Your client’s crop yielded poorly, and they assume it was due to drought. But you suspect it may be a fertility issue. How can you accurately diagnose the problem?
Let us first examine what dry and drought mean.
A dry year means reduced crop growth and reduced nutrient uptake, since biological, chemical and physical processes are altered, resulting in a reduced soil nutrient availability to crops. Further, depending on management, dry and drought mean altered residual fertilizer nutrients.
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Let us use nitrogen as an example by examining the plant nitrogen-use efficiency. This is the total dry matter or grain yield produced per unit of N absorbed. This physiological parameter, also called physiological efficiency (PEN), is defined as kilograms (or pounds) of grain/kg (or lb.) of fertilizer N used by a plant and is calculated by subtracting the yield of the control treatment from the fertilized treatment and dividing it by the difference between the uptake of N by the fertilized treatment minus that of the control one:
PEN = YN – Y0 / UN – U0
The PEN represents the ability of a plant to transform a given amount of acquired fertilizer N into grain yield and thus depends on genotype characteristics (e.g. harvest index) and environmental and management factors, particularly during reproductive growth.
Low PEN usually suggests suboptimal growth conditions, often caused by nutrient deficiencies other than N and/or by drought stress, insect predation and disease.
As a result, the soil nitrogen status is greatly affected and variability within a field is increased. Obviously, fertilizer does not move very far from where it was placed when it is dry. Also, poor crops do not use nitrogen and lack of moisture means no nitrogen movement.
But as has happened in many parts of the Prairies, late rains and subsequent regrowth of crops change the nutrient status. Also, you may notice a decrease in soil pH, which can be followed by an increase in EC (salts) and available P. These will correct themselves in time.
See examples below from my time as director of the Saskatchewan Soil Testing Laboratory following the dry years of the late 1980s and early ’90s and then wet mid-’90s:
This type of variability will also occur during wet and dry periods within the same growing season.
Other factors influencing crop response to fertilizer application
Applied fertilizer use depends on corresponding soil nutrient availability and potential losses of applied fertilizer. However, several other agronomic factors can cause a poor response to applied nutrients.
Crop cultivars often have varying nutrient requirements depending on yield potential and agroecological conditions.
Available soil nutrient supply at planting will influence crop response to fertilizer. Fertilizer management requires proper assessment of both the soil’s nutrient status and the crop’s nutrient requirements. It is widely accepted today that soil testing allows producers to make more qualified fertility management decisions based on soil nutrient inventory and interpretive criteria of this inventory.
Hence, soil testing has to be an integral part of an attempt to obtain optimum yields. However, like any other assessment tool, soil testing is subject to the laws of statistics and has many limitations. Understanding those allows an understanding of its usefulness.
Choose laboratories based on knowledge of how its staff assesses the soil nutrient inventory (from chemistry to quality of analyses) and how they interpret the results.
The steps in soil testing also come into play. An examination of the four steps involved in the traditional soil testing process suggests current techniques would:
- result in a statistical error of ±22 per cent due to the common sampling schemes (20 samples per field/unit);
- result in varying errors due to analysis (for example on a soil N content of 50 kg N ha-1 in the 0-30 cm depth the maximum analytical error is approximately 12-15 per cent and this would increase as the soil test value decreases and vice versa);
- calibration techniques based on yield curves can commonly result in our ability to describe approximately 50 per cent of changes in the yield by the changes in soil test levels; and,
- fertilizer recommendation models can add wide variation in recommended nutrient levels.
Therefore, it is obvious that conventional soil testing databases have been developed to address “fields” as whole units, or better yet, “large geographic areas” from which deductions can be made for individual fields. This is important when databases thus derived are used for variable rate fertilization in precision farming.
It is remarkable that calibrations of soil tests took place in the 1980s and early ‘90s and a limited number in the early 2000s. Take phosphorus (P), for example. The most recent calibration and response experiment was conducted in Alberta from 1991 to 1993 with wheat, barley and canola.
It was very thorough research carried out on a wide range of soil types across Alberta to determine the frequency at which each crop will respond to phosphate fertilizer. Another thing to remember is that soil testing criteria developed in one region of North America (e.g., southern U.S. or Ontario) are often not relevant to western Canadian farms.
Finally, the quality of sampling can play an important role in deriving proper recommendations. Therefore, make sure a sample is taken from the depth you are sampling. Special care must be given in dry and wet soils. Why? Often samples are taken with probes attached to a truck with the sampler sitting at the front. Soil can fall off the probe in a dry soil situation or become compressed in a wet soil situation, resulting in false readings in both cases.
The nutrient status, especially that of N, of a field, can also be estimated from the previous cropping history, but is more accurately determined by a soil test.
It is generally accepted that improvement in the accuracy of N recommendations requires a reliable estimate of soil N-supplying capability. Mineralization of N is a function of environmental conditions; it can vary from as low as nil under drought conditions to as high as one-third greater than average under favourable conditions.
Late seeding usually results in lower yield potential and reduced response from N fertilizer due to moisture/heat relationships. Also, there is a greater risk of crop loss from increased disease pressure, insects, frost and poor harvest conditions.
Weeds compete with plants for moisture, nutrients and light. Applied fertilizers may stimulate the growth of weed seedlings almost to the same extent as a crop. It is important to control weeds to minimize the competition between weeds and crop plants.
Banding fertilizers or placing fertilizers with the seed makes them less accessible to weeds during the early growing season. However, if too much fertilizer is seed-placed, injury to the seedling will reduce emergence, resulting in higher weed competition.
Another issue is that seedling damage can lead to delayed maturity and increase the risk of damage from fall frosts in northern areas where the growing season is short. In canola, increased seed chlorophyll content indicates delayed maturity and influences crop quality.
Disease infestation also comes into play. Well-nourished, healthy plants have some resistance to many disease organisms. Inadequately nurtured wheat plants seem predisposed to certain diseases such as common root rot. Take-all root rot, for example, is reduced when wheat plants absorb ammonium N and is increased when the plants take up excessive amounts of nitrate-N.
Soil moisture can’t be ignored. Water-holding capacity/movement through the soil profile will depend on the soil’s physical properties. Once water enters the soil, it will move under the effect of gravity or capillary suction. In lower rainfall environments, soil moisture reserves must be considered when choosing fertilizer rates.
The risk of crop damage or failure is higher on poorly drained or flood-prone fields. Lower N fertilizer applications are advised on these fields if adequate drainage cannot be provided. Although well-fertilized crops usually withstand more water, if water stands for more than two or three days, causing saturated conditions, considerable crop damage or complete failure may result.
Coarse textured soils with water tables deeper than 1.2 to 1.8 metres below the surface are often droughty. Yield potential, to a large extent, is restricted by lack of moisture. High rates of N fertilizers are generally not recommended in these soils.
Other parameters influenced by changes in water regime
In addition to target yields, many other parameters vary with water regime, including mineralization, immobilization, leaching, denitrification as well as soil residual, mineralizable, and fertilizer N use efficiency and N content in plants.
Mineralization of N is also a function of environmental conditions (water) and can be effectively reduced to zero under drought conditions or become as much as one-third higher under favourable (moist) conditions compared to “normal” conditions.
Karamanos and Cannon (2002) used a limited amount of data from the work by others to derive a relationship between the mineralization rate constant (k35C) and organic carbon content that allowed the estimation of mineralizable N. For organic matter levels less than eight per cent, an average estimate can be made by multiplying the percentage of organic matter from the soil test by 14. In general, under “normal” conditions, 80 per cent of mineralizable nitrogen is available to the crops.
Immobilization: We all understand that decomposition of plant residues requires extra N. How much N is required to fuel the decomposition process? It depends on how much crop residue is incorporated into the soil.
There is information to suggest that in high-residue situations, as much as 20-40 lbs. of broadcast-applied N can be immobilized during straw decomposition. The obvious way to prevent significant immobilization losses is to place the N fertilizer in compact bands. Broadcast-applied N is extremely vulnerable to immobilization losses when it is incorporated into the same soil layer as the straw.
Once the grain is harvested, incorporating the remaining crop residue will immobilize a significant amount of N from the soil in the following year. In these situations, loss of N fertilizer can be greatly reduced by concentrating the fertilizer in bands rather than broadcasting and incorporating it so that it is in intimate contact with the decomposing residue.
Leaching is not a major issue or an issue at all in most of the Prairies, with the possible exception of the Red River Valley and sandy soils. For example, nitrate leaching occurs when nitrate is carried below the crop root zone by water draining through the soil.
Unlike ammonium (NH4+), nitrate (NO3-) is a negatively charged ion and is not held onto soil particles. Further, unlike the other processes, nitrate leaching is not driven by microbes and is not as temperature dependent.
Nitrate leaching is most likely to occur under these conditions: sandy textured soils, high rainfall (or excessive irrigation), summerfallow (no crop present for uptake, therefore, significant amounts of surplus water drain through the soil profile), and high levels of soil nitrate.
Interestingly, periods of the highest water drainage correspond closely to periods of highest nitrate N content. Because subsoils are still frozen during the early part of the spring thaw, leaching is usually not of major concern.
In continuously cropped situations, nitrate leaching can be minimized because crop uptake prevents the accumulation of surplus soil nitrate in soils. Fall fertilizers should be banded after soils are cool, and nitrate-containing fertilizers should be avoided except on vigorously growing crops.
Denitrification is the anaerobic microbial process that converts nitrate (NO3-) into nitrogen gases including N2 and N2O.
Denitrification rates increase under anaerobic conditions when the soil is warm (at least 5 C, optimum above 20 C) and pH between six and eight and, of course, when high levels of nitrate are present in the soil. Losses from denitrification are most likely in poorly drained soils with high nitrate levels.
Although denitrification losses occur slowly on cold soils, cumulative losses in cold, wet soils can be significant during early spring thaw conditions. As I mentioned in my 2022 article on fall fertilization, research conducted at the University of Saskatchewan in the 1980s showed about 35 per cent of autumn-applied fertilizer nitrogen was lost via denitrification during snowmelt the following spring.
During the growing season, denitrification losses are much more rapid if soils are subject to flooding during summer months when soil temperatures are much higher. It can also happen later in the growing season after ammonium fertilizer has converted to nitrate, but before it has been used by the growing crop, especially on clay-textured soils in the black or gray soil zones.
Banding provides drought protection
Extensive research conducted on the Prairies, originally by the University of Alberta, led researchers to conclude that banding provides a form of drought-proofing. The research results indicated clearly that the advantage of banding over broadcasting became more significant as growing season moisture supply became less favourable.
The researchers suggested deep banding’s “drought buffering” should be regarded as a management tool to lessen drought’s adverse effects. Yields fluctuated much more dramatically with broadcast treatments than band treatments as the amount of water available to the crop was varied.
Since no region of the Prairies is immune to drought, band application of fertilizer would be beneficial in all regions. However, the greatest benefits would accrue in the brown, dark brown and thin black soil zones.
But this research does not suggest that banding is a substitute for moisture. Banding is only beneficial when there’s sufficient moisture to produce a crop. It’s of no benefit in the case of a total crop failure. Banding also slows the conversion of urea to ammonium and ammonium to nitrate, which can reduce losses by denitrification and leaching.
Good soil moisture opens new opportunities
Recent precipitation may ease drought in many Prairie regions. Remember that soil moisture is an important component of the crop yield equation and it is an invaluable tool for the producer for targeting crop yield for the upcoming growing season.
Targeting for low yields when yield potential is great will result in under-fertilizing crops, reducing crop yields and protein content.
Although precipitation is hard to predict, soil moisture is easy to estimate with a soil moisture probe. A soil moisture probe is simply a half-inch rod that is 3.5 to four feet long and has a 5/8-inch ball bearing at one end and a T-handle at the other. Soil moisture is measured by pushing the probe into the ground. When the soil runs out of moisture, you will not be able to push the probe any further. However, make sure you haven’t hit a stone or ice in the spring.
Here is the available water based on the texture and depth:
What are the opportunities?
Knowing soil-available moisture allows you to fertilize accordingly, resulting in target yields that are average, below or above average. Choose the right target and fertilize accordingly.
With improved moisture conditions, fertilization of crops that are to be grown on drought-stricken fields with moderate nitrogen rates will undoubtedly result in higher yield and/or protein content.
Make sure you have a soil test and probe several fields or management zones with different textures to establish the moisture status in your area and obtain a good estimate of potential yield. Supply the crop with adequate phosphorus and any other nutrients required.
