Impact of pH on nutrient availability

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Potash Development Association says there is still some way to go to ensure that soils are managed appropriately.

UNDER the Farming Rules for Water, introduced in 2018, there is a requirement to test soils for nutrient levels (and pH) every three to five years.

Of those that are sampling, it is primarily the basic soil analysis that is completed, where measurements of phosphorus, potassium, magnesium and pH are taken. However, soil analysis is only the starting point and what is done with the information gathered is the important part.

The Potash Development Association says there is still some way to go to ensure that soils are managed appropriately for the measurements recorded.

When it comes to crop nutrition, soil pH must be considered the starting point, as the availability of all nutrients are affected by the pH of the soil. The optimum availability of most plant nutrients occurs around a neutral pH of 6.5 to 7, however the optimum pH for soils will vary depending on the soil type and the cropping.

Since soil pH is measured on a logarithmic scale, a drop of just 1 pH value increases the level of acidity 10-fold. For this reason, any drop in pH from the optimum range can have a large impact on the availability of some nutrients.

There can be large variation in soil pH across a field, so the accuracy of the soil sampling is also important to ensure that the results are truly representative of the area measured.

Rainfall, which is naturally slightly acidic, ammonium nitrogen and root exudates will all contribute to the natural acidification of soil, but not to the extent of reducing the pH of high pH soils. These factors are more likely to be detrimental to naturally acidic and neutral soils than they are beneficial in naturally alkaline soils.

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Only half of all soils are at or above the target index for both phosphate and potash. This means a significant proportion of soils are below the target level for one (35%) or both (14%) nutrients. 

The availability of nutrients from the soil reserves will be reduced if the pH is sub-optimal, with this problem being more serious if nutrient levels are below Index 2. On soils with low nutrient reserves, especially where they are also acidic, if nutrient applications are being reduced or omitted, yields are going to suffer as a result, increasing the cost of production in a time of increasing input costs and lower returns.

Correcting a nutrient deficiency is challenging to achieve in the short term, however it can be even more costly if not addressed. Fertiliser recommendations for phosphate and potash are built up from immediate removal by the crop being grown (a maintenance or removal figure) plus an adjustment for soil index (additional for a 0 or 1, a reduction for anything over 2/2-). The standard adjustment values can be modified to build up an index over a shorter or longer time period using the PDA P&K Nutrient Calculator.

The first step to ensuring crops receive optimal nutrition, and thereby helping to reduce the unit cost of production, is to ensure the correct soil pH. Once this is corrected, achieving target soil index is the next consideration. Where soils are below the target, one option in times of high input prices could be to extend the number of years to build-up the index to reduce the annual cost. Although it should be noted that this will leave the crops grown over that time period vulnerable to sub optimal yields, it would at least reduce the possibility of soils slipping further and costing more in the long run to build back up.

Source: PDA

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