The best time to apply lime

Peter Gambara —
Command Agriculture kicked off in earnest three weeks ago as farmers started collecting inputs. Farmers were given vouchers to collect diesel, seed, lime and Compound D fertiliser. Whilst diesel, seed and — to some extent — Compound D collection was smooth, there were challenges regarding the collection of lime as it was not available at the designated points.

Most contracted farmers were, therefore, in a dilemma as to what they were to do. Should they wait for the lime to be available or proceed with land preparations without it? Lime is applied to the soil to neutralise soil acidity so as to enhance the release of plant nutrients in the soil. Soil acidity is measured in pH units, which is a measure of the concentration of hydrogen ions in the soil. Soil pH is measured on a logarithmic scale of 1 to 14, with 7 being neutral.

Most plants prefer a pH of between 5,5 and 6,5 and if the pH is too low (below 5,5) and therefore acidic, or too high (greater than 65) and therefore alkaline, plants face problems in absorbing nutrients from the soil. Below a pH of 5,5, aluminium may be concentrated in the soil to the point of limiting root development; hence the roots fail to absorb water and nutrients.

In very acid conditions, nitrogen, phosphorus, potassium, sulphur, calcium and some trace elements may become unavailable.  Poor crop growth, yield reduction and small grain size will occur as a result. The plants can become stunted and may exhibit phosphorous deficiency symptoms.
Maize prefers a pH of around 6.

Soil acidity is caused by leaching of nitrogen below the root zone from excessive rains, excessive use of nitrogenous fertilisers like ammonium nitrate, organic matter decay and the harvesting of high yielding crops. Whilst nitrogen is an important plant nutrient, excessive nitrate in the soil can lead to a risk of draining or leaching below the plant roots into the ground water system, leaving behind hydrogen ions in the top soil, thereby increasing soil acidity.

Organic matter decay produces hydrogen ions that cause acidity. However, this is insignificant in the short-term. Most plant material is alkaline and the removal of plant material through harvesting of high yielding crops contributes to soil acidity.  As the plants grow, they absorb a lot of the desirable elements like calcium, magnesium and potassium, leaving residual hydrogen ions in the soil and the higher the yield, the more elements removed at a time.

The amount of lime applied to a soil depends on the existing pH, hence the general recommendation that soils should be tested first before applying the lime. The pH can be determined through the use of a portable kit. However, such kits are rare in Zimbabwe. Farmers have to take their soils to the Department of Research and Specialist Services for testing and recommendations.

Soils can also be tested at Kutsaga Research Station, at the Zimbabwe Fertiliser Company and other private laboratories. Command Agriculture is recommending the application of 700kg per hectare this year. Generally, lime should be applied two to three months prior to planting to allow time for it to neutralise soil acidity.

The lime should be incorporated into the soil for maximum benefit. However, even if it is incorporated into the soil, the lime will not have any effect on the soil pH if the soil is dry. Some moisture is required for the lime-soil reaction to occur.  After applying the lime, one can only expect to get measurable effects on the soil pH after about four weeks, but it takes six weeks to two months for the lime to dissolve completely and the farmer to get full benefits.

The benefits of lime are well-documented.  Properly limed soils will readily release plant nutrients with phenomenal benefits to crop yields. It was going to be very easy for the contracted farmers to achieve the targeted 5t/ha if the lime had been availed and applied on time. The dilemma that Command Agriculture-contracted farmers are facing is due to the delay in availability of the lime.

Should they continue to wait for lime or proceed with land preparations so that they can plant early?  Depending on the nature of the lands, some farmers plough or rip and then disc, whilst others simply disc twice. Therefore, such farmers can still apply lime after the first application and disc it during the second operation.

Ideally, farmers should plough their land in winter whilst there is still some moisture in the soil. If lime is applied in that period, then there is adequate time for it to take effect before planting starts around September/October. This dilemma is compounded by the fact that most farmers will need to plant early.

Some farmers had already picked late maturing varieties which require up to 158 days to maturity. Any further delay in land preparation whilst waiting for the lime eats into those days to maturity. Those in red soils also face the additional danger that if heavy rains come before they have prepared land, they will have to wait even longer as they will not be able to work on their lands because they become sticky.

The second dilemma that farmers are facing relates to the bulkiness of the lime. Based on the Command Agriculture recommended rates, a farmer contracted to grow 20 hectares needs 14 tonnes; one growing 50ha needs 35 tonnes, whilst one growing 120ha needs 84 tonnes. It is not easy to move such large volumes.

In many instances where lime has been promoted as a necessary intervention, the issue of its bulkiness compared to its cost of US$6 per 50kg bag makes it difficult to convince farmers to adopt it. However, Command Agriculture is a three-year programme; those who fail to apply the lime this year can still apply it next year.

In some cases, it is necessary to apply lime for three consecutive years to correct the pH.  What has happened this year also provides some learning points for next year, ie lime should be availed on time, well before the rest of the inputs. Secondly, the logistics to move the lime should also be clearer. Very few farmers will be able to hire lorries to pick it up from far away depots.

Thirdly, some consideration should be made to purchasing portable pH testers that can be used on the ground by extension officers to service contracted farmers. Lastly, through sensitisation, farmers should be encouraged to take their soils for analysis at laboratories immediately after the rain season as soil samples should be taken when the soil is dry.

Besides getting recommendations on how much lime they should apply, farmers will also get to know how much fertilisers they need to apply. This year, blanket rates on both lime and fertilisers were being applied to all farmers and yet individual circumstances differ. I hope with this information, farmers will be able to make the right decision based on their individual circumstances like what tillage systems they are using, choice of maize seed variety, etcetera.

Mr Peter Gambara is an agricultural economist/consultant based in Harare.

1,155 total views, no views today