Dr John Basera, Agronomist
Soil pH is an excellent indicator of soil condition (quality and its ability to avail both macro and micro nutrients to the crop) on top of other soil structural quality properties. Soil pH also affects microbial activities in the soil which can impact crop growth and yield.
The objectives of soil sampling and analysis are to determine:
◆ average nutrient status in a field;
◆ pH and recommend soil conditioning;
◆ clay content for herbicide application;
◆ crop types to be grown and for irrigation purposes; and to obtain a measure of nutrient variability in the field.
When variability is known, fertiliser application can be adjusted to meet the supplemental nutrient needs of a crop for specific field areas. Correct fertiliser nutrient use can result in increased yield, reduced cost and reduced potential environmental pollution.
When to sample soils
Winter (just after harvesting a summer crop) is ideal for soil sampling except for testing for nitrate-nitrogen in sandy soils. This allows more time to get results from the testing laboratory and avoids the busy laboratory schedule in the spring.
Getting results on time will allow timely actioning of the recommendations eg, if lime is to be applied; the best time is 3-6 months before crop establishment and concurrently with winter tillage.
Mid to late summer is the appropriate time to collect soil samples for winter wheat.
Phosphorus level in the soil should be determined prior to seeding winter wheat.
Nitrate-nitrogen tests made prior to planting winter wheat help predict nitrogen fertiliser needs for the crop.
Farmers can improve soil quality by liming to adjust pH to levels needed by the crop to be grown.
Soil pH is the measure of the degree of acidity or alkalinity of the soil. It is determined by measuring the concentration of the hydrogen ions in the soil solution and is expressed in terms of scale with a range of 0 to 14.
A soil with a pH of 7 is considered neutral while less than 6 is considered acidic and a soil with pH greater than 7 is considered alkaline.
Soils may become more acid as a result of harvested crops removing differing levels of bases such as calcium and magnesium from the soil. This is normal and natural.
Rainfall also affects soil pH – water passing through the soil leaches basic nutrients (calcium, magnesium) beyond the root zone into drainage water replacing them with acidic elements (hydrogen, manganese, aluminium).
Application of nitrogen fertilisers (Ammonium Nitrate or Urea and, to a lesser extent, basal fertilisers), contribute to soil acidity by nitrification of ammonium to nitrate through a process which releases hydrogen ions. Organic matter breaks down naturally in the soil and hydrogen ions are released, which causes an increase in soil acidity. Plants release hydrogen ions to the soil which contributes to the soil acidity.
Why does soil acidity matter to crop productivity?
As the pH decreases below 5,5, the availability of aluminium and manganese increase and may reach a point of toxicity to the plant. Excess aluminium ions in the soil solution interfere with root growth and function, as well as restricting plant uptake of certain nutrients.
Acid soils cause phosphorus to form insoluble compounds with aluminium and iron. Liming of soils with low pH dissolves these insoluble compounds and allows phosphorus to be more available for plant uptake.
Acidic soils affect the availability of micronutrients in soil and general crop development and productivity.
Some micro-organisms e.g. important bacteria and fungi in the soil associated with nitrification require a certain soil pH level to function efficiently in acidic soils (low pH).
Liming improves soils physical structure by reducing soil crusting/capping. This promotes better emergence of small seeded crops and results in better crop stands. Population stand is key in attaining higher yields generally in all crops. NPK uptake efficiency can also be affected. The following table illustrates NPK uptake efficiency vs pH levels. This is critical information:
Lime should be applied at least 3-6 months before crop establishment since it takes a significant amount of time for lime to dissolve and react with the soil to cause desired pH adjustments – a week after harvesting is the best time to apply lime. However, it can still be done even during crop establishment. The lime can act as a buffer which works as a conduit for nutrient uptake from soil to crop through the roots and adjust the pH during the later stages of the crop cycle. Frequency of subsequent liming should be determined by soil tests.
Lime should be evenly incorporated in the 15-25cm zone as this is a root range of most grain crops grown in Zimbabwe e.g. maize. For maximum effectiveness, lime should be uniformly spread and incorporated into and with the soil.
Incorporation can be achieved through dishing or harrowing followed by a roller. In Zimbabwe, liming agents are in powdery formulations to increase surface area for quicker reaction with the soils. Some liming agents are now coming in sprilled form and these can be easily broadcasted.
Liming can be spread by hand through broadcasting or banding in furrows or incorporating in planting holes in non-mechanised systems. Some small holder farmers mix lime with a basal fertiliser before application and giving commendable results.
In mechanised farms a lime spreader of lime box can be used, which normally gives the best results. A lime box should always be operated when at least half full to full to ensure uniform application of lime.
The use of Nitrogen containing fertilisers increases soil acidity levels. We recommend whenever any form of Nitrogen, be it basal (compound D, L, S or J, blends and high analysis blends etc.) OR top dressing (e.g. AN or Urea) is applied, a maintenance lime application of about 1,8kg is needed for every 1kg of Nitrogen applied.
This is applicable if one does not carry out a soil analysis before the next crop and is based on nutrient removal by the previous crop. However, the top recommendation is to sample your soils for analysis to determine the lime rates and agents after at least every 3-4 years of soil use.
What liming agents are available in Zimbabwe?
Dolomitic lime (magnesium carbonate). Ideal for adjusting pH in magnesium deficient soils.
Calcitic lime (calcium carbonate). Suited to adjusting pH in calcium deficient soils.
Farmers should not blindly apply lime without qualification of the liming agent from soil analysis results. This can have detrimental effects to the soil and crop productivity. It can result in preferential uptake instead of a balanced uptake of bases (when a certain nutrient is take up at the expense of the other or a balanced uptake due to concentration differences).
A good example is when calcitic lime is applied blindly instead of dolomitic lime — this will cause an increased uptake with other elements e.g. magnesium, and therefore a crop will show magnesium deficiencies. Yield will be affected.
Lime vs Gypsum
Lime (calcium carbonate/magnesium carbonate) adjusts soil pH and supplies soil with either calcium or magnesium and carbon, depending on liming agent used.
Lime “sweetens” acidic soils.
Gypsum (calcium sulphate) is a supplementary source of calcium and sulphur which farmers apply whenever there is a deficiency of these two elements in the soil.
Gypsum does not adjust soil pH but rather supplements the soil. It also improves soil’s physical structure by removing hard setting clodiness, removes surface crusting/capping and improves soil workability.
Benefits of liming
Improved soil structure, nutrient availability and crop productivity.
Improved fertiliser use efficiency of crops. Use of fertiliser alone without lime results in poor fertiliser uptake and reduces the economic benefits. Lime reduces availability of toxic elements in soil such as aluminium and manganese.
Improved soil physical structure, resulting in good crop emergence and stand, greater root proliferation and an improved nutrient uptake.
Improved environment for beneficial soil micro organisms. Liming acidic soils to optimal levels creates a conducive environment for micro organisms to carry out necessary processes in the soil e.g. nitrification.
In a soyabean crop, for the rhizobium (inoculant) to function properly in nitrogen fixation there must be a conducive pH range of 5.2 to 6. That is why soyabean and most other legumes and food crops are sensitive to low soil pH.
A more rapid breakdown of organic materials in the soil, releasing nutrients to growing plants.
Pre emergence herbicides efficacy can be affected if soils are acidic.