Plant nutrients in the soil

Maintaining plant nutrients at the proper levels in soil, so that they are readily available to plants as needed is not so straightforward as many fertilizer makers would have it appear, but it is the key to achieving balanced plant nutrition.

But what are plant nutrients and what do the fertilizer numbers mean?

For the very best results, it is important to apply nutrient requirements according to the principles of the Mikhail System. This treats soil as a ‘Living System’ in which all aspects need to be managed together as an integrated process.

Unfortunately, many people have been led to believe that soil tests are only about fertilizer and so they do not send in samples until they are ready to buy fertilizer. Using fertilizer without first improving soil structure and friability (either alone or in combination with the Lime and Gypsum), will not give the best result.

These notes explain the basis for the greatest benefit from applied fertilizer, how plant nutrients can be more effectively managed during the season and what you can do to get as good a result as possible if you need to start with a ‘fertilizer first’ approach.

Soil has needs of its own

Even with no production – just to maintain essential plant cover and microbial activity – soil has a need for plant nutrients. How much of each nutrient is needed will depend on the Cation Exchange Capacity (or the capacity of the soil to provide nutrients in plant-available form).

The first step in preparing nutrient recommendation must, therefore, be to look at the levels of available nutrients on a soil test report and make a provision for any that are below this essential ‘base fertility’ level. Soil tests from SWEP Laboratories always include this in their recommendations.

However, these soil tests do not recommend specific fertilizers - only the actual amounts of each nutrient required. This means you will need to convert recommendations for plant nutrients into appropriate fertilizers using materials available in your area. Your local Nurseryman or fertilizer supplier may also be able to help you with this.

This principle applies to all living systems – not just the base fertility of soil. For instance, each of us requires a certain calorie intake just to stay alive – before we even begin to work out what is needed to support our particular activity level. If we do not make any allowance for this, we will find ourselves unable to meet the goals or objectives we would like in sport, work or family life. We see the same thing in our plants when, even in good seasons, they fail to meet our expectations.

Plant nutrients for the entire season

Every plant has its own requirement for various plant nutrients and every soil has its own capacity for providing these nutrients – the two cannot be separated! What is more, the needs of a crop will vary throughout its life. For much the same reason that you do not expect a new-born baby to eat a three course steak dinner (much less the its complete food requirement for a lifetime), newly emergent seedlings and shoots should not be expected to properly utilize an entire season’s nutrient requirement all at once.

Of course, the soil is an important factor here. Light, sandy soils have only a small capacity to hold nutrients in plant-available form, compared with heavier soils. As a result, smaller and more frequent fertilizer applications become more important as the soil texture becomes more sandy.

On the other hand, a strongly structured, well-prepared clay soil can have a large capacity to hold and supply plant nutrients over an extended period. For these soils, one or two fertilizer applications per year may be sufficient.

For these reasons, heavier soils (given adequate preparation) will be suited to crops for which changes in requirement over the season are small and/or repeated applications are impractical for some reason. Conversely, sandy soils are best for irrigated crops with special nutrient needs at certain critical stages of their development. In both cases, well-balanced soils respond best to Organic fertilizers, rather than synthetic, chemical ones.

A soil test can give you the overall nutrient requirement for a given crop in a particular soil, but you may still need some help working out how and when to best apply those nutrients. Advice from your local Nurseryman or an agronomist with experience on the crop in question and how it performs in your region will be essential to getting close to the optimal ‘crop potential’ for the season.

Responses limited by soil balance relationships

So why is it many people find simply applying more fertilizer fails to deliver improved results?

The short answer is that plant growth can be limited by more than just the amount of nutrients available. There are also aspects of the soil environment that affect how well plants access the nutrients that are there - things like drainage, moisture retention, how easily roots can penetrate the soils and so on.

This is why the first step should always be the application of any required Lime, Dolomite or Gypsum (for improved soil structure and function) according to the Mikhail System. The application of fertilizer is the second step - done six months later. You can get more information about this by reading about the 5-steps to Soil Improvement. To summarize, the important steps to improve any soil are:

1. Soil test to identify the problem
2. Adjust the Cation Balance first (usually in Autumn)
3. Apply fertilizer six months later
4. Stimulate soil biology after every disturbance
5. Monitor and Adjust

An important aspect of this method is that it leads to progressively reduced nutrient inputs, until only small maintenance applications of fertilizer are required each season for both sustainable and cost-effective production.

If you must use a ‘Fertilizer First’ approach

Since you will be applying controlled amounts of all the nutrients actually needed by your plants – rather than just more and more of the same old NPK – you can expect to get reasonable results from a SWEP soil test (all other factors aside) if you use fertilizer before improving your soil. However, getting the best results will mean waiting until you can apply any required Lime, Dolomite &/or Gypsum and giving these adequate time to take effect with improved cation balance, soil structure and friability.

Right now, while you are waiting to do this, but still needing to maintain production, there are a few things you can do to get the best possible response from your fertilizer:

1. Increase the major nutrient recommendations (N, P, K and S) on your soil test report just a little.
2. If the soil pH (measured in water) is less than 5.7, apply the trace elements (and extra P if you can) as a foliar spray, but consult your local Agronomist or SWEP agent about the best timing for this.
3. If your soil pH (in water) is above 8.0 foliar applications of plant nutrients will be essential.
4. If your recommendations include a high Nitrogen requirement, consider splitting this into two or three applications at critical crop stages. Again, your Nurseryman or Agronomist can help you with this.
5. Don’t overlook the Sulfur. Using low analysis phosphate and the sulphate forms of other fertilizers will usually be enough to meet any requirement you have for this nutrient.

Further Reading & Useful Links

What is soil and how does it function?
Information on what soil is composed of and how these fractions interact to create a medium that is suitable for plant growth.

An explanation of the Calcium-Magnesium ratio in soil
Contrary to popular belief, this ratio is only indirectly involved with the availability of plant nutrients. Its real importance lies in its affect on soil structure and friability.

Toxic elements in the soil
Apart from the plant nutrients, there are also some elements that are toxic to plants.

Collecting soil samples for analysis
A soil test is only ever as good as the sample that is tested, so it is critically important that you collect your samples properly.

Using Lime and Gypsum to improve soil
Again, popular belief is that Gypsum is a "Clay Breaker" and Lime is used to adjust the pH of soil. In fact, these are only side effects. Here you can find information on what they actually do in the soil.

Reading soil test reports
Once you have a soil test, you may find the report you get a bit confusing. Here are some tips on how to read and understand it properly.