Monday, August 9, 2010

Soil Test results- How to interpret and use them.

Soil pH

Soil pH is measured either in a solution of Calcium chloride or water. These methods give slightly different readings and the difference between water and calcium chloride is in a range of 0.5-1 unit. Water always gives a higher reading. When soil testing kit from garden shops is used for testing soil pH, water is used.

A soil pH of 9 is strongly alkaline, 7.9-8.9 is alkaline, 7 neutral, 6.5-5.6 slightly acidic and below 5 would be strongly acidic to very acidic. If soil pH in Calcium chloride method is above 5.5, liming is not needed but below this level lime application is needed. Generally vary acidic soils may have Aluminium and or Magnesium toxicity however it is not always the case. Aluminium and Magnesium available to plants are usually measured as Cation Exchange Capacity (CEC) in soil testing laboratories.

Plants such as lucerne (alfalfa) and canola are very sensitive to acidity in soils. Plants such as wheat and barley are sensitive, sub-clover, cocksfoot and rye are tolerant, lupins, oats, triticale and serradella are very tolerant to acidity in soils. Most plants including vegetables and flowers will grow in pH range of 5.3- 6.4. For garden plants, look at the label on the plant pots to ensure you select right plants for your soil.

Nitrogen (N)

Nitrogen is generally added in the mineral format either as nitrate or ammonium. If the total nitrogen in soil is less than 0.15 then it is low, 0.15-.25 is medium, 0.25-.5 is high and more than 0.5 is very high. Much of the total nitrogen is held in the organic matter in the soil and not available to the plants for use. Nitrogen which is really available to plants is measured as Nitrate (NO-3) which is variable in soils. Nitrate requirements of crops vary. If you look at the amount of nutrients removed in the later table then you will know how much nutrients are required.

Table below gives nutrient weight %.

Urea has 46.0% N therefore for 1 unit of Nitrogen = 2.17 is the multiplier.

Synthetic Nitrogen fertilizers are manufactured and therefore use fuel for its manufacture. Do not use excess of Nitrogen as it only leaches out from soil and contaminates waterways, rivers and groundwater. High levels of Nitrates in ground water can be toxic. Infants under 3 months have an upper limit of 50mg/liter and for children and adults 100mg/liter.

Another way to add nitrogen to soils is through biological nitrogen fixation. Plants such as legumes (peas, beans, lucerne (alfalfa) capture nitrogen from the atmosphere and “fix” in the soil through the use of a naturally occurring bacteria called rhizobia which reside in small bundles of tissue on the roots called as nodules. These plants when used in rotations with other crops provide enough nitrogen required for many crops for 1-2 years without adding synthetic fertilizers. They also provide organic carbon to the soil in addition to nitrogen through the roots and death of rhizobia. Therefore legumes in general and perennial legumes especially provide both carbon and nitrogen to the soil keeping the soil healthy and conditioned.
Phosphorus (P)
Phosphorus levels in the soil is measured to decide if phosphate fertilizers need to be used to amend the soil (DAP, MAP). Phosphorus required will depend on the soils. Generally Lactate and Brays test methods are commonly used to assess phosphorous levels.
Fertiliser requirement = P requirement/P content (proportion) of fertilizer (e.g., superphosphate requirement = P requirement/ 0.095).

For 20Kg per hectare = P/ 8.8% P x 100 = 227kg per hectare of single super required.
MAP (11% N, 22%P), DAP (18%N, 20% P) and Single Super Phosphate (0% N and 9% P, 11% S and 19% Ca). Use the table below for other fertilizers. See table below for Phosphorous content of fertilizers.

Potassium (K)

For each kilogram of soil 156mg of Potassium is required. In 1 hectare of soil to 10cm depth, there is 1,000,000 kg of soil. Therefore 156kg K/hectare is required. If for example muriate of Potash is used (48.5% of P) as a fertilizer then approximately 156 x 2 = 312kg/hactare of the fertilizer is required. Use table below for calculations for other the requirement calculations. The table below provides values in wt%.

Fertilizer nutrient composition information is from

Exchangeable Cations

Cation Exchange Capacity is the capacity of soil to hold and exchange cations. This capacity provides buffering effect for various changes in the soil including pH. Testing is generally conducted on most abundant cations in the soil. They are calcium (Ca2+), magnesium (Mg2+), potassium (K+), sodium (Na+) and in strongly acidic soils aluminium (Al3+). Cations such as manganese (Mn2+), iron (Fe2+), copper (Cu2+) and zinc (Zn2+) are not present in high amounts although in some soils it may be. The individual cations are expressed as a percentage of the Effective Cation Exchange Capacity (CEC).

The desirable proportion of CEC as a percentage for many plants are Calcium (65-80%), Magnesium (10-15%), Potassium (1-5%), sodium (0-1%) and Aluminium (less than 5%). Exchangeable Aluminium is important in pH less than 5.5 (water tested) and in pH less than 4.7 in Calcium chloride testing.

If the Calcium: Magnesium ratio is less than 4 then either calcium is low or deficient, if it is 4-6 it is balanced and good and if it is in the range of 6-10 or higher Magnesium is deficient or low. 

The following table provides the amount of nutrients removed/used by 1 ton of production of the crops.

Soil Organic Matter (SOM) and Soil Organic Carbon (SOC)

Organic matter in soil is derived from plants and animals to the soils and it is important to have organic matter as it supports useful soil bugs. Organic matter and organic Carbon are very important to maintain soil health, conditioning and to store carbon in soils. Organic matter and organic Carbon are expressed as a percentage of the soil by weight. Organic matter = Organic Carbon x 1.72.  Organic matter in soil is considered to have a constant Carbon composition of 57%.

Organic matter less than 1.7 g/100g will have organic carbon ranging 0.60-1 g/100g. this soil will be low in organic carbon and soils are considered to have poor structure and very low stability. Organic matter of 1.7-3g/100g will have Organic carbon of 1-1.8g/100g and this is considered as moderate value and soils are considered to be have average structural condition and stability. Organic matter of 3-5.15g/100g will have Organic Carbon of 1.8-3.00 g/100g is considered as high value and soils are considered to have good structural condition and stability.

Soil Salinity

Accumulation of various salts mainly sodium in water causes salinity in soils. Soil salinity is variable and if salinity is high it affects plant growth and production. Salinity levels are measured by electrical conductivity of 1:5 soil/water suspensions. Multiplier factors are used to covert the value to obtain ECe (deciSiemen/meter, dS/m). Multiplier factors are dependent on the soil properties and type.

Non saline soils have a lower value of less than 2, slightly saline will have 2-4, moderately saline 4-8, Highly saline 8-16 and extremely saline more than 16.  In water, rain water and soil surface normal water will have values of 0.02-0.03, river water, 0.6-.9, bore water of poor quality 1, saline soil water will have 1.6, water from highly saline soil will have 8. Sea water has a value of 46.6 dS/m.

Crops such as barley, cotton, safflower, sorghum, soybean, sugarbeat, wheat, ryegrass, phalaris and beats are tolerant to salinity levels of ECe 3.9 dS/m. Crops such as beans, broad bean, cowpea, flax, maize, sugar cane, clovers, cabbage, cauliflower, celery, lettuce, onion, peas, potato, apple, apricot, bananas, avocado, grape, lemon, orange, peach, pear, plum, strawberry are sensitive to ECe 0-1.9 dS/m.

Rice, peanut, alfalfa, cucumber, spinach, tomato watermelon, grape and olives are tolerant to salinity levels of ECe 2-3.9 dS/m.

Waterlogging in soil is one of the first indicators of soil salinity problems. Look out for bare patches in soil, plant and tree death appearance of salt crystals on soil are all indicators of salinity problem.

Soil Acidity

There are two types of acidity in soils Acid sulphate soils which generally and naturally occur in the coastal areas. Here I will write only about agriculturally induced acidity in soils. Use of nitrogen fertilizers and some plants such as clovers cause soils to become acidic. Generally farmers use lime to adjust soil pH and this works for a while. However when soils become acidic with pH less than 5.1-5.2 then it is more difficult to grow crops and plants in such soils. Liming may or may not work. Preventing is the best way to stop soil acidification. One method is to use less synthetic fertilizers and use plants such as legumes to biologically fix nitrogen and supplement with other nutrients. Testing soils will help to assess how much of residual nitrogen is there in the soil.

Acidic soils generally have problems with toxic levels of aluminium and/or magnesium. Toxic levels of Aluminium prevents uptake of other nutrients by plants and either prevents plant seeds from germinating or growth once germinated. Toxic levels of Magnesium can suppress growth of plants and production. In some coastal parts there may not be problems with Aluminium but may be toxic levels of Magnesium.

If your soil is acidic you can use crops which are genetically tolerant to acidic soils

Soil Physical Properties

Soil physical properties such as particle size distribution, water holding properties, moisture balance, hydraulic conductivity (water flow), bulk density and air porosity, soil strength, texture, aggregate stability and water repellence all affect the way crops can be produced on soils.

If you have heavy clay soils then you can fix the soil when you dig it out (see blog article for garden bed preparation) by adding gypsum and allowing nature to break down clay particles. Consider adding graded sand to create a texture and mulch to add organic matter.

If you soil is sandy you will have problem of water drainage and evaporation and therefore need to add mulch, green manure or potting mix (for garden beds) to add some texture. You can also purchase soil and add it to the sandy soil.

I do not add mulch on top but I add grass clippings from my lawn to the soil when I dig it for preparation to create a texture to the soil.

If you have a large area grow some cheap legume and dig it back into the soil before flowering commences and add organic matter to the soil. This is called as green manuring. It can be done in strips for large acreages or garden beds. If soil does not have texture it will lose its air filled porosity and capacity to hold water. This can lead to lots of issues especially if nutrient uptake becomes a problem. 

The information in this blog is summarized from the book Interpreting Soil Test Results. Authors are Pam Hazelton and Brian Murphy. Published by CSIRO, Australia in 2007.

If you want to know about the soil tests more there is plenty of information in the internet. This is provided as a guide only to help gardeners and farmers who want to understand these issues.