P & K and pH management issues following drought
With a prolonged drought, low yields and less P and K removal will tend to increase post-harvest soil-test P and K levels, but at the same time less short-term recycling from maturing plants and crop residues will result in lower soil-test levels, mainly for K. Also, dry soil slows down the normal equilibrium between different soil nutrient pools, which often results in lower soil-test P and K levels, but most importantly for K. This is because plants are like pumps taking up P and K from the most available soil pools, and normal rainfall allows for a replenishment of the available nutrient pools from the less available pools. With dry soil, however, this replenishment is limited. So the end result from these three processes makes prediction of what will happen with soil-test values uncertain. Most likely, however, there will be significantly lower than normal soil-test K, and probably also some lower soil-test P results.
Very dry soil conditions may result in lower soil pH values (more acidic in neutral to acidic soils). Differences from of 0.1 to 0.3 pH units are common with very dry conditions. This is because small concentrations of soluble salts present in the soil solution are not leached by rainfall, which result in higher hydrogen ion concentration and greater acidity in the soil solution. On the other hand, the dry soil effect on Buffer pH, which is used to estimate lime requirement, is not large or consistent. Therefore, the main issue with dry soil is taking into consideration that the pH result may over-estimate acidity to decide if lime should be applied or not, but will not affect the amount of lime to apply.
Suggestions about what to do
1. Consider estimates of P and K removal with harvest to decide maintenance fertilization rates for the Optimum soil-test category. Also consider if more than expected P and K applied before this year's low-yielding crop can be accounted for the next crop.
2. Try to delay soil sampling until meaningful rainfall occurs because it will result in a better sample and more reliable soil test results. It is not possible to say how much rainfall would be helpful, but we believe it should be enough to thoroughly wet the sample depth for some time before sampling.
3. If you have to take soil samples with the current dry conditions:
- Be careful with sampling depth control and that you get the complete soil core.
- Soil K test results may be lower than they would be with normal conditions due to less recycling to the soil and less replenishment of the soluble or easily exchangeable soil K pools.
- Soil P test results probably will be affected little by the recycling issue.
- Soil pH test result may be a bit more acidic than in normal conditions.