By RUTH VAUGHAN Technical Director CROPNUTS
A healthy avocado tree has a taproot that can penetrate to one meter, with most of the feeder roots in the top 20cm of soil in the drip zone. Its roots are generally shallow and spread in the soil surface in search of water and nutrients. The taproot holds the tree firmly only releasing water when it is extremely dry. The branches tend to follow the roots towards the source of water therefore keep the bottom of the tree moist to maintain the tree within manageable spread, with a clear distance between plants and rows for sufficient lighting and feeding.
The feeder roots are very delicate. Root rot (Phytophthora) can develop very fast when damaged or waterlogged. It is important to determine, in advance, the fitness of soil for avocado production.
Soil profile pits should be dug throughout the farm, at least 1.5 m and a minimum of one pit per ha (more in hilly or non-homogenous areas). One should look at soil color, texture, hard pans, sitting water, structure, patches, concretions, gravels and stones. Soil samples should be analyzed for chemical and textural properties. Samples should include both the topsoil (0-20cm) and subsoil (20cm-1m). Drainage and soil depth can be increased by mounding the soils up.
Red / brown soils are preferable, yellow /grey/light brown soils are often prone to temporary or permanent water logging. Very dark or black soils tend to have too much clay or a large percentage of organic matter that may result in acidic conditions and aluminum toxicity.
The clay content in soil can be determined by conducting a soil texture analysis in the laboratory. Avocados do best in soils with 20-40% clay. In soils with a low clay content (<20%), the water holding capacity is generally much lower, requiring frequent irrigation otherwise the roots may suffer from temporary drought. Very heavy clay soils have high water retention and lower infiltration rates. These soils may become oversaturated during heavy rainfall or over irrigation which promotes root rot.
Soil structure plays a big role in production and root health. Strongly developed block structures, soils that break into hard clods and soils that have large cracks when dry are unsuitable. Soils should only show small, fine cracks when the soil dries up.
Avocados can grow in soils from pH 5-7, with an optimum PH of 6.2 t0 6.5. A complete soil analysis, done in good time well in advance of planting is essential. Lime is added to acidic soil to increase the soil PH to 6.5. Dolomitic lime is part substituted for calcitic lime if magnesium is deficient. If phosphorous is deficient it should be added along with the lime and mixed thoroughly into the soil prior to planting. If the soil is too alkaline, the PH may be reduced by adding Sulphur and keeping the soil moist for 6-12 months.
Calcium is a critical nutrient in avocado production. Soil levels need to be higher enough to suppress root rot. Plant levels are important for the nutrient content and storability of the fruit.
Story page 26 on Aug -Sep issue .