Advice on lime for farmers
Agriculture, Mechanisation and Irrigation Development Joseph Made
By Peter Gambara
THE command agriculture programme kicked off in earnest these past two weeks as farmers started collecting the necessary inputs. Farmers were given vouchers to collect diesel, seed, lime and Compound D fertiliser.
While the collection process was smooth for diesel and seed and to some extent Compound D, there were challenges as far as the collection of lime is concerned because it was not available at the designated points.
Most contracted farmers were therefore in a dilemma as to what they should do; should they wait for the lime to be available or should they proceed with land preparations without the lime.
Lime is applied to the soil in order to neutralise soil acidity so as to enhance the release of plant nutrients in the soil.
Soil acidity is measured in pH (potential of hydrogen) units, which is a measure of the concentration of hydrogen ions in the soil.
Soil pH is measured on a logarithmic scale of one to 14, with seven being the neutral.
Most plants prefer a pH of between 5,5 and 6,5 and if the pH is too low (below 5,5) and hence acidic or too high (greater than 6,5) and hence alkaline, the plants face problems in absorbing plant 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 six.
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.
While 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. This is however, 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 soil depends on the existing pH, hence the general recommendation that soils should be tested first before applying the lime.
The pH can easily 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 laboratories for testing and recommendations. Soils can also be tested at Kutsaga Research Station, at ZFC and other private laboratories.
The command agriculture programme is recommending the application of 700 kilogrammes per hectare this year.
Generally lime should be applied two to three months prior to planting to allow time for it to neutralise the soil acidity. The lime should be incorporated. 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 five tonnes per hectare, 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 it to be available or should they proceed to do their land preparations so that they can plant early.
Depending on the nature of the land, some farmers plough or rip and then disc, whilst other simply disc twice, therefore such farmers can still apply the lime after the first application and disc during the second operation. Ideally farmers should plough their lands in winter while there is still some moisture in the soil. If the lime if applied at that time, then there is adequate time for the lime to take effect before planting of crops starts around September or October.
This dilemma is compounded by the fact that most farmers will need to plant early.
Some farmers had already picked late maturing varieties at SeedCo like SC 719 or SC 727, which require up to 158 days to mature. Any further delay in land preparation while 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 done land preparations, 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 recommend rates, a farmer contracted to grow 20 hectares (ha) needs 14 tonnes, one growing 50ha needs 35 tonnes, while 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, the command agriculture programme is a three-year initiative.
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 can be encouraged to take their soils for analysis at laboratories immediately after the rain season because 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 and so on.
Peter Gambara is an agricultural expert based in Harare