: N ratio of soil influences the activity of soil microbes.
This ratio is considered an important variable related to soil fertility.
Affects the rate of release of nutrients from decomposing organic matter.
In general, soil C:N ratios between 10 and 15 are considered favorable for aquaculture and ratio of 20:1 or narrower gives good results.

5.What is the levels of C:N ratio and their action in nutrient release?

Nutrient release - very fast - C:N ratio - less than 10
Nutrient release - medium- C:N ratio - 10-20
Nutrient release – More - C:N ratio – more than 20

6. Range of C: N ratio

The composition of soil organic matter averages about 5 percent nitrogen and 52 percent carbon. A soil with 1 percent organic matter contains about 0.05 percent nitrogen and 0.52 percent carbon – a C:N ratio of about 10:1 (range 8:1 to 12:1).