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Soil Management and Fertilizer Use:
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| Author: | OMAFRA Staff |
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| Creation Date: | 01 March 2002 |
| Last Reviewed: | 01 March 2002 |
| Agronomy Guide > Pub 811: Soil Management and Fertilizer Use > Fertilizer Recommendations: Soil Acidity & Liming |
Fertilizer requirements on the OMAFRA soil test report are for specific crop and management. *See Updates section below. Adjustments in fertilizer requirements may be needed if changes are made in manure application or if legumes are going to be incorporated. For fertilizer adjustments, see Table 2-15. Adjustment of Nitrogen Requirement Where Crops Containing Legumes Are Plowed Down. Changing the crop from the original soil test recommendation will require a new fertilizer recommendation. This can be obtained by looking up the appropriate table under the specific commodity chapter in this publication.
The pH scale ranging from 0 to 14 is used to indicate acidity and alkalinity. A pH value of 7.0 is neutral; values below 7.0 are acid and those above 7.0 are alkaline. Most field crops grow well in a soil pH range from 6.0 to 8.0.
To correct soil acidity, ground limestone should be broadcast and worked into the soil at rates determined by soil test. Table 2-10. Soil pH at Which Lime Is Recommended for Ontario Crops, above, shows the pH values below which liming is recommended and the target soil pH to which soils should be limed, for different crops. In Ontario, most crops grow quite well at pH values higher than the target pH to which liming is recommended.
1Where a crop is grown in rotation with other crops requiring a higher pH (for example, corn in rotation with wheat or alfalfa), lime the soil to the higher pH.
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The soil pH measures the amount of acidity in the soil solution and indicates whether liming is necessary for crop production. It does not measure the amount of reserve acidity held on the clay and organic matter particles in the soil, which will dictate how much lime is needed. Different amounts of reserve acidity will mean that two soils at the same pH value will need different amounts of lime to raise the pH to the desired level. The reserve acidity is measured in a separate test: the buffer pH. The greater the amount of reserve acidity, the lower the buffer pH and the more lime is required to raise the pH. For soils needing lime (based on soil pH), Table 2-11. Lime Requirements to Correct Soil Acidity Based on Soil pH and Soil Buffer pH, may be used to determine the amount of lime required to reach different "target" soil pH values required for different crops.
1Buffer pH in Ontario is measured using the Shoemaker,
MacLean and Pratt (SMP) buffer. Other jurisdictions may use different
buffers, which will give similar but not identical results.
2Lime if soil pH below 6.1
3Lime if soil pH below 5.6
4Lime if soil pH below 5.1
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Calcitic limestone consists largely of calcium carbonate, and dolomitic limestone is a mixture of both calcium and magnesium carbonates. Dolomitic limestone should be used on soils with a magnesium soil test of 100 or less, as it is an excellent and inexpensive source of magnesium for acid soils. On soils with magnesium tests greater than 100, calcitic or dolomitic limestone may be used.
The two main factors that affect the value of limestone for soil application are neutralizing value and particle size. Neutralizing value is the amount of acid a given quantity of limestone will neutralize when it is totally dissolved. It is expressed as a percentage of the neutralizing value of pure calcium carbonate. A limestone that will neutralize 90% as much acid as pure calcium carbonate is said to have a neutralizing value of 90. In general, the higher the calcium and magnesium content of a limestone, the higher the neutralizing value.
The second factor that affects the value of limestone as a neutralizer of acidity is the particle size. Limestone rock has much less surface area to react with acid soil than finely powdered limestone and, hence, it neutralizes acidity much more slowly; so slowly that it is of little value. The calculation of a fineness rating for ground limestone is illustrated in Table 2-12. Example Calculation of the Fineness Rating of a Limestone.
1A #10 Tyler
sieve has wires spaced 2.0 mm apart.
2A #60 Tyler sieve has wires spaced 0.25 mm apart.
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This index has been developed in Ontario as a means of combining the neutralizing value and the fineness rating to compare various limestones that are available.
The Agricultural Index = (neutralizing value x fineness rating) ÷ 100
The Agricultural Index can be used to compare the relative value of different limestones for neutralization of soil acidity. Lime with a high Agricultural Index is worth proportionately more than lime with a low index because it may be applied at a lower rate.
For example, if two ground limestones, A and B, have Agricultural Indices of 50 and 80 respectively, the rate of application of limestone A required for a particular soil will be 80 ÷ 50 x the rate required for limestone B. Limestone A spread on your farm is worth 50 ÷ 80 x the price of limestone B per tonne.
Recommendations from the OMAFRA soil test service are based on limestone with an Agricultural Index of 75. If the Agricultural Index is known, a rate of application specifically for limestone of that quality can be calculated. This can be done using the following equation:
Limestone
application rate from soil test x (75 ÷ Agricultural Index
of limestone)
= Rate of application of limestone
For example, if there is a limestone requirement by soil test of 9 t/ha, and the most suitable source of limestone from a quality and price standpoint has an Agricultural Index of 90, then 7.5 t/ha (75 ÷ 90 x 9) should be applied.
The Agricultural Index does not provide information about magnesium content. Dolomitic limestone should be used on soils low in magnesium.
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Lime recommendations presented here should raise the pH of the top 15 cm (6 in.) of a soil to the listed target pH. If the soil is plowed to a lesser or greater depth than 15 cm, proportionately more or less lime is required to reach the same target pH. Where reduced tillage depths are used, rates of application should be reduced proportionately. More frequent liming will be needed.
On soils with pH values below 6.5, it is possible to lower the pH (make the soil more acid) by adding sulphur or ammonium sulphate. This may be desirable for some crops, for example, potatoes for scab control, but usually will not be suitable for rotation crops. Soil pH can not be adjusted from a low pH to a more moderate pH year to year. If the soil pH is above 6.5, it is not advisable and also usually quite impractical to lower the soil pH because of the very large amounts of sulphur or ammonium sulphate required.
Addition to Changes in Crop or Management box - "Fertilizer rates on farms required to complete a nutrient management plan may be limited by regulatory requirements. Refer to the NMAN software or Nutrient Management Workbook for details."
OMAFRA Publication 811: Agronomy Guide for Field Crops Table of Variance
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