author= mowlid Hassan student of college of agriculture and environmental science school of animal and range science

haramaya university

…approved by Arayaselassie Abebe (m.sc of plant biology and biodiversity)

topic

Soil acidityKey points

Soil ph is measure of the concentration of the hydrogen ions in the soil solution. The lower the pH of soil, the greater the acidity.

PH should be maintained at above 5.5 in the top soil and 4.8 in the supsurface. A well maintained soil pH will maintain the value of the soil resource, maximize crop and

pasture choice and avoid production losses due to low pH.

SOIL Ph Soil acidity is measured in PH units. Soil pH is a measure of the concentration of hydrogen ions in the soil solution. The lower the pH of soil, the greater the acidity. PH is measured on logarithmic scale from 1 to 14, with 7 being neutral. A soli with pH of 4 has 10 times more acid than a soil with Ph of 5 and 100 times more acid than soil with pH of 6.

Effects of soil acidity Plant gowth and most soil process, including nutrient availability and microbial activity, are favoured by a soil Ph range of 5.5-8. Acid soil, particularly in the subsurface, will also restrict root access to water and nutrients. Aluminium toxicity When soil PH drops,aluminium becomes soluble.A small drop in pH can result in large increase in soluble aluminium. That form aluminium retards root growth, restrictingAccess to water and nutrients. Poor crop and pasture growth, yield reduction and smaller grain size occur as a result of in adequate water and nutrition. The affects of aluminium toxicity on crops are usually most

noticeable in reasons with a dry finish as plants have restricted access to stored subsoil water for gain filling.

Nutrient availability In very acid soils, all the major plant nutrients (nitrogen, phosphorus, potassium, sulphur, calcium, manganese and also the trace element molybdenum) may be unavailable ,or only available in insufficient quantities. Plants can show deficiency symptoms despite adequate fertilizer application.

Microbial activity Low PH in topsoil may affect microbial activity, most notably decreasing legume nodulation. The resulting nitrogen deficiency may be indicated by reddening of stems and petioles on pasture legumes, or yellowing and death of oldest leaves on grain legumes .Rhizobia bacteria are greatly reduced in acid soils. Some pasture legumes may fail to persist due to the in ability of reduced Rhizobia populations to successfully nodulate roots and form functioning symbiosis

Causes of soil acidity Soil acidification is a natural process accelerated by agriculture. Soil acidifies because the concentration of hydrogen ions in the soil increases. The main cause of soil acidification is inefficient use of nitrogen, followed by the export of alkalinity in produce.Ammonium based fertilizers are major contributors to soil acidification .Ammonium nitrogen is readily converted to nitrate and hydrogen ions in the soil. If nitrate is not taken up by plants, it can leach away from the root zone leaving behind hydrogen ions there by increasing soil acidity.Most plant material is slightly alkaline and removal by grazing or harvest leaves residual hydrogen ions in the soil. Over time, as this process is repeated , the soil becomes acidic.Major contributors are hay, especially Lucerne hay and legume crops. Alkalinity removed in animal products is low, however, concentration of dung is stock camps adds to the total alkalinity exported in animal production. As conceptual, There are four main causes of soil acidity are

Removal of product from the farm or paddock Leaching of nitrogen below the plant root zone In appropriate use of nitrogenous ferlisers Build up in organic matter

Removal of product. Obviously the main aim of any agricultural production system is to produce saleable products. However most agricultural products are slightly alkaline so their removal from a paddock or farm leaves the soil slightly more acidic. The degree of acidification will depend on how alkaline the product is and how many kilograms of product are removed. Where little actual product is

removed from the farm, such as in wool production, the system remains largely in balance. The most acidifying forms of agricultural producing are operations.

Leaching of nitrogen. Leaching of nitrogen in the form is a very important factor in soil acidity. Nitrate is major nutrient for plant growth. It is supplied either from nitrogenous fertilizers or armospheric nitrogen fixed by legumes. When there is more nitrate than the plant can use , the nitrate isat risk of draining-leaching, below the plants roots and into the ground water system. This leaves the soil more acidic.

Leaching of nitrate can happen through inappropriate use of nitrogen fertilizers and is more common in intensive production like horticulture or because the plants are not at asuitable stage of growth to use the available nitrogen.Nitrogenous fertilizers. The amount of acid added to the soil by nitrogenous fertilizers varies according to the type of fertilizer. Soil acidification due to use of phosphorus fertilizers is small compared to that attributed to nitrogen, due to the lower amounts of this nutrient used and the lower acidification per kg phosphorus.

.Management of soil acidic Soil testing

. Knowledge of how soil pH profiles and acidification rates vary across the farm will assist effective soil acidity management. Ideally, soil samples should be taken when soils are dry and have minimal biological activity. It is standard to measure PH using one part soil to five parts 0.01 m cacl2. Soils with low total salts show large seasonal variation in pH if it is measured in water can read 0.6-1.2 PH units higher than in calcium chloride (moore et al,1998) soil sampling should take paddock variability into consideration . for example, clays have greater capacity to resist pH change(buffering) than loams, which are better buffered than sands. Samples should be taken at the surface and in the subsurface to determine soil PH profile. This will detect subsurface acidity, which may underlie topsoils with an optimal PH.

Interpreting PH of soil acidity Depending on soil PH test results, agricultural lime may need to be applied pH, or to recover pH to an appropriate level. If the top soil pH is above 5.5 and the subsurface pH above 4.8 only maintenance levels of limiting will be required to counter on-going acidification caused by productive agriculture if the topsoil pH is below 5.5,recovery limiting is recommended. Keeping the top soil is above 5.5 will treat the on-going acidification due to farming and ensure sufficient alkalinity can move down and treat subsurface acidity.limiting is necessary if the subsurface pH below 4.8 whether or not the topsoil is acidic.. if the 10-20 cm layer is below 4.8 but the 20-30 cm layer above 4.8, limiting is still required .in this case the hand of acidic soil will restrict root access to the more suitable soil below.

LimingLiming is the most economical method of ameliorating soil acidity. The amount of lime required will depend on the soil pH profile, lime quality,soil type,farming system and rainfall,limesand,from castal dunes,crushed limestone and dolomitic limestone are the main sources of agricultural lime, carbonate from calcium carbonate and magnesium carbonate is the component in all of these sources that neutralises acid in soil. The key factors in lime quality are neutralizing value and particle size. The neutralising value of the lime is expressed as a percentage of pure calcium carbonate which is given value of 100%. With higher neutralising value , less lime can be used , or more area are treated , for the same Ph change. Lime with a higher proportion of small particles will react quicker to neutralize acid in the soil, which is beneficial when liming to recover acidic soil.

THE IMPACTS OF SOIL ACIDITYFarmers see the direct impacts of soil acidity as lost productivity and reduced income through:• reduced yields from acid sensitive crops and pastures• poor establishment of perennial pastures• failure of perennial pastures to persist.Acidic soils also impact on the community.

• There is permanent degradation of the soil when the acidity leaches to a depth where it cannot be practically or economically corrected

• Recharge of aquifers is due to less water use by plants affected by soil acidity. This can lead to dryland salinity and damage to infrastructure such as the break up of roads.

• There is an increase in soil erosion and addition of silt and organic matter to waterways as annual vegetation predominates on acidic soils, leaving the soils exposed to erosion for a significant part of the year.

RECOGNISING SOIL ACIDITYThe signs of soil acidity are more subtle than the clearly visible symptoms of salinity and soil erosion.

Cereal growers may predict that their soil is acidic when acid sensitive crops fail to establish, or crop production is lower than expected, particularly in dry years. In pasture paddocks poor establishment or lack of persistence of acid sensitive pastures such as lucerne, and to a lesser degree phalaris, is an indication that the soil may be acidic.

Protecting soils at risk of becoming acidicWhere soils are at risk of becoming acidic the future impact of soil acidity can be reduced, but not eliminated, by slowing the rate of acidification. This can be achieved by:

• minimising leaching of nitrate nitrogen • using less acidifying fertilisers• reducing the effect of removal of product • preventing erosion of the surface soil.

Reducing the leaching of nitrate nitrogenNitrate nitrogen (NO3-N) is easily leached as it is highly soluble. When NO3-N is leached away it leaves that part of the soil more acidic. If the NO3-N is taken up further down the profile there can be an increase in pHCa at the point of uptake. However when the NO3-N is leached below the root zone it leaves the soil profile more acidic. Table 8 lists the factors that affect NO3-N leaching in order of importance. It qualifies the effect of each factor and indicates how the effect can be influenced. The absolute effect of each factor will increase with higher rainfall. Using less acidifying fertilizers Acidification of the soil can be reduced by avoiding the use of highly acidifying fertilisers such as sulphate of ammonium and mono-ammonium phosphate (MAP) . Nitrogen fertiliser (including urea) that is pre-sown should be drilled into narrow bands to slow nitrification and subsequent leaching. Surface application of nitrogenous fertiliser for crops before sowing, even if harrowed, can result in acidification due to nitrate leaching. Nitrate nitrogen will be better utilised by applying top-dressed nitrogen to actively growing rather than dormant crops, resulting in less acidification risk.

Written by student of range science in haramaya university

Contact= hassanmowlid@yahoo.com