SoilPH: Balancing nutrients in the soil


Nutrient deficiency affects crop yields whether they are trees grown for timber, fruit trees, cereals, flowers or vegetables.
A shortage (or excess) of nutrients can cause serious reductions in crop growth, yield and the quality of the crop produced. Essential major nutrients such as nitrogen, phosphorus, potassium, magnesium, sulphur and calcium are required in relatively large quantities, whilst trace elements such as manganese, copper and boron are required in very small quantities.
Many crops show large and very profitable responses to the correct use of lime and fertiliser in terms of both the yield and quality of the crop produced.
An important part of farming is providing plants with proper amounts of lime and essential nutrients. Soil testing can be used to indicate if additional nutrients are needed to achieve optimal yield.
Soil analysis is the most accurate guide to fertilizer and lime requirements. It is especially important to determine soil fertility and pH levels before planting a crop, so that the necessary lime and fertilizer can be applied to the soil.
Managing Soil pH
pH is a measure of the acidity or alkalinity of the soil and affects the availability of nutrients to the plant. Most floriculture crops do not respond to fertilization when the pH is very low (extremely acid soils, pH less than 5.0) or very high (extremely alkaline soils, pH above 7.5).
Calcium, phosphorus, magnesium, and molybdenum are the nutrients that are most likely to be deficient under acid soil conditions. Test the soil to determine pH before planting and every 2-3 years to monitor changes. Soil pH can usually be modified to obtain a suitable pH.

raising Soil pH
For acidic soils lime application to raise soil pH is usually required. When the soil pH is not known, a soil test should be performed.
On extremely acidic soils, flowers and most crops will not respond to fertilization or other management factors. Agricultural grade limestone (calcium carbon¬ate) is generally recommended to correct soil acidity.
Note that lime should not be applied within one week of applying nitrogen fertilizer or manure. The high soil pH that occurs shortly after liming will increase the loss of ammonia.
Lime does not move through the soil, it must be incorporated.
Some soils limed heavily over a period of years may not require further applications. Some light-textured soils that have an adequate pH occasionally test very low in calcium, and therefore require lime. If calcium levels are low, gypsum or fertilizers such as calcium nitrate may also be used to supply calcium, rather than using lime. Gypsum (CaSO4) is not a liming agent. It will not increase soil pH, and under certain conditions it is used to lower soil pH. The use of some dolomitic limestone is recommended since it contains a significant quantity of magnesium, an essential and often deficient plant nutrient.
The positive effects of lime application include:
• Reduce soil acidity,
•Improve the physical condition of the soil,
•Provide calcium and magnesium (if dolomitic limestone is used),
•Favour bacterial action and, thereby, hasten the decomposition of organic matter and the release of nitrogen,
•Improve conditions for availability of other nutrients, notably phosphorus and some minor elements, and
• Reduce the toxicity of some elements such as manganese and aluminium.

Growers need to be careful when applying lime. If applied at too high a rate (above 5 tonnes per ha), lime may tie up some micronutrients (e.g. boron) or cause nutrient imbalances.

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Lime application may aggravate magnesium deficiencies, especially in sandy soil. Where this is a problem, some dolomitic lime should be used. Liming can also
increase the rate of organic matter depletion and encourage the germination of some weeds. Lime should always be used in conjunction with a planned soil testing and fertilizer program.

lowering soil pH
Sometimes it is advantageous to lower or acidify the soil pH. Alkaline mineral soils may need to be acidified for crop production.
The principal materials used to lower soil pH are elemental sulphur, sulphuric acid, aluminum sulphate and iron sulphate (ferrous sulphate). Ammonium sulphate, ammonium phosphate and other ammonium containing fertilizers are also quite effective when the soil receives sufficient water, though they are primarily sources of plant nutrients

Soluble salts in soil
Elevated salt levels in soil will interfere with water uptake and eventually plant growth. The effects range from delayed or non-germination of seed to death of new transplants and serious reduction in growth of new or established plants. The problem with soluble salts is most severe when soil moisture is low and salt concentration is high.


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