Soil Fertility

Soil Fertility capability or ability of soils to supply elements essential for plant growth without a toxic concentration of any element. It is the inherent capacity of a soil to supply 15 of the 18 essential nutrient elements to the growing crop. It is the quality of soil that enables it to provide compounds or elements in adequate amounts and in proper balance for the growth of specified plants when other growth factors like light, moisture, temperature and the physical conditions of the soils are favourable. So, fertility is the potential nutrient status of a soil to produce crops. As plants have evolved in different climates and on different soils, they have different needs for the essential nutrients and different tolerance to the toxic elements. As such, a soil can be fertile for one plant and at the same time be unfertile for another plant. On the other hand soil productivity is a measure of the soils ability to produce a particular crop or sequence of crops under a specified management system. The major constraints of bangladesh soils as regards general fertility and health are briefly enunciated below.

Soil Reaction The average pH of Bangladesh soils could be taken on the acidic side of the pH scale, between 5.5 and 6.5. The Gangetic alluvium soils, particularly the calcareous one, have pH greater than 7.5, reaching at times up to 8.3. These contain free carbonates and bicarbonates. Soils in plateaus, raised lands and hills are usually acidic in nature. Because of pH variations, the nutrient availability, particularly that of phosphorus (P) and some micronutrients, is affected. Otherwise, lowland rice cultivation is not affected by original soil reaction, as the pH tends to come to a value between 6.5 and 7.5 on submergence. liming is needed for crops other than tea in soils having pH less than 4.5, which is more prominent in acid sulphate soils and hill soils. Upland crops are adapted to local soil ph. Soils are categorized by SRDI as very strongly acidic having PH below 4.5; strongly acidic pH 4.6-5.5; slightly acidic pH ranges 5.6-6.5; neutral pH 6.6-7.3; slightly alkaline pH ranges 7.4-8.4; strongly alkaline pH ranges 8.5-9.0 and very strongly alkaline pH 9.0. The United States Department of Agriculture (USDA) categorized soils on the basis of pH values are extremely acid having pH below 4.5; very strongly acid- pH ranges 4.5-5.0; strongly acid- pH ranges 5.1-5.5; medium acid- pH ranges 5.6-6.0; slightly acid- pH ranges 6.1-6.5; neutral- pH ranges 6.6-7.3; slightly alkaline- pH ranges 7.4-7.8; moderately alkaline- pH ranges 7.9-8.4; strongly alkaline- pH ranges 8.5-9.0; very strongly alkaline pH ranges 9.1 and above.

Organic matter status Organic matter (OM) status of Bangladesh soil is one of the lowest in the world. About 3.7 million hectares of land contain' 1.75% organic matter; soils of the low-lying areas contain' 5.5% organic matter with the exception of peat soils which contains not less 20% organic matter; and rest of the soils contain medium to high amounts of organic matter. Organic matter supply in soil is one of the major constraints to the agriculture of the country. Yet, the country has been producing good crops. This has been possible only due to the use of high doses of synthetic fertilisers and improved varieties of seeds. Most of the Bangladesh soils show an improved response when OM is incorporated along with inorganic fertilisers. The recommended doses vary between 5 to 10 tons/ha of fresh or partially decomposed cowdung. Use of green manuring plants like Sesbenia rostrate is also encouraged. Use of compost is increasing gradually. The organic matter status has been classified as: very low (1%); low (1.1-1.7%); medium (1.8-3.4); high (3.4-5.5); and very high >5.5.

Nitrogen status Because of low level of OM the nitrogen status of Bangladesh soils is substantially low and most crops on all soils respond to nitrogen applications. In fact, nitrogen (N) fertilisers are the most commonly used fertilisers in the country. The country has as many as 6 fertiliser factories producing mostly urea from natural gas. Compared to production with no fertilisers, a 2 to 3 fold increase is common in most crops including rice with N-fertilisers.

Phosphorus status The available phosphorus in Bangladesh soils could be considered between low and medium. Approximately 6% of the total land is severely deficient, 50% are strongly deficient and 44% are moderately deficient. Phosphorus availability is pH dependent. The source of P supply in soils is mainly inorganic fertilizers and to a limited extent organic manure. This again is not proportionate to the supply of synthetic chemical N fertilizers. The sales of N fertilizers during 2010-11 were 2,657 thousand metric tons of urea, 323 tons of DAP and 8800 tons of ammonium sulphate whereas in 2000-01 the sales of the respective fertilizers were 2,121 thousand tons, 90 thousand tons and 3 thousand tons.

Potassium status Bangladesh soils are deficient in potassium and many soils are found to respond to K-fertiliser application. These are particularly non-alluvial soils and the coastal saline soils. About 0.12 meq percent of NH4OAc extractable K is considered critical limit for the soils of Bangladesh. Potassium is severely deficient in 6% of the land, strongly deficient in 45% of soils, and moderately deficient in 33% soils. It has been estimated in 2007 that potassium contents varied in the range low to very low in 3.76 million hectares of land in comparison with 2.96 million hectares in the year 2000.

Sulphur status Response to sulphur (S) application is common in most soils except in coastal saline soils, acid sulphate soils and some acidic soils. Irrigated crops in the northern districts respond markedly to S-application. About 70-80% of the soils are deficient in sulphur. Gypsum is the principal source of sulphur.

Zinc and Boron During the recent past, soils, particularly those under constant waterlogging and irrigation have been found to respond to zinc (Zn) and boron (B) applications. The calcareous floodplain soils are one of them. The demand for zinc fertilizers has been increased from 3 thousand metric tons in 2000-01 to 42 thousand metric tons in 2010-11, which indicates a widespread zinc deficiency in the soils of Bangladesh.

Other micronutrients Response to micronutrients other than Zn and B has not yet been reported in any soil for any particular plant. However, it has been suggested that in some peat land soils and other soils, Mn application might have a positive response. It has not yet been confirmed. Crop response to Mo application is also found in some agroecological zone (AEZ).

Cation exchange capacity In Bangladesh soils the organic matter content is low and the majority of soils contain 1:1 clays and illites. As such, the CEC of Bangladesh soils in general is not appreciably high. Classification of Bangladesh soils on the basis of CEC (meq%) is: very high (>30); high (15-30); medium (7.5-15.0); low (3.0-7.5); and very low (<3.0).

Soil salinity A vast area of land in the coastal area is subjected to seasonal salinity. The salinity is mainly of Cl-SO4 type. As it is caused by marine water intrusion, the Ca:Mg ratio in the coastal saline soils is less than 1.0, which creates severe fertility problems. Most lands in saline areas are under single crop.

Fertility status of the different agroecological zones of Bangladesh are as below:

Old Himalayan Piedmont Plain Strongly acidic in topsoil and moderately acidic in subsoils. Organic matter contents are relatively higher than in other floodplain areas. The natural fertility of the soil, except the coarse textured, is moderate but well sustained. Soil fertility problems include rapid leaching of N, K, S, Ca, Mg and B.

Active Tista Floodplain Moderately acidic throughout, organic matter content is low and CEC is medium. Soil fertility level is low to medium.

Tista Meander Floodplain Moderately acidic throughout, low in organic matter content on the higher land, but moderate in the lower parts. Fertility level, in general, is low to medium but the status of K and CEC is medium in most of the places.

Karatoya-Bangali Floodplain Moderately acidic throughout. Organic matter content is low in ridges and moderate in basins. General fertility is medium.

Lower Atrai Basin Acidic; organic matter low to medium, moderate CEC and fertility status of soils.

Lower Punarbhaba Floodplain Acidic; organic matter status is medium to high with high CEC. General fertility level is medium with high K status.

Active Brahmaputra-Jamuna Floodplain Slightly alkaline; organic matter status is low and fertility status is low to medium. Nitrogen and P are, in general limiting whereas K, S and Zn status is reasonable.

Young Brahmaputra and Jamuna Floodplain Neutral to slightly acidic; organic matter content is low in ridges and moderate in basins. Soils are deficient in N, P and S but the status of K and Zn is reasonable.

Old Brahmaputra Floodplain Topsoils moderately acidic while subsoils are neutral. Organic matter content low in ridges and moderate in basins. General fertility level is low. Phosphorus and cation exchange capacity (CEC) is medium and K status is low in highlands and medium in lowlands.

Active Ganges Floodplain mildly alkaline; organic matter content low. General fertility level is medium with high CEC but deficient in N and available P and Zn.

High Ganges River Floodplain Slightly alkaline; organic matter content in the brown ridge soils is low but higher in the dark grey soils. General fertility level is low although CEC is medium.

Low Ganges River Floodplain Neutral to slightly alkaline; organic matter low in ridges and moderate in the basins. General fertility level is medium with high CEC and K status.

Ganges Tidal Floodplain Most topsoils are acidic and subsoils are neutral to mildly alkaline. General fertility level is high with medium to high organic matter and very high CEC. Limitations are due to high exchangeable Na and low Ca/Mg ratio.

Gopalganj-Khulna Beels Potentially strongly acidic; organic matter content is medium to high. Low in P status. Fertility level is medium.

Arial Beel Moderately acidic; organic matter content generally exceeds two per cent. High CEC and general fertility level is medium to high.

Middle Meghna River Floodplain Topsoils are strongly acidic and subsoils slightly acidic to slightly alkaline. General fertility level is medium with low N and organic matter.

Lower Meghna River Floodplain Topsoils are moderately acidic and subsoils are neutral. General fertility level is medium to high with low to medium organic matter.

Young Meghna Estuarine Floodplain Mildly alkaline. General fertility is medium but low in N and organic matter. Sulphur status is moderate to high.

Old Meghna Estuarine Floodplain Topsoils are moderately acidic, but subsoils are neutral. General fertility level is medium. Potassium status is low in uplands and low to moderate in lowlands.

Eastern Surma-Kushiyara Floodplain Strongly acidic to neutral. Organic matter content is moderate. CEC and Zn level are high, other essential nutrients medium.

Sylhet Basin Mainly acidic; organic matter content is moderate. Fertility level is medium to high with low P and high Zn content.

Northern and Eastern Piedmont Plain Slightly acidic to strongly acidic. General fertility level is low to medium.

Chittagong Coastal Plain Moderately acidic. Organic matter content is low to moderate. General fertility level of the soils is medium, but N and K are limiting. Status of S is high.

St Martin's Coral Island Neutral; general fertility level is low.

Level Barind Tract Slightly acidic to acidic; organic matter status is very low. Most of the available nutrients are limiting.

High Barind Tract Acidic to strongly acidic; low organic matter status. General fertility status is low. Zinc level is medium to high.

North Eastern Barind Tract strongly acidic; low organic matter. General fertility level is poor with high Zn status.

Madhupur Tract strongly acidic with low organic matter status. Low fertility level. Soils are mainly phosphate fixing and low in K, S and Ca.

Northern and Eastern Hills acidic; low organic matter. General fertility level is low.

Akhaura Terrace strongly acidic with low organic matter. General fertility is low.

Contaminants At present arsenic (As) is to be considered the single most important contaminant in Bangladesh soils. The source of arsenic in the groundwater is geologic. The depth of arsenic laden groundwater is variable, depending upon the depth of the layers containing oxidisable-reducible arsenic-containing minerals. Soils irrigated with arsenic laden water are being contaminated with the element. Accumulation of arsenic on the upper horizons is also likely to occur through capillary rise of water during lean periods. The maximum values obtained for arsenic in soils are in the range of 10-12 ppm. Pb, Cd, Cr and many organics are also contaminating our soils. These mainly occur around the industrial belts and sewage disposal locations in the peri-urban areas. Motor vehicle exhaust also contributes to substantial lead accumulation in agricultural lands. Disproportionate use of nitrogenous fertilisers sometimes leads to NO3 pollution in the ground and surface water. Accumulation of pesticide residue is a source of contamination, which, though not marked, cannot be ignored, and needs to be properly and timely addressed. Another source of soil contamination is the use of adulterated fertilizers.

Misuse and abuse of soils It is high time for the planners and policy makers to frame rules/laws to protect arable soils from ruination. Many agricultural lands are brought under urbanisation or industrialisation. One can find numbers of brick kilns set up on fertile agricultural lands along highways. As these lands are private owned, the government has practically no control on their use. Forestlands are also brought under urbanisation and industrialisation. Only about 17.09% of the total land area is under forest cover now. Overexploitation, ie using the soil for intensive cultivation without replenishing it, is causing nutrient mining to an extent that ultimately will make it barren. Excessive greed, mingled with lack of farsightedness, proper awareness and absence of punitive laws, has aggravated the misuse and abuse of our limited land and this is taking a heavy toll on our soils.

Even with all these constraints, Bangladesh has been going ahead with the production of cereals and other crops for the last few years. The country has now achieved self-sufficiency in food production. [SM Imamul Huq]

Bibliography Bangladesh Agricultural Research Council, Fertiliser Recommendation Guide for Most Bangladesh Crops, Bangladesh Agricultural Research Council (BARC), 1985; Bangladesh Agricultural Research Council, Fertiliser Recommendation Guide for Most Bangladesh Crops, Bangladesh Agricultural Research Council (BARC), 1997.