Seed Bed: Can you fertilize your way out of poor soil conditions?

Thursday, June 4, 2009

Using extra inputs to compensate for poor soil conditions has its limitations.
Here's a primer to help you sort out how far this strategy can take you

by KEITH REID

"I know conditions weren't quite right, but I couldn't wait any longer. What should I do now to make up for it?" 

This is a common question anytime we have been pressured for time and went to the field despite poor soil conditions. The culprit may have been a wet harvest season, a late spring combined with a full manure tank, or even making that first pass with the cultivator just to keep up with the neighbours. Whatever the cause, the end result is a compacted, crusted or cloddy seedbed, with a much-reduced ability to provide nutrients and water to the crop.

The first reaction is often remorse, kind of like a hungover college student on a Saturday morning swearing he will never drink again. Most farmers know that building a solid foundation of good soil will avoid a lot of these problems in the future, making the soil more resilient and able to absorb the occasional insult. But that doesn't stop them from looking for short cuts.

The steps that farmers will use to try and overcome poor soil conditions fall into two basic categories – sound agronomic practice that may actually help the crop overcome adverse soil conditions, or burnt offerings to the gods of agronomy so they will forgive your transgressions and give you a good crop anyway. In this article, I'll try to sort out which class any extra inputs might fit into.

Higher fertilizer rate. Since the plants will have a smaller root system,  increasing the concentration of nutrients in the soil should allow the crop to meet its nutrient requirements by taking up more nutrients per inch of root length. This will actually work, up to a point. But two factors get in the way of generating a profitable response to extra fertilizer. The first is the physical limit to the amount of nutrient each inch of root can absorb, but the second, more important factor is the cost of fertilizer that would be needed to make any significant difference to the nutrient concentration in the soil.

Better fertilizer placement. Banding nutrients where plant roots can reach them easily does significantly increase the concentration of nutrients, so that plants with a small root system can meet the crop requirements more easily. It is well documented that the response to banded nutrients is greater in compacted or poorly structured soils than in soils with good tilth. The limitation here is the capacity of each inch of root to absorb nutrients, so a very restricted root system cannot keep up with the nutrient demand, no matter how concentrated the supply is.

Foliar nutrients. Some agronomists will suggest that, since the roots are restricted, you should apply the nutrients directly to the foliage. This will be effective for some micronutrients, where the total requirements are very small, but it is not effective for the macronutrients. You can only apply about a one-day supply for a crop that is growing actively before you start to burn the leaf tissue, so it cannot overcome a serious deficit of nutrients from the soil. The plant may take on a darker green colour, but this is usually cosmetic rather than reflecting a healthier plant.

Irrigation. Adding more fertilizer ignores the biggest limitation of a restricted root system, and that is the ability of the plant to get enough water out of the soil. Crops in compacted or cloddy soils will suffer moisture stress much sooner than crops in good soil, and suffer much greater yield loss in dry years.

For field crops, it would take an extreme yield loss to justify the capital expense of irrigation equipment (assuming there was a source of water available), but it is not uncommon to use irrigation to compensate for less than ideal soil conditions in some horticultural crops. What these growers need to consider is the operating costs of the equipment, and the potential savings for fuel and maintenance from better soil management.

Creating 'tilth' with cultivation.
To some extent, tillage can open up a soil that is too tight or break down clods that are too large. But the tougher the soil, the more difficult it is to get the soil conditions that you want. Pulverizing a cloddy soil usually creates dust rather than a granular seedbed, and this increases the risk of crusting from heavy rains. Deep tillage of wet soils brings up slabs rather than opening cracks into the subsoil. And the extra tillage does increase the loss of soil organic matter, reducing the ability of the soil to withstand the next indignity. BF

Keith Reid is soil fertility specialist with the Ontario Ministry of Agriculture Food and Rural Affairs, based in Stratford. keith.reid@ontario.ca