Agriculture occupies a central place in 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’. Again soil conditioning is a critical component of soil management.

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 (bottom-line story) 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, Mechanisation and Irrigation Development, Honorable, Dr. Joseph Made has always reiterated the importance of soil management in attaining high and sustainable productivity levels in all cropping programmes. The minister specifically refers to the liming issue as a critical game-changing component which leads to quantum yield gains for farmers.

Of all the 10-tonne plus yielders whom I have interacted with in the 2015/16 season, it seems soil analysis and liming are a standard practice that they do after every 3 years in sandy to loamy soils and 4 years in heavier soils. Even with farmers who are registering for the 11-tonne Plus Club this season, we are realising the same secrete – they have conditioned their soils with lime. It seems like it’s a “standard secret” to their success.

Soil pH is an excellent chemical indicator of soil condition (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 another bumper season. Luck and success favour the prepared farmer and therefore we encourage our farmers to start summer preparations as soon as they finish harvesting the current summer crop. This is very critical.

A Bumper season is possible and a Green Revolution is imminent – everyone could see it coming. Let’s get ready for it. Let’s condition our soils!

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 calcific lime) is recommended as this enhances Fertiliser 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 any cropping concern. This is the top secret to enhancing yield per unit area. It is my strong feeling that it must be promoted and encouraged among farmers. Some would even argue for the legitimation of this critical aspect.

Secondly, application of fertilisers is heavily recommended if we are to get good crop productivity levels and returns on investment. The principle is to apply the right quantities of the right type of fertiliser at the right time and place. Fertilisation 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 type. We recommend three years for lighter soils and four for heavier soils. In a season the best time to sample soils is the first week after harvesting a summer crop. This will give the farmer a good lead-time to correct any deficiencies in the soil, 3-6 months before establishing the next crop in summer. Farmers should note that lime can be applied even on the day of planting and still act as a “buffer” before correcting the soil acidity condition but the ideal/optimum time to apply is at least three months before planting the next crop.

How to sample soils?

The most commonly used procedure for soil sampling would be based on soil type.  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 organise 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 zigzag, random, the cross diagonal methods are commonly used and recommended where samples are taken in a zigzag 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/cores 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). This is also called the tillage layer. The most commonly used tools for taking samples are augers, probes, hoes or sometimes shovels. Samples from different sites in a block are then mixed thoroughly and bagged into 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 fertiliser houses do sampling free of charge or at a paltry sum. Other approved laboratories include the Department of Specialist Services-Soil Chemistry laboratories. Other options include Soil Testing Laboratory at the University of Zimbabwe’s Soil Science Department; Tobacco Research Board, AgLabs etc. 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 come 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 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 acidic and a soil with pH greater than 7 is considered alkaline. A good liming programme is based on soil test that determines the degree of soil acidity and the correct amount of a liming material needed to neutralise 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:

• 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 fertilisers (e.g. Ammonium Nitrate or Urea and to a lesser extent 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 contribut 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 functionas well as restricting plant uptake of certain nutrients.

Effect on phosphorus availability: acidic 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 micro-organisms 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 micro-organisms do not function efficiently in acidic soils (low pH).

Soil physical condition: liming improves soil physical structure by reducing soil crusting/capping and this promotes better emergence of small-seeded crops and ultimately results 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 three to six 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, a week after harvesting is the best time to apply 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’ which acts 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 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/tillage layer is essential and must be achieved. We generally recommend lime to 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 discing or harrowing followed concurrently by a roller. In Zimbabwe, liming agents are in powdery formulations to increase surface area for quicker reaction with the soils.

Lime can be spread by hand or by lime spreaders, which normally gives the best results. Hand application is normally not recommended when the weather is windy. However, I have seen some smallholder farmers mixing lime with a basal fertiliser before application and giving commendable results. However, my only concern with this method is on the timing of lime applications (basal fertilisers are normally applied during planting and yet we recommend lime to be applied 3-6 months before crop establishment). But still it acts as a buffer.

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 and differs from one soil type to another. 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

General interpretation: it means a farmer with a sandy soil requires 1000kg (1 tonne) 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 fertilisers increases soil acidity levels. So we generally recommend that whenever any form of Nitrogen, be it from basal (compound D, L, S or J etc.)  OR top dressing (e.g. AN/Urea) is applied, a farmer needs a maintenance lime application of about 1.8kg for every kg of Nitrogen applied. This is applicable if one does 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 three years of soil use.

What are the liming agents available in Zimbabwe?

It is important to sample your soils for analysis to determine pH and also 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 this should be noted. Preferential uptake is when a certain nutrient is taken 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 of Calcium at the expense of a balanced uptake with other elements e.g.  Magnesium, and therefore a crop will show Magnesium deficiencies – yield will be affected.

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. Lime ‘sweetens’ acidic soils. On the other hand Gypsum (Calcium Sulphate) is a supplementary source of Calcium and Sulphur which farmers apply whenever there is deficiency of these two elements in the soil. It should be noted that Gypsum does not adjust soil pH but rather supplement the soil with Calcium and Sulphur. Gypsum also improves soil physical structure i.e. removes hard setting clodiness, removes surface crusting/capping and improves soil workability.

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 tonne) of lime may cost a paltry $100 or less. Imagine this is only 8% 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 benefits 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. These are in fact ‘complimentary benefits’.
• Liming improves Fertiliser Use Efficiency (FUE) by crops. Use of fertiliser alone without lime result in poor fertiliser uptake and reduce the economic benefit of using fertiliser. In economic terms, lime is an enabler to get the best return on fertilizers investment.
• Lime reduces availability of toxic elements in the soil such as aluminum and manganese. This results 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, [delete it] liming acidic soils to optimal levels creates a conducive environment for microorganisms to carry out necessary processes in the soil e.g. 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 suppresses some specific problematic weeds such as witch weed/karoyi/bise.
• 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.

NB: Generally most cereal and legume crops in Zimbabwe require an optimum pH range of 5.2 – 6.8.

Conclusion

A green revolution is upon us. Another Bumper Harvest is possible! It can be done! Let’s approach this coming season with the right set of preparations and attitude. Sample your soils after harvesting, correct soil pH deficiencies by liming and incorporate the liming agent into the tillage layer before planting the summer crop.

Also register for the 11 Tonne Plus Club and win BIG this year.

About the author

John Basera is Seed Co Head of Agronomy Services. He can be contacted on +263 772 413 184/ john.basera@Seed Cogroup.com.