Improving soil fertility

For a gently sloping garden or orchard (≤9%), soil conservation strategies will mainly focus on protecting the soil surface from being exposed to the sun and drying out, thereby losing water, reducing biological activity and facilitating quick loss of organic matter. The risk of water and wind erosion is quite low on gentle slopes. Such strategies to reduce soil exposure include:

• Mulching. Mulching is the process of covering the topsoil with plant materials such as leaves, grass, twigs, crop residues or straw. This protects the topsoil from being washed away by strong rain and from drying out by the sun. The mulch reduces evaporation of water and thus keeps the soil humid. As a result the plants need less irrigation and can use the available rain more efficiently. A moist soil also enhances the activity of soil organisms such as earthworms, and microorganisms as rhizobia and mycorrhiza. Organic mulch material is an excellent food source for soil organisms and provides suitable conditions for their growth. As the mulch material decomposes, it also releases its nutrients, while part of the mulch material is transformed to stable humus, contributing positively to the soil’s organic matter content. A thick mulch layer further suppresses weed growth by inhibiting their germination. Mulching is especially useful during early stages of growth of mango trees, i.e. the first 3 years of establishment, by spreading the mulch materials around the growing trees.
• Use of cover crops or intercrops. Mango trees grow slowly, taking 3 to 5 years depending on the variety before the trees cover the space in-between the trees. Besides the exposure of the soil to erosion and water loss, normally weeds will grow in this space requiring regular ploughing or digging to remove them. 
• Cover crops are low-growing perennial plant species, which can be sown in the alley spaces between the mango trees with the main intention to protect the soil, prevent weed growth and improve soil fertility. They therefore require regular slashing, mowing to avoid competition with the growing mangoes. The slashed materials are left in the field where they contribute to improving the physical properties of the soil, reduce runoff and erosion, suppress weeds and, if the cover crop is a legume, transfer nitrogen to the main crop, when left as mulch in the garden. There are different plant species that can be used as cover crops. However a good cover crop should be low-growing and not climbing; grows fast and covers the soil in a short time; resistant against common pests and diseases, and does not transmit any to the main crop (mango); tolerant to drought; nitrogen-fixing; and easy to sow and to manage, slash, and or cut for fodder.

Besides cover crops, an economic crop may be grown in alley spaces of the mango trees, a practice referred to as intercropping. Intercropping with seasonal crops like beans, maize, peanuts, vegetables is preferable, however upon harvest all crop residues should be left in the garden to protect the soil. Use of intercrops or cover crops increases biodiversity and thus assist in pest and disease management. Maintaining biological diversity within and around the orchard is an important feature of organic systems. Areas of remnant vegetation can also be protected on the garden peripheries as shelterbelts providing an important habitat for natural predators of insect pests, which reduce harm to crops and the need for direct pest control measures.

For a moderately sloping field (10 to 15 %), the biggest challenge for the farmer is to protect the soil from loss from erosion. Therefore, a lot of investment will go into construction of water catchment structures like contour ridges and bunds, combined with grass strips planted on the contour ridges, and water pits/catchments to reduce the movement of water along the slope. Water pits/ditches may be dug along the contour to trap the running water and encourage infiltration into the soil. Mangoes can then be planted on the contour ridges.

Fodder grasses may be planted as grass strips, such as vetiver grass (Vetiver zizanioides), napier grass (Pennisetum purpureum) and guinea grass (Panicum maximum), Bahia grass (Paspulum notatum) can be planted in strips at intervals across the slope to slow down water runoff. In addition to reducing soil erosion, the grasses provide feed for the animals. The grass strips can be mixed or replaced with a hedge row of leguminous fodder trees  such as Leucaena diversifola, Calliandra calothyrsus, Sesbania sesban, Gliricidia sepium. Then manure from the animals can be used to add organic matter to the soil.

The A-frame is a simple, cheap and easy-to-construct tool for marking contour lines along a slope, making it readily accessible to farmers. The A-frame is made from three poles, some rope, a stone and a supply of stakes.

How to build and use an A-frame: 

(i) Fix three poles of about 2.5 meters long each in a position forming an even ‘A’. If rope is not sufficient to tie the ends, use nails.

(ii) Tie one end of a piece of cord to the top of the A and fi x a stone tied to the other end so that the stone is at some distance from both the ground and the crossbar.

(iii) Put the A-frame upright and mark the position of both legs. Then, mark the point where the string passes the crossbar of the A.

(iv) Turn the A-frame so that the placement of the legs is reversed. Again mark the point where the string passes the crossbar. If the two marks are not at the same point, mark a third point with a knife exactly halfway between the first two.

(v) Drive the first stake at the edge at the top of the field. Place one leg of the A-frame above and touching the stake. Place the other leg in such a position that the string passes the level position point on the crossbar.

(vi) Drive another stake into the ground just below the second leg. Move the A-frame and continue in the same way across the field.

(vii) The next contour line is placed 3 to 6 meters below the first line. The steeper the slope, the closer the lines should be.

Mango production on steep slopes

On steep slopes (>15 %), reduced or no tillage should be applied to minimize soil disturbance. Due to the very high risk of erosion, soil disturbance should be minimized. Bush clearing/slashing followed by direct digging of planting holes for the mangoes is highly recommended.

These practices aim at enhancing the organic matter content of the soil as the basis for efficient management of plant nutrients and water. They can be applied wherever mango is grown irrespective of the slope of the garden or orchard.

Soil organic matter benefits can be grouped into three categories:

• Physical benefits: Soil organic matter enhances aggregate stability, improves water infiltration and soil aeration, reduces runoff; improves water holding capacity; reduces the stickiness of clay soils making them easier to till; reduces surface crusting, and facilitates seedbed preparation.
• Chemical benefits: Soil organic matter increases the ability of the soil to hold onto and supply over time essential nutrients such as calcium, magnesium and potassium – also known as Cation Exchange Capacity (CEC); it improves the ability of a soil to resist pH change – this is also known as buffering effect capacity; accelerates decomposition of soil minerals over time, making the nutrients in the minerals available for plant uptake.
• Biological benefits: Soil organic matter provides food for living organisms in the soil; it enhances soil microbial biodiversity and activity, which can help in the suppression of crop diseases and pests; and enhances pore space through the actions of soil microorganisms. This helps to increase infiltration and reduce runoff.

In addition to mulching and cover cropping (described in step 1) which also contribute to soil organic matter to some extent, farm yard manures or compost from within or outside the farm can be used. These materials are a good source of humus substances to the soil and thus improve its structure and water holding capacity of the soil.

Well-decomposed manure or compost can be applied directly in the planting holes of mangoes, to aid quick root development. Additional quantities of manure can be top dressed directly around the growing plants  if well decomposed or composted, or spread in shallow trenches in between the mango planting lines.

The nutrient requirements of mangoes are dependent on the stage of growth. Mangoes especially require a good supply of nutrients during the active growth stage until about 4 years when they start fruit production and thereafter, more nutrients are required, especially during flowering and fruit formation.

The following general nutrient supply measures can be adopted:

• During the early development of the mango tree (during the first four years), a regular supply of compost and green manure is provided to improve the foliar development.
• As soon as the mango tree produces fruit (from the 4th year on), application of organic fertilizers (compost made of farm yard manure or other organic material) should always follow the flowering, so that enough nutrients are available for fruit formation and fruit development. 
• Mango trees under optimal conditions can achieve yields between 10 to 30 tons per ha and year. The yield varies per tree from 50 to 500 kg depending on the planting system, size of the tree and variety.

In situations of heavy nutrient depletion or unfavourable growing conditions such as extreme pH levels, based on soil or leaf test results, soil supplements of macro- and micronutrients may be used. These supplementary measures include:

• Use of self-made liquid manures that are easily available to plants.
• Use of soil amendments such as lime to correct soil pH, and microbial inoculations to enhance biological activity of the soil and nitrogen fixation in the soil.
• Use of fertigation water to supplement both water and nutrient requirements. Fertigation water must be adequately managed, scheduled and monitored to reduce problems with contamination of the water table, leaching of nutrients and salinity inducement.
• Use of commercial organic and selected mineral fertilizers (Tables 1 and 2) to satisfy specific nutrient needs. In organic production, mineral fertilizers shall be used as a supplement to methods described in Steps 1 and 2. The use of mineral fertilizers shall be justified by appropriate soil and or leaf analysis. However only naturally occurring mineral fertilizers are allowed by most organic standards and certification schemes.