Essay on Soil Fertility Status | Soil Fertility | Soil Management

In this essay we will discuss about the assessment of soil fertility status.

Soil fertility is the inherent capacity of soil that enables it to provide essential plant elements in quantities and proportions for the growth of specified plant when other growth factors are favourable. It indicates plant growth in relation to nutrients available in soil. Soil productivity is the ability of a soil for producing a specified plant or sequence of plants under a specified system of management.

It is usually expressed in terms of crop yield. All fertile soils may not be productive due to adverse climate, water-logging, salinity, alkalinity, acidity, etc.

Soil fertility status can be accessed from visual symptoms of nutrient deficiency in plants as well as from plant analysis, soil analysis or soil test.

Visual Symptoms:

The cheapest diagnostic technique for identifying nutrient disorders in crop plants is visual symptoms. However, visual symptoms are sometimes confused with disease, insect or soil moisture stress.

There are three steps in identifying nutrient disorders by visual symptoms:

(1) Observing Plant for its Normal Growth and Development:

Stunted growth may be due to deficiency or toxicity of all elements, but N and P deficiencies have more influence on growth reduction

(2) Plant Part Affected:

Whether foliar symptoms appear on lower or older leaves, or on younger or growing points of the plant

(3) Recognition of Nature of Symptoms:

(i) Chlorotic,

(ii) Necrotic or

(iii) Deformed.

If symptoms appear on lower leaves, they may be due to deficiency of mobile nutrients such as N, P, K and Mg. Mobile nutrients are those which can be translocated within plants. Hence, deficiency symptoms occur first on the lower part of the plant.

If deficiency symptoms first appear on the upper young leaves, they may be due to deficiency of immobile nutrients such as Ca, Fe, Cu, S, B, Mn and Mo. Immobile nutrients are not translocated to the growing region of the plant but remain in older leaves where they were originally deposited after absorption from the soil.

Nutrient Mobility:

Nutrient Mobility in Soil

1. Very mobile (prone to leaching)—Nitrate nitrogen, sulphate sulphur, boron.

2. Moderately mobile—Ammonium nitrogen (ammonium nitrogen is temporarily immobile), potassium, calcium, magnesium, molybdenum.

3. Immobile—Organic nitrogen, phosphorus, copper, iron, manganese, zinc (chelated forms of copper, iron, manganese and zinc are mobile and resistant to leaching).

Nutrient Mobility in Plants:

1. Very mobile—Nitrogen, phosphorus, potassium, magnesium (deficiency symptoms appear first in older leaves and quickly spread throughout the plant).

2. Moderately mobile—Sulphur, copper, iron, manganese, molybdenum, zinc (deficiency symptoms first appear in new growth but do not readily translocate to old growth).

3. Immobile—Boron, calcium (calcium is very immobile).

Indicator plants are also used as a diagnostic tool for plant nutrient deficiencies.

More commonly used indicator plants are given below:

Key points in identification of nutrient deficiency or toxicity symptoms in crops are given in Tables 6.3 and 6.4 Practical applicability of this method is rather limited.

1. Symptoms are often vitiated by the interaction of elements and also by the intensification of pests and diseases.

2. The very fact that crop is showing deficiency symptoms indicates that its growth and development has already been hindered and optimum yield may not be possible even after the remedy.

3. Deficiency symptoms may vary from species to species and even from variety to variety.

4. Plants may not exhibit deficiency symptoms due to hidden hunger.

Plant Analysis:

Plant analysis is based on the assumption that the amount of a given element in plant is an indication of supply of that particular nutrient and as such is directly related to quantity in the soil.

There are two types of plant analysis:

1. Tissue test on fresh tissue in the field and

2. Total analysis in laboratory.

1. Tissue Tests:

Rapid tests are made in the field on the above ground parts of growing plants for determining nutrient elements in the plant sap of fresh tissue. The sap from ruptured cells is tested for unassimilated nitrogen, phosphorus and potassium. These are semi-quantitative tests for verifying or predicting deficiencies of N, P and K. The sap from the ruptured cells is tested with reagents.

The intensity of colour developed is compared with standards and used as a measure of the supply of the nutrient in question. In general, latest mature leaf is used for testing. The plant should be tested at the time of bloom or from bloom to early fruiting stage early in the morning or late in the afternoon. Tissue tests have gained considerable importance due to their low cost and simplicity.

2. Total Analysis:

Plant analysis aims at determining the concentration of an element or extractable fraction of an element in a sample from a particular part at a certain stage of crop growth. With the development of instruments such as atomic absorption spectrophotometers, spark emission spectrometers and inductively coupled plasma (ICP) emission spectrometers, plant analysis has become more sensitive and simplified.

Simultaneous analysis of up to 22 elements is possible by ICP. With total analysis, almost all the elements associated with plant nutrition can be determined. Plant parts to be sampled at different growth stages are given in Table 6.5. When sampling, precautions should be taken to avoid soiled, diseased and insect or mechanically damaged plants and to exclude dying or dead tissue.

Interpretation:

The basis for plant analysis as a diagnostic technique is the relationship between nutrient concentration in the plant and growth and production response. This relationship should be significant to have complete interpretation in terms of deficient, adequate and excess nutrient concentrations in the plant.

A hypothetical curve showing the relationship between nutrient concentration and productivity is shown in Fig. 6.2:

When the nutrients are in deficiency range, yields are significantly reduced and there will be sharp increase in yield even with very little change in nutrient concentration in plant. In the marginal range, there will be reduction in yield, but plants do not show deficiency symptoms and both nutrient concentrations and growth increase as more nutrient is absorbed.

Within the marginal zone lies critical nutrient concentration (CNC) at which growth and yield of crop begins to decrease significantly. The CNC is usually estimated on the basis of 5, 10 or 20 per cent reduction in maximum yield. The third range is adequate zone, in which there is no increase in yield, but nutrient concentration increases.

In the excess range, between adequate and toxic, fertiliser use should be reduced until the nutritional status of plants lies in the adequate range. Growth and yield of crop is reduced but nutrients concentration continues to increase in the toxic range leading to toxicity symptoms.

Adequate concentrations for essential nutrients in crop plants vary with soil, climate, crop, management practices, etc. and is very difficult to make generalisations. However, such generalised values may give an idea about adequate levels of nutrients in crop plants (Table 6.6)

Diagnosis and recommendation integrated system (DRIS) is a comprehensive system which identifies all the nutritional factors limiting crop production for improving crop yields through an ideal fertiliser schedule. Index values which measure how far particular nutrients in the leaf or plant are from the optimum are used in the calibration to classify yield factors in the order of limiting importance.

It has several advantages over CNC approach:

1. Importance of nutritional balance is taken into account.

2. Elemental content in leaf tissue can be universally applied to a particular crop, regardless of where it is grown.

3. Diagnosis can be made over a wide-range in stages of crop development, irrespective of cultivar.

4. Nutrients limiting yield can be identified and arranged in order of their limiting importance on yield.

Crop logging is a graphic record of progress of crop containing a series of chemical and physical measurements. These measurements indicate the general condition of the crop and suggest changes in management that are necessary to obtain maximum yield. It was developed in Hawaii for sugarcane crop. Critical nutrient concentration approach is used in crop log system.

The plant tissue is sampled at regular intervals during the crop season and analysed for nitrogen, sugar, moisture and weight of the tissue. Analyses are made for P and K at critical times and adjustments in management practices are introduced as needed.

Isotope dilution technique consisting of radio-chemical analysis of plants grown on soils which have been treated with fertilisers containing elements such as radioactive phosphorus may be used to calculate phosphorus supply of original soil. Equations, based on the concept that a plant will absorb a nutrient from two sources in direct proportion to the amount available from each source, reduce essentially to identities in which A = Y.

A value: A =B(1-y)/y

where, A = available phosphorus in soil

B = amount of phosphorus applied to soil

y = fraction of phosphorus taken by plant from B.

Soil Analysis or Soil Testing:

Fertility status of the soil, for essential plant nutrients limiting the growth, can be known well in advance through soil analysis. As such, deficiencies can be corrected before sowing/planting through application of manures/fertilisers. In soil testing, the nutrient element is extracted from the soil with appropriate extraction solutions.

The soil test values are correlated with per cent response of crops to the applied nutrients under field or pot conditions. The critical levels of deficiency, moderate or marginal deficiency and adequacy of the nutrients worked out. Corrective measures are taken when once the deficient soils are identified through application of manures and fertilisers or both.

Soil Test and Fertiliser Recommendations:

Soil testing and plant analysis are useful tools for framing fertiliser recommendations to different crops. Soil testing gives a measure of the availability of nutrients to crops, whereas plant analysis indicates the actual removal of the nutrients from the soil.

The degree of regression of crop responses (in terms of dry matter or economic yield) to the added fertiliser nutrient and the soil test value with a particular method assesses whether a soil test method extracts the nutrients in proportion of the amount extracted by the crop. The value of soil test method that gives a very high degree of regression is calibrated with a crop response.

Correlation of soil test values of each method with percentage yields or nutrient uptake is made. The method that gives the highest coefficient of correlation is categorised as the best method.

Critical values of the nutrient element for deficiency (highly responsive), moderate deficiency (moderately responsive), no deficiency or adequacy (non-responsive) or into categories of low, medium and high available nutrient level as measured by the efficient method are determined.

For this purpose, a graph is prepared where the soil test values are plotted on X-axis and corresponding response to applied nutrient on Y-axis either as a point or bar. A careful look at the scatter diagram or the histograms will show a clear separation of the highly responsive soils from the moderately responsive and non-responsive soils.

The soil test method that proved efficient in measuring the available nutrients, adopted by most of the soil testing laboratories in the country (ICAR 2009), their ratings are given in Table 6.7.

Nanotechnology in Soil Fertility, Fertiliser and Plant Nutrition:

Nanotechnology is defined as understanding and control of matter and dimensions at 1-100 nm where the unique physical properties make novel applications possible (EPA 2007). Soils contain matter of dimensions of 1 – 1000 nm, the colloids, (inorganic, organic humic substances and large biopolymers) of nanoparticles (NPs) size.

NPs have extremely high specific surface areas for their volume as well as high proportion of atoms at their surface than other particles. Weathering of silicates, oxides and their minerals produce number of NPs in soil such as amorphous silica, hydrous aluminosilicates and oxides and sulphides but their precise functions and effects are still poorly defined and understood (ICAR 2009).

Nano-enhanced products like nano-fertilisers, with nano-based smart delivery systems (like halloysites) are available in advanced countries to provide nutrients at desired site, time and rate to enhance nutrient or fertiliser use efficiency and productivity.

Studies have shown the importance of nano-practices in increasing the availability of bio-availability of nutrient elements and transport of pollutants in soils. Synthesised NPs like amphiphilic polyurethane, zerovalent iron (nZVI) and nano-sized zeolite are used for reclaiming heavy metal and other polluted soils.

Recent studies show that inorganic NPs of ZnO SiO₂ and TiO₂ have toxic effects on bacteria, other organisms and that of TiO₂ on green algae having cell wall similar to plants but had a beneficial effect on growth of spinach when applied through seed or spray. Interaction effects of NPs with plants are not fully known and required detailed investigations. Investigations in this direction are in progress at several SAUs in India.

Fertiliser use Based on Targeted Yield:

Basis of this approach is the linear relationship between nutrient uptake and yield at economic level. Required parameters are nutrient requirement in kg per 100 (NR). per cent contribution from soil available nutrients (CS) which is obtained from the nutrient uptake from control plots, their soil-test values and per cent contribution from fertiliser nutrients (CF), which is obtained from the uptake data of targeted plot, the CS value and the fertiliser dose applied to the plot.

The total nutrient requirement for a target T is given by:

T x NR = FD x CF + ST x CS

where FD is fertiliser dose of a particular nutrient and ST is the soil test value of that nutrient. By rearranging the above equation, the following equation is obtained, which is called adjustment equation:

FD = [(T x NR)/CF] – [CS/CF] x ST or FD = k₁ – k₂ ST

where k₁ and k₂ are constants. These constants vary with the crop, soil and climate. Such equations on a number of crop or varieties have been generated under AICRPs throughout the country on soil test crop response correlations for different soil-crop-climate conditions for making fertiliser recommendations for the specific yield targets of crops.

Permanent Manurial Experiments:

Long-term field experiments can only give precise information on changes in soil properties and soil productivity. The earliest long term field experiment called permanent manurial experiment was started at the Rothamsted Experimental Station, Harpenden, England in 1843. Sir John Lawes was the first to start investigating agricultural problems. Herbert Gilbert later assisted Lawes.

In India, such experiments were started at Kanpur in 1885, at Pusa in 1908 and at Coimbatore in 1909. Except at Coimbatore, the experiments have been discontinued. However, new permanent manurial experiments were started at several places during 1930s.

The ICAR sponsored Coordinated Project on Long Term Fertiliser Experiments in 1970-71 for critically assessing the effect of modern intensive agricultural systems on soil productivity and crop yield obtained at Bangalore, Barrackpore, Bhubaneswar, Coimbatore, Delhi, Hyderabad, Jabalpur, Ludhiana, Palampur, Pantnagar and Ranchi. Results so far obtained indicate importance of balanced NPK along with manures, sulphur and zinc in maintaining soil fertility and productivity.

Sources of Plant Nutrients:

Soil is the major natural source of plant nutrients. Soil may support growth and development of wild flora just sufficient for their survival and regeneration. However, intensive crop production aimed at high levels of productivity needs supplemental plant nutrition from other sources.

Sources of plant nutrients include:

1. Organic sources (manures).

2. Mineral fertilisers.

3. Biological nitrogen fixation.

4. Aerial deposition.

5. Irrigation water, floodwater and groundwater.

6. Soil amendments.