John Basera
Agriculture occupies a central place in the Zimbabwe’s economy and has the potential to significantly reduce poverty, enhance economic recovery and growth and also entrench economic stability.
Because of the continual increase in the county’s population and the consequent necessity for the intensification of food production, the management of soils will become increasingly important today and in the coming years. In other words to achieve food security today and in the future, soil management must be treated as a ‘necessary and central necessity’.
Soil management is a critical aspect as far as improving agricultural (cropping) productivity is concerned. The goal of soil management is to protect soil and enhance crop performance, so that farmers can farm profitably and sustainably. In general soil fertility management aspect is a fundamental cog for restoration, improvement and maintenance of soil and crop productivity in agricultural industry in Zimbabwe.
The Minister of Agriculture, Mechanization and Irrigation Development, Honorable, Dr. Joseph Made has always reiterated the importance of soil management in attaining high and sustainable productivity levels especially in the staple crop-maize. He is always passionate about soil management and specifically the issues to do with addressing soil acidity/alkalinity status (pH). Specifically he always talks about liming whenever he speaks at farmer events since most of the soils in Zimbabwe are now acidic.
Soil pH is an excellent chemical indicator of soil quality and its ability to avail nutrients (both macro and micro) to the crop on top of other soil structural quality properties.
This article will demystify issues around soil sampling and analysis, soil acidity, causes of soil acidity, addressing soil acidity (liming), application (methods, timing and rates) of lime among other issues as we prepare our farmers for the coming summer season which is being forecasted to be normal to above normal in terms of rainfall. Lucky and success favors the prepared farmer and therefore we encourage our farmers to Plan, Prepare and Pray (a success oriented mode) rather than Spray and Pray (an ad hoc mode).
Why is soil sampling and analysis important?
Firstly farmers MUST sample their soils for pH and fertility analysis. If there are any imbalances in the soil pH and fertility, they must be corrected promptly e.g. low pH is corrected by liming. Use of appropriate liming agents (dolomitic or calcitic lime) is recommended as this enhances Fertilizer Use Efficiency (FUE). Agricultural lime is a relatively cheap soil conditioner whose many benefits to the farmer far outweigh the cost of procuring and applying it. Hence the continuous and consistent use of lime will enhance the profitability of cropping business. This is the foundation in commercializing farming in Zimbabwe and it is my feeling that it must be promoted and encouraged to farmers.
Secondly application of fertilizers is heavily recommended if we are to get good crop productivity levels and ROIs. The principle is to apply the right quantities of the right type of fertilizer at the right time and place. Fertilization regimes management should follow prescriptions from the soil analysis recommendations.
It is important to sample your soils for analysis after every 3-4 years depending on soil types. We recommend 3 years for lighter soils and 4 for heavier soils. In a season the best time to sample soils is the first week after harvesting a summer crop (NOW). This will give the farmer a good lead time to correct any deficiencies in the soil, 3-6 months before preceding crop establishment.
The most commonly used procedure for soil sampling would be based on soil types. Fields are split into sampling blocks that contain similar soils e.g. block A, B, C and so on. Hillsides are kept separate from bottoms since the soil types will vary greatly. Soil survey maps, if applicable, can help organize the soil types throughout the sampling area. Samples will not necessarily need to be collected for every soil type; however, similar soils should be kept together. The zig zag or the cross diagonal methods are commonly used and recommended where samples are taken in a zig zag or at cross diagonal format from a block. This will result in a sample which scientifically represents the whole block.
The sampling block will be dependent on the soils and topography. Generally, a block of 10-20 ha is considered the maximum size. Smaller sampling blocks may be needed if the soils are quite variable or a production problem is apparent and evident.
Once the sampling block is determined, a sufficient number of sites should be taken to acquire a representative sample. This is generally 10 to 20 sites. The depth of sample for surface soils would be about 20cm or as deep as the primary tillage or specifically as deep as the root zone of intended crop(s). The most commonly used tools for taking samples are augers, probes, hoes and sometimes shovels. Samples from different sites in a block are then mixed thoroughly and bagged into a khaki pockets and labeled. Information on the labels should include farmer name, farm name, contact details, block name, date taken and intended crop before they are submitted for analysis to approved laboratories.
Most fertilizer houses do sampling free of charge. Other approved laboratories include the Department of Specialist Services-Soil Chemistry laboratories. Always prescribe the intended tests before submission. We strongly recommend farmers to do a full analysis (pH and soil nutrient profiling).
Soil analysis results normally comes with recommendations. We recommend farmers to understand the recommendations from the laboratory tests and also to seek technical guidance in the interpretation of results from Agritex extension personnel in their respective holding areas. Seed Co Agronomy Services also assist in interpreting results from laboratories. Contact you’re a Seed Co Agronomist in your province.
What are Acid Soils?
These are soils with a pH measure of less than 7 on a Calcium Chloride Scale in Zimbabwe. These soils contain high levels of active hydrogen and or aluminum in relation to calcium and magnesium levels. Farmers can improve the soil quality of acid soils by liming to adjust pH to the levels needed by the crop to be grown.
Soil pH is the measure of the acidity or alkalinity of the soil. The degree of acidity or alkalinity is determined by measuring the concentration of the hydrogen ions in the soil solution. This is expressed in terms of a scale with a range of 0 to 14. A soil with a pH of 7 is considered neutral while less than 6 is considered acid and a soil with pH greater than 7 is considered alkaline. A good liming program is based on soil test that determines the degree of soil acidity and the correct amount of a liming material needed to neutralize that acidity. Once this amount is determined, a liming material must be selected that will economically satisfy the soil test recommendation and result in maximum and efficient crop productivity levels.
What are the causes of soil acidity?
There are basically three causes of soil acidity:
Soils may become more acid as a result harvested crops removing bases such calcium and magnesium from the soil. This is a normal and natural process. Different crops remove different amounts of Calcium and Magnesium from the soil.
Rainfall also affects soil pH, whereby water passing through the soil leaches basic nutrients such as Calcium and Magnesium beyond the root zone into drainage water replacing them with acidic elements such as Hydrogen, Manganese and Aluminum and thereby acidifying the soil.
Application of nitrogen fertilizers e.g. Ammonium Nitrate or Urea and to a lesser extend though basal fertilisers, contribute to soil acidity by nitrification of ammonium to nitrate a process which releases hydrogen ions. Organic matter breaks down naturally in 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?
Toxicity to crop: as the pH decreases below 5.5, the availability of aluminum and manganese increase and may reach a point of toxicity to the plant. Excess Aluminum ions in the soil solution interferes with root growth and function, as well as restricting plant uptake of certain nutrients.
Effect on phosphorus availability: acid soils cause Phosphorus to form insoluble compounds with aluminum and iron. Liming of soils with low pH dissolves these insoluble compounds and allows Phosphorus to be more available for plant uptake.
Micronutrient availability: acidic soils affect the availability of micronutrients in the soil and affects general crop development and productivity ultimately
Soil organisms: some microorganisms e.g. important bacteria and fungi in the soil associated with nitrification require a certain soil pH level to function efficiently. In other words the beneficial microorganisms do not function efficiently in acidic soils (low pH).
Soil physical condition: liming improve soil physical structure by reducing soil crusting/capping and this promotes better emergence of small-seeded crops and ultimately result in better crop stands. Remember population stand is key in attaining higher yields generally in all crops.
When is the right time to lime?
Lime should be applied at least 3 to 6 months before crop establishment since it takes a significant amount of time for lime to dissolve and react with the soil to cause the desired adjustments in pH. In other words ‘NOW’ is the best time to apply your lime. However farmers should note that, application of lime can still be done even during crop establishment and the lime can act as a ‘buffer’ responsible as a conduit for nutrient uptake from soil to crop through the roots and later adjust the pH during the crop cycle. Frequency of subsequent liming should be determined by soil tests.
Lime Placement and incorporation
The most important factor determining the effectiveness of lime is placement and incorporation. Placement for maximum contact with the soil into the root zone of the intended crop is essential and must be achieved. We generally recommend lime should be applied in the 15-25cm zone as this is a root zone range of most food crops grown in Zimbabwe e.g. the staple crop-maize. For maximum effectiveness, lime should be uniformly spread and incorporated into and with the soil. Incorporation can be achieved through disking or harrowing followed by rolling. In Zimbabwe liming agents are in powdery formulations to increase surface area for quicker reaction with the soils.
Lime can be spread by hand, adjusted vaicon, or by lime spreaders which normally gives the best results. Hand and vaicon are normally not recommended when the weather is windy. The best method is a lime spreader. However I have seen some smallholder farmers mixing lime with a basal fertilizer before applications and giving commendable results. However my only concern with this method is on the timing of lime applications (basal fertilizers are normally applied during planting and yet we recommend lime to be applied 3-6 months before crop establishment).
What amounts of lime can one apply?
We recommend farmers to follow recommendations on the soil analysis results with regards to amounts and type of lime to be applied. The amount is depended on the acidity levels of the soil. However the following general recommendation can be useful and is depended on the soil type.
Amount of lime required to raise soil pH by 0.1 units for different soil types-a gentle guide:
| Soil type | Lime rates to raise by 0.1 pH units |
| Sandy soils | 100kg/0.1 pH units |
| Sandy Loamy soils | 120kg/0.1 pH units |
| Clay soils | 200kg/0.1 pH units |
General interpretation: it means a farmer with a sandy soil requires 1000kg (1 tone) of lime to raise his pH from 4.5 (acidic) to 5.5 (optimum for maize and most food/cash crops).
What is maintenance liming?
The use of Nitrogen containing fertilizers increases soil acidity levels. So we generally recommend that when you apply any form of Nitrogen, be it from basal (compound D, L, S or J etc.) OR top dressing (e.g. AN/Urea), you need a maintenance lime application of about 1.8kg for every kg of Nitrogen applied. This is applicable if you do not carry out a soil analysis before the next crop. However the top recommendation is to sample your soils for analysis to determine lime rates and type after at least every 3 years of soil use.
What are the liming agents available in Zimbabwe?
It is important to sample your soils for analysis to determine your pH and also to determine the liming agent to be used. We strongly discourage farmers to blindly apply lime without qualification of the liming agent from soil analysis results. This can result in some detrimental effects to the soil and crop productivity. It can result in what we call preferential uptake. In Zimbabwe we basically have two types of liming agents i.e. Dolomitic Lime (Magnesium Carbonate) which is ideal for adjusting pH in magnesium deficient soils. The other agent available in Zimbabwe is Calcitic Lime (Calcium Carbonate) which is suited for adjusting pH in calcium deficient soils. There is no blanket recommendation for a liming agent and hence this should be noted. Preferential uptake is when a certain nutrient is ‘uptook’ at the expense of the other due to concentration differences. A good example is when Calcitic lime is applied (blindly) instead of Dolomitic lime- this will cause an increased uptake of Calcium at the expense of a balanced uptake with other elements e.g. Magnesium, and therefore a crop will show some magnesium deficiencies.
Lime vs Gypsum
This is a frequently asked comparison. Lime (Calcium Carbonate/ Magnesium Carbonate) adjust soil pH and at the same time supply the soil with either Calcium or Magnesium and Carbon, depending on the liming agent used. On the other hand Gypsum (Calcium Sulphate) is a supplementary source of Calcium and Sulphur which farmers apply whenever there is deficiency of these 2 elements in the soil. It should be noted that Gypsum does not adjust soil pH but rather supplement the soil with Calcium and Sulphur.
How expensive is lime in Zimbabwe?
In Zimbabwe lime is the cheapest form of soil conditioner. Normally a 50kg bag of lime will not cost an excess of $6 and therefore its benefits outweighs its cost by a huge margin. The following is a long list of the bundle of benefits of liming. 1000kg (1 tone) of lime may cost a paltry $100. Imagine this is only 10% or less of the ideal cost structure for a crop like maize on a per hectare basis. It is a minor cost driver in frank terms!
What are the FABs of liming?
- Liming generally improves soil structure and nutrient availability in the soil and ultimately crop productivity and to a greater extend improves livelihoods and the economy at large. Liming also provides some plant nutrient such as Calcium or Magnesium and Carbon. My favorite term for these is ‘complimentary benefits’.
- Liming improves Fertilizer Use Efficiency (FUE) by crops. Use of fertilizer alone without lime result in poor fertilizer uptake and reduce the economic benefit of using fertilizer. In economic terms, lime is an enabler to get the best ROI on fertilizers.
- Lime reduces availability of toxic elements in the soil such as aluminum and manganese. This result in improved root development and ultimately nutrient uptake.
- Liming improves the soil physical structure, resulting in good crop emergence and stand, greater root proliferation and an improved nutrient uptake.
- Liming acid soils improves the environment for beneficial soil microorganisms. In simpler terms it liming acidic soils to optimal levels creates a conducive environment for microorganisms to carry out necessary processes in the soil eg nitrification. A good example we always mention to farmers is of a soya bean crop. For the rhizobium (inoculant) to function properly (trap atmospheric nitrogen and fix into usable nitrate-process widely known as nitrogen fixation), there must be a conducive pH range of 5.2 to 6. That is why soya bean and most other legumes and food crops are sensitive to low soil pH. So generally liming will increase crop productivity in all crops (food, plantations and cash crops).
- Liming promotes a more rapid breakdown of organic materials in the soil, realizing nutrients for growing plants.
- Liming improves the palatability of forages.
- Some herbicides and soil based chemicals will not work properly in low pH soils, hence liming soils will enhance the efficacy of some herbicides especially pre-emergent herbicides.
Optimum pH ranges for common crops in Zimbabwe
| CROPS | Optimum pH Range (Calcium Chloride Scale) |
| Maize | 5.0 – 5.8 |
| Cotton | 5.5 – 6.0 |
| Wheat | 5.5 – 6.0 |
| Groundnuts | 5.5 – 6.0 |
| Tobacco (Burley) | 6.0 – 6.5 |
| Tobacco (Virginia) | 5.5 – 6.0 |
| Common beans | 5.3 – 6.5 |
| Baby corn | 5.3 – 6.5 |
| Sweet corn | 5.3 – 6.5 |
| Soyabeans | 5.5 – 6.0 |
| Sorghum | 5.0 – 5.5 |
| Sugarcane | 6.1 – 7.7 |
| Paprika | 5.5 – 5.8 |
| Bananas | 5.5 – 5.8 |
| Sunflowers | 5.5- 6.0 |
| Vegetables | 5.8 – 6.5 |
| Brassicas | 5.8 – 6.3 |
| Chilies | 5.5 – 7.0 |
| Potatoes | 4.2 – 5.8 |
| Tomatoes | 5.5 – 6.8 |
| Asparagus | 6.2 – 6.8 |
| Lawns | 5.5 – 6.0 |
| Flowers | 5.5 – 6.0 |
| Onions | 5.5 – 6.0 |
| Jatropha | 4.0 – 5.0 |
| Apples & Pears | 5.0 – 5.5 |
| Grapes | 5.0 – 5.3 |
| Garlic | 5.5 – 6.5 |
| Beat root | 5.5 – 6.5 |
| Celery | 5.5 – 6.5 |
| Spinach | 6.5 – 6.8 |
| Cape Gooseberry | 5.3 – 5.5 |
| Watermelons | 6.0 – 7.0 |
| Strawberries | 5.5 – 6.5 |
| Lucerne | 6.0 – 7.2 |
| Peaches | 5.0 – 5.5 |
| Carrots | 5.5 – 6.5 |
Table adapted from several sources
Conclusion
IT CAN BE DONE! Let’s approach this coming season with the right set of preparations and attitude. Sample yours soils NOW, correct pH deficiencies by liming NOW, procure your HYBRID (Improved genetics for climate resilience) seed NOW, buy your inputs NOW, prepare your fields NOW, and success favors the prepared one. Plan, Prepare and Pray!
John Basera is a seasoned Agronomist and Agribusiness expert working for Seed Co as Head of Agronomy Services, a department which provides technical support to farmers to improve their productivity levels and to fatten their pockets. He is a PhD researcher in the Agribusiness field with research interests in Agricultural Technology Adoption and Agribusiness Modelling among others. He can be contacted on +263 772 413 184/ john.basera@seedcogroup.com.
