Soil Testing: How to Interpret Results and Calculate Fertilizer Needs

Soil, being composed of billions of organisms with their complex relationships and nutrient exchanges. It can be a bit overwhelming to understand it all and nor do I claim to. At a minimum, basic understanding of some of the relationships and nutrient exchanges is however, a critical piece in any agricultural endeavor. 

If the law of conservation of mass and energy holds to be true, then we can not create something out of nothing. In short, we should not expect to have bountiful crops when essential nutrients (building blocks) are missing from the soil in which the crop requires to hit its maximum potential. 

If we follow Liebig’s law of the minimum which states that growth is dictated not by total resources available, but by the scarcest resource (limiting factor), then it is imperative that we know our limiting factors/resources/nutrients. Knowing those limits requires a good old fashioned soil analysis by a lab. 

Having a BSc in Agriculture and having studied agronomy, you would have thought I would have started my farm journey at the top of my soil game but… I didn’t. I knew basic soil science and spent several years digging soil pits in forests around BC for silviculture prescriptions, and yet, in our beginning years, I didn’t really do any soil tests other than texture and organic matter content. I was growing crops on very little available nutrients from the decomposing turf layer that had been plowed under. Crops did “ok” but it was apparent very soon that I couldn’t  1) Guess on soil fertility and 2) Had to do more robust testing and create fertility plans exact to the crops needs to ensure maximum yields were possible.

Maybe I just needed to see the truth to what I had been taught?

When the weather and climate are limiting factors in production of crops to begin with (and highly unpredictable these days), it is wise to remove as many controllable risks and limiting factors as possible to enhance the odds of turning a net revenue on your efforts.  Soil is something you can work at, reducing your risks with amendments and drainage (this may take time if your starting with super dense gleysol like us)

When each plant in the ground is performing well due to appropriate pH for said crop as well as appropriate nutrient levels, you earn more per square foot of growing area. In terms of efficiency this is super important.

To visualize the importance, picture a well amended soil with healthy plants in a 100 foot bed compared to a 100 foot bed with a pH that isn’t best for the crop and lacks nutrients available to said crop. Yields can be half as much or even much worse in the bed without proper Ph and available nutrients yet the expenses for weeding, planting, growing transplants, seed etc will be virtually the same for each bed. You can often find the required nutrient levels and pH for most crops online via university resources in your region and if you have trouble finding these resources, reaching out to your regional agrologist can be very helpful. 

Test don’t guess

Each element exists in several different chemical forms within the soil but only a few are able to be taken up by plants. Which chemical form an element is in is determined, in part, by soil pH. As pH changes, so does the availability of each element to be taken up by plants. For instance, the micronutrients iron, manganese, and zinc become more available as pH decreases but molybdenum becomes less available. In our greenhouse production we try to keep our calcium levels high to prevent blossom end rot in our tomatoes but doing so adjusts the pH in a direction that ties up more iron so we must have ample blood meal in our amendment plan to supply plants with their iron or they develop Iron Chlorosis (yellowing of leaves). 

Testing soil is fairly simple and requires only trowels, ziplock bags and a lab to send the samples to. I don’t advocate for home testing kits from garden centers. They lend themselves to too much human error. Remember when it comes to soil tests, there are professional labs that can give accurate results. The accuracy of results can keep you from over applying amendments or under applying. Underapplying can be a waste of money and so can over applying which can also damage the local ecosystem. 

If you just need to understand soil texture analysis you can do at home go here for an easy to read how to.  Soil texture is important as it helps you understand some limiting factors your soil may have. For instance if your soil is very heavy on the sand content, it will likely have a low Cation exchange capacity and will likely need addition of fertility and compost often to keep helps happy. Conversely if your soil is heavy on clay it will likely have drainage issues and can have  a very high cation exchange capacity. 

When and how

Sampling should be done prior to planting and after harvest, but can also be done throughout the season multiple times in greenhouses or long season crops. 

Sampling before planting and after harvesting allows you to know if there are any unused nutrients left in the soil after application. If weather wasn’t a big factor in the lack of nutrient uptake causing nutrients to be left over, adjustments on your application rates may need to be done based on your particular soil conditions/biology and equipment. 

Out of caution, I often sample before applications and right after to “check” myself on my own fertility application rate calculations. 

In our greenhouse with long season indeterminate tomatoes, I’ll test every couple weeks to constantly adjust micronutrients, pH etc. We do this because our plants are growing so vigorously in the greenhouse. They take up nutrients quickly and we don’t want them to starve for any one nutrient or element slowing their production during their long season. 

For each production area multiple random samples in a zigzag pattern should be taken to ensure accuracy. 

The samples should be blended/mixed together for the lab to test to get the best average of the field or production area.  

Sample depths should be no more than 6 inches into the soil profile to accurately show potassium and phosphorus levels. 

Want to learn more about soil testing? Check out our step-by-step guide.

Numbers on the fertilizer bag and what do they mean?

I recall when moving to our region and seeing people spreading vast amounts of fertilizer on their lawns from fertilizer bags that the owners likely had no idea on how to read or calculate all the while there were salmon stencils painted on all the rain gutters in their streets. Cosmetic use of fertilizers can be a discussion for another day where I mostly rant. 

Understanding fertilizer amounts in bags and your crop or lawn’s needs is important to not only protect the environment but also to not waste money. 

The math part

Is your calculator ready, the next step is the fun part.  A bag of fertilizer states nutrient values as N-P-K but in actuality it is %N–%P2O5–%K2O.  

For example, a bag of 10-10-10 fertilizer is 10%N, 10% P2O5, and 10% K2O by weight. 

A conversion factor of 0.43 is required to convert P2O5 to P and a conversion factor of 0.83 to convert K2O to K.  

No conversion factor is necessary for nitrogen.

The following formulas will help you calculate pounds of P or K, which are color-coded to the following example:

Interpreting test results and the magic of simple math

So you’ve done your testing and your soil results are in from the lab, now what? 

If it’s your first time looking at your soil results from a lab it may be a bit intimidating to interpret what all the numbers mean. 

Some tests may provide trace elements as well as the big three (N,P,K) along with soil organic matter, electrical conductivity and pH. 

For the cost of it,  I don’t mind paying for more detailed tests. More known variables allow for more accurate predictions in  crop yields and needs. 

Soil reports will often list your nutrients in pounds per acre or PPM (parts per million). 

The formula lbs/acre = ppm x 2 can be used to convert between the two numbers.

If we look at the report above, K (Potassium) is listed at 762ppm. To know how many pounds of K are in the soil on one acre,   we multiply the ppm x 2 and we get 1524 lbs per acre of K.

1524 lbs of K is at about twice as much as the recommended amounts needed in the soil for something like potatoes which consume a lot of K. Based on this, no application of K would be needed and may not be for a couple years.

As we can see the Organic Matter (OM) content is at 25% (which is fairly high) and all nutrient levels are high, we can tell that this test was done after an application of manure and bedding material.

To calculate your fertilizer needs, you simply look at your suggested ppm of the specific nutrient for the specific crop (often you can find this info in university research and extension papers) and compare it to your soil test results.

 Example:  if your fertility report has a potassium value of 280 lbs/acre and you plan to grow vegetables that require 340 lbs/acre, then you would apply based on the formula below: 

Now you know how many pounds of potassium you need to amend your soil but how many pounds of actual potassium are in your bags of fertilizer? 

I typically use sulphate of potassium (potash) for my potassium needs in our organic operation. Bags are typically listed as 0-0-50 meaning they are heavy on the potassium so you don’t need too many bags. 

If each bag weighs 50lbs then we can expect 25lbs to be K2O or 20.75lbs of K (Potassium) using our conversion factor of .83.

If we need 60 lbs of Potassium (K) according to our formula above based on our soil test results then we need 2.8 bags. 

50 lbs bag x .5 x .83=20.75lbs of K per bag

60lbs of K required / 20.75lbs of K per bag

= 2.8bags  

Electrical Conductivity and salty situations

Soil electrical conductivity (EC) is a measure of the amount of salts in soil (salinity of soil). It is an excellent indicator of nutrient availability and loss, soil texture, and available water capacity.

Often in your soil tests from labs they will let you know when your EC is too high meaning too much salts/fertilizers which can become toxic to your plants and also a hazard to the environment. In greenhouses that don’t receive periodic rain storms or snow to help push salts back into the soil profile, salts can become crystallized in the surface of the soil from evaporation making a poor place for seeds and transplants to start their life. 

Organic fertilizers and synthetic, what’s the difference?

Organic fertilizers are often derived from natural organic sources such as plant or animal wastes as well as ground rock minerals while synthetic fertilizers are made from human-made compounds like ammonium nitrate, ammonium phosphate.

Synthetic fertilizers are often available immediately to the plants and also tend to leave the soil quickly which means multiple applications may be necessary through the season to ensure peak performance of crops. 

Organic fertilizers are often slow release and require healthy soil organism populations to break them down and change them into forms that are available for the plants to uptake.  Because of the slow nature of organic fertilizers, we ensure that we prep our soil and incorporate fertilizers a minimum of two – three weeks ahead of planting to ensure organisms have made the nutrients available. In our greenhouse we can watch this nutrient release happen as mycelial growth quickly coats the soil surface under our landscape fabric and we then know that soon these fungal networks will release the nutrients into a form that can readily be absorbed by our crops. 

I hope this primer on interpreting your soil test results and calculating your needs has been helpful to make your production more efficient and more bountiful. 

Remember, if you have any questions or need help interpreting your soil test results and calculations, reach out to your regional ministry agrologist for assistance.

So roll up your sleeves, get your TI-83 calculator and your pocket protector and get to sciencing your soil, because there is no room for guessing in such a tight margined industry. 

If you are interested in furthering your experience and knowledge in farming check out the Young Agrarians apprenticeship program.