How to calculate the fertilizer requirements from soil analysis

Calculating fertilizer requirements for a crop is quite often more of an art than exact science.  The reason is that soil is a dynamic micro world on its own that changes continuously.  It is full of microscopic living organisms such as bacteria, viruses and insects that feed on it and break complex compounds down on which the plants then feed.  You might add 30 t/ha organic material today but by the end of the year it will all be gone. Not only do the micro-organisms change the soil but mechanical preparation completely changes soil structure.

I will present a general method of calculating the fertilizer requirements for any crop based on a soil analysis. If you don’t have a soil analysis I will discuss it at the end of the article.

Only 3 major elements are calculated in open field production since most of the other essential elements are already in the soil at high enough concentrations.

The major elements that need to be calculated are

1. Nitrogen
2. Phosphorus &
3. Potassium

Step 1: Determining Nitrogen fertilizer requirements

You should have an idea of what your yield should be or what yields are potentially realistic in your environment.  A grower knows this either through experience, gets the info from experienced growers or through literature.  Let’s assume that a grower wants to plant potatoes. So nitrogen fertilizer requirements are not calculated using soil analysis, but by yield potential.  The reason is that nitrogen fluctuates too much to provide accurate measurements.

Below is a general estimate of the nitrogen fertilization requirements of potatoes. Don’t use it as is. You have to adapt the table according to your climate, soil history, soil type, planting density, variety and plant date. The type of fertilizer and spread of application is a totally different matter and will be discussed in a separate article.  For now it is important to know the total amount that is required for the growing season.

Nitrogen fertilizer application for potatoes

Looking at the table above.  If you know that you can achieve 30 t/ha and your potatoes are planted on irrigated land, the recommended total nitrogen application should be ±150 kg/ha. It is important to know the yield in your area because that will determine the amount of nitrogen removed from the soil.

Potential yields are available from your local seed company, bear in mind that these yields are for the harvested product only and does not include the roots and leaves that are not harvested and sold.  One has to add a little more to compensate for the non marketable component as they also used up nutrients from the soil.

Use the nutrient uptake table below to assist with N uptake per type of crop

Step 2: Determining Phosphorus fertilizer requirements

Since phosphorus (P) does not move a lot in the soil it can be applied in the beginning of the crop season. Again, just as for N, we use potential estimated yields.  The P level in the soil must be calculated by a lab. They use one of four different methods and it should be stated clearly on the soil analysis report.  The four are Ambic, Bray 1, Bray 2 and Olsen. Once you have received the soil analysis report, for instance Bray 1 with 12 mg/kg (or ppm) in the soil you can correlate the fertilizer recommendation with the potential yield, for instance, 30 t/ha beetroot.  Where the row and column intersects at high yields, ie. 80 %, so the total amount of P added is 100% x 30 t/ha x 0.75 kg/ton P removed = 22.50 kg/ha of P must be added

So what happens if you don’t have this complicated table.  Well then you have to use the estimated p uptake of the crop. For instance it has been determined that tomatoes use approximately 0.75 kg P per ton yield.  So if you expect a yield of 100 tons/ha, then you should add 100 x 0.75 = 75 kg P/ha.  These are very rough estimates and over time one will get a good idea of how much P must be added for a specific piece of land.

Note that there is a vast difference between tomato and beetroot yields, thus there will be a big difference in P application. The calculations do not take into account what the ideal level of P must be in the soil. If the P is very low, one should try and increase it over time by adding a little extra with each application. Keep the P concentration above 60 ppm. These are the ideal levels for P.

For every 40 kg P added to the soil, that element’s concentration will increase with 10 ppm

Use the nutrient uptake table above to assist with N uptake per type of crop

Step 3: Determining Potassium fertilizer requirements

Use the cation exchange capacity of the soil. The cation capacity of the soil is calculated by using 4 essential cations and titratable acid content of the soil.  So you will need to analyze the following:

• Potassium (K) in mg/kg
• Calcium (Ca) in mg/kg
• Magnesium (Mg) in mg/kg
• Sodium (Na) in mg/kg and
• Hydrogen (H) in me%

All these values should be in the soil analysis.

Now convert to me% if not already done so.

Conversion is easy by dividing each element by the molecular weight as follows:

• Potassium (K) in mg/kg ÷ 390
• Calcium (Ca) in mg/kg ÷ 200
• Magnesium (Mg) in mg/kg ÷ 120
• Sodium (Na) in mg/kg and ÷ 230

By adding the results together with the hydrogen value we get the CEC of the soil.

%K of CEC is calculated as follows: %K = me % K / CEC × 100.

Once we have the % K of the CEC we can calculate the K fertilizer requirement of the crop with the following table.

Thoughts on fertilizer calculations

Calculating fertilizer recommendations is an art and not science. If you are not sure, or have absolutely no idea what you are doing, you can add 500 kg 2:3:2 (22) at the beginning of the season and top-up with 200 kg KNO₃ after 3 weeks and again after 5 weeks. You should be fine, honestly. You might not achieve 100% of the potential yield of your crop, but it will not be a failure.

Fertilizer recommendations can become so technical that even I have lost track. Other factors are:

• Clay content of the soil
• Roughness of the soil
• Variety planted
• Texture type
• Soil type
• Organic content

And the list can go on. At the end. Climate is the biggest influence on yield potential and crop quality. Then secondary factors such as soil type, fertilizer etc. ( as long as the secondary type factors are not limiting).